The Standard Model has three fundamental forces, the electromagnetic force, the weak force, and the strong force. I outline the categories of particle interactions with these forces below, with the most plausible missing particle categories to explore in bold.
* Standard Model quarks which are fermions interact with all three of these forces. They have weak force interactions (in beta decay) mediated by W and Z bosons, they have strong force interactions mediated by gluons, and they have electromagnetic interactions mediated by photons. There are no known bosons that interact with all three of these forces (if there were, they would presumably be charged gluons with some rest mass and might be hard to distinguish experimentally from light charged mesons like charged pions and charged kaons, for example, perhaps either the short kaon or the long kaon is really a charged gluon with rest mass and not a composite particle).
* Charged leptons (which are fermions) and W+/W- bosons interact with two of these forces, the electromagnetic force and the weak force, but not with the strong force.
* Neutrinos (which are fermions), Z bosons and hypothetical Higgs bosons interact with one of these forces, the weak force, but not with the electromagnetic force or the strong force.
* Gluons (which are bosons) also only interact with one of these forces, the strong force, but not with the electromagnetic or the weak forces. No fermions are known that interact with the strong force, but do not interact with the weak force or the electromagnetic force. Such fermions would presumably have no charge and no rest mass, but would have a color charge.
* Photons (which are bosons) interact with one of these forces, the electromagnetic force, but not with the strong force or the weak force. No fermions are know that interact only with the electromagnetic force. Such fermions would presumably have charge, but no rest mass.
* Hypothetical gravitons (which are bosons) interact with none of these three forces. They have no charge, no rest mass, and color charge. There are no fermions that have no charge, no rest mass, and no color charge, although until recently the neutrinos were believed to be particles of this type and right handed neutrinos, if they existed would fit in this category. A scalar gravitational boson that interacted with no force but gravity could also have no rest mass, but without a force to carry, could be the prototypical dark energy carrier, making it a natural complement to the graviton as the implementation of the cosmological constant component of general relativity. Its absence of non-gravitational effects and constant speed of light movement would probably make the direct detection of such bosons impossible even if they did exist. This might be equivalent to the hypothetical graviscalar.
Three of the four gaps in these categories could be resolved if there was a law of physics that provided that there could be no massless fermions (a gap the might also explain the non-existence of right handed neutrinos, which would interact with nothing but gravity and might not even interact with gravity as rest mass seems to be linked to weak force interactions). There are several other conceivable combinations of interactions and fermion/boson status that have no particles associated with them.
* There are no fermions or bosons that interact with the two forces that are the weak force and the strong force, but not the electromagnetic force. The fermions would be electrically neutral quarks. The bosons would be gluons with rest mass.
* There are no fermions or bosons that interact with the two forces that are the electromagnetic force and the strong force. The fermions would be quarks with no rest mass, almost like up quarks except that nothing would ever decay into them. A no massless fermion rule would exclude them. The bosons would be electrically charged gluons.
Thus, to recap, the missing particle categories that would make sense if there was a no massless fermion rule would be as follows:
Fermions:
1. electrically neutral quarks
Bosons:
1. charged gluons with some rest mass
2. gluons with rest mass
3. electrically charged gluons
4. a scalar gravitational boson (perhaps associated with dark energy)
It is worth observing that there are composite analogs to three of the four strong force interacting missing categories.
* There aren't any electrically neutral quarks (basically neutrinos with color charge), but there are quite a few electrically neutral baryons that are fermions, most famously, the neutron.
Electrically neutral quarks, if they existed, might be good dark matter candidates, since their stable hadrons might be lighter than ordinary baryonic matter for deep reasons that somehow match the reality that neutrinos are lighter than charged leptons. Since they would have mass, they would have weak force interactions, at least through Z bosons. To have W+ or W- interactions, there would have to be quarks with an electrical charge of +/- 1 as well (basically electrons with color charge), to mimic neutrino-charged lepton couplings. But, their absence from the Z boson decay spectrum argues against their existence.
* There are electrically charged mesons (e.g. charged pions and charged kaons) that are electrically charged bosons with rest mass that have color charges that can lead to strong force interactions within them.
* There are electrically neutral mesons (e.g. neutral pions and neutral kaons) that are bosons and have rest mass that have color charges that can lead to strong force interactions within them. Glueballs are also hypothetical bosons that have rest mass and color charges that can lead to strong force interactions within them, even though isolated gluons lack mass.
One could imagine that Nature lacks fundamental particles that simply do what composite particles could out of some sort of cosmic avoidance of superfluous particles rule. Alternately, one could imagine that some of the so called fundamental particles are themselves composite particles made out of preons, so that all of the available categories (or at least many of them) are actually filled by composite particles.
There are no electrically charged gluons without rest mass. Indeed, there are no electrically charged particles of any kind without rest mass; all massless particles lack electromagnetic charge. Perhaps, as a consequence of the deep links associated with electroweak unification and the apparent role of the weak force in giving rise to fundamental particle mass (since all massive particles have weak force interactions and no massless particles have weak force interactions), it is impossible for charged particles to lack rest mass entirely.
The converse proposition, that massive particles must have electromagnetic charges, is not true, unless neutrinos and Z bosons and Higgs bosons are actually all composite particles (a not completely absurd possibility).
The Higgs boson mass is remarkably close to the sum of the W+, W- and Z boson masses divided by two (or perhaps more generally, the sum of all four electroweak boson masses including the massless photon), suggesting some sort of linear combination of them, which has a composite character to it.
It is not hard to imagine a scenario in which the Z boson is a composite of the W+ and W- bosons although it doesn't fit the story of electroweak unification very well, indeed, electroweak unification imagines the Z boson and photons as non-trival linear combinations of a B boson and a neutral W boson via Higgs boson interactions, which makes both Z bosons and photons, in some sense, quasi-composite even in the Standard Model.
Composite neutrinos are a harder pill to swallow, but the neutron certainly provides an analogy, and if light particles can turn energy into mass to emit W and Z bosons, while W and Z bosons can turn mass into energy to emit particles lighter than themselves (or at higher energies, particles heavier than themselves), it certainly doesn't defy nature to imagine that composite neutrinos could have masses less than their component parts.
This isn't to say that any of these particles are actually composite, although they might all be varying manifestations of a single fundamental particle in different configurations a la string theory. The notions of democratic Z boson decay and quark-lepton complementarity, for example, discourages the idea of thinking about any fermions as composite. They all seem to be on a equal footing (or at least 3-1 proportionality due to color charge distinctions) in some sense.
Thursday, December 8, 2011
How Do We Know A 125 GeV Signal Is A Spin Zero Higgs Boson?
Suppose that we learn that there is evidence of a 125 GeV mass particle at LHC. How do we know that it is a spin zero particle, as the Higgs boson is hypothesized to be? It turns out that this isn't too difficult, as comments in response to a question from Doug Sweetser at this blog post disclose.
Q: "Let's say there is a signal around 125 GeV. What kind of work is then required to show that the signal has a particular spin? The Higgs must be spin 0, but if this was some really odd process that was not in the selection code, and it had a spin different from 0, then one see a real signal that was not the Higgs boson. The analysis of this huge amount of data must presume our understanding of what can happen is almost perfectly complete. Doug Sweetser | 12/08/11 | 14:04 PM"
Answer 1: "'What kind of work is then required to show that the signal has a particular spin?'
A simple way is the measurement of the angular distribution of the Higgs particles. Higgs boson is a scalar, and so we have to expect a spherical angular distribution. Nick (not verified) | 12/08/11 | 14:57 PM"
Answer 2: "If it decays into two photons you are almost done. There is no angular momentum of the two particles in the centre of mass frame so the spin of the original particle is the sum of the spins on the particles it decayed into. A photon has spin +1 or -1 so you already know the source was spin 0 or 2. If they can get the polarization of the two photons they would know which. PhilG (not verified) | 12/08/11 | 18:07 PM."
The rumors have it that much of the evidence for a Higgs boson to be announced on December 13 is from the diphoton channel, so this makes Answer 2 immediately applicable. Also, while neither of the answers above mentions the fact, due to charge conservation, any diphoton signal (as well as any other electrically neutral set of decay products) must also have an electrically neutral source.
The data should be consistent with either a Standard Model Higgs boson or lightest SUSY Higgs boson
Both of these features (spin zero, electromagnetic charge zero), in a particle with 125 GeV rest mass, would be identical to those of the hypothetical Standard Model Higgs boson and unlike any other previously observed particle, although the lightest neutral SUSY Higgs boson of the supersymmetric set of five Higgs bosons would be, so far as I know, indistinguishable from a Standard Model Higgs boson in all respects. SUSY differs from the Standard Model in the Higgs sector by having more of them, not by having a lightest Higgs boson any different from the Standard Model one.
Per Wikipedia: "the Higgs boson has no spin, hence no intrinsic angular momentum. The Higgs boson is also its own antiparticle and is CP-even." The main distinction between CP-even and CP-odd particles observed experimentally is the distinction between the short kaon (CP-even that decays into two pions) and the long kaon (CP-odd that decays into three pions). Presumably, examination of the number of decay products in the presumed Higgs boson signals (which are probably already part of the screening process looking for a Higgs boson signal) would reveal its CP-odd v. CP-even character as soon as a signal is detected.
The already observed and measureable Higgs field vacuum expectation value also supports the existence of a Higgs boson to create it, and as discussed in previous posts at this blog, both much lower mass Higgs bosons, and much higher mass Higgs bosons over a wide mass range have been experimentally ruled out. So, if there is a Higgs boson, to give rise to a Higgs field vev it must be at approximately this mass range, which also conveniently makes the vacuum stable, or at least metastable, up to or almost up to Planck scale energies.
Known Particles Considered As Alternative Signal Sources
All of the Standard Model particles are either spin 1/2 or spin 1, so diphoton signals effectively rule out any Standard Model fundamental particles or higher mass versions of them.
No Standard Model baryons have spin 0 or 2, so they are ruled out.
There are experimentally known peudoscalar mesons with spin 0, but all of them are far too light. All of the possible pseudoscalar mesons in the Standard Model have been observed experimentally, and all of them have a rest mass of under 6 GeV. The most common pseudoscalar mesons have only about 0.1% of the rumored Higgs boson mass. There are no Standard Model mesons with spin 2.
Hypothetical Particles Considered As Alternative Signal Sources
Since we are looking for something never before observed, it is appropriate to consider hypothetical particles other than the Higgs boson to see if those particles could also match the signal that has apparently been seen. The other possibilities, with one exception, are inconsistent with the rumored signal, and in the last case, the hypothetical supersymmetric particle called the sneutrino seems to have been experimentally ruled out in the relevant mass range by other means.
The only predicted hypothetical particle with spin 2 of which I am aware is the graviton (not even supersymmetry predicts any other spin 2 particles), which is also predicted to have a zero rest mass, which would be inconsistent with a 125 GeV rest mass signal. Thus, if the commentator above is correct on conservation of spin, and the particle is a hypothetical particle other than a Higgs boson, a diphoton signal at a 125 GeV rest mass much must still presumably be some other kind of previously unobserved spin 0 particle aka a scalar particle.
There are hypothetical supersymmetric particles with spin 0 in addition to a lightest Higgs boson with a neutral electrical charge called sneutrinos, but they are predicted to have masses in the tens of TeV by most versions of these theories which have a supersymmetric boson mass multiple that is very large. But, those mass estimates are particularly speculative in the case of sneutrinos since we have only a dim idea of the mass of the neutrinos themselves and those masses are many orders of magnitude lower than the masses of other fermions which have bosonic superpartners in SUSY models. It wouldn't be absolutely inconceivable that an electrically neutral, spin zero signal at 125 GeV could be produced by an electron or muon sneutrino, but it would be very surprising. Recent LHC searches for the sneutrino in other channels distinctive to the sneutrino appear to have ruled it out at the 125 GeV mass, although not absolutely definitively. One non-standard supersymmetric theory proposed in a preprint this summer even hypothesizes that the Higgs boson is an sneutrino.
The hypothetical spin zero, electrically neutral axion is far too light (about the mass of a neutrino or less) to fit this signal and has been ruled out experimentally by other means. Supersymmetric and extradimensional theories also offer up the saxion aka dilaton as an electrically neutral, spin zero hypothetical superpartner of the axion with an expected relatively low mass, but the fact that the neutral axion has been effectively ruled out experimentally makes the existence of a supersymmetric partner to it implausible as well. Not everyone is convinced that the "strong CP problem" that it was proposed to solve is really a problem with QCD physics at all, and the strong CP problem can be solved, mechanically in the Standard Model equations, simply by setting one of the possible constants in those equations to zero.
A spin zero graviscalar, which is predicted in theories to explain the weakness of gravity by assuming that there are five or more dimesions to time-space, would be expected to have no mass, since gravity is an infinite range force, and also would not be expected to not arise in LHC conditions where gravitational fields are not particularly strong.
The spin zero, charge zero Majoron (predicted to play a part in some versions of a hypothetical seesaw mechanism of neutrino mass generation) would look very much like a Standard Model Higgs boson, but given the strong theoretical motivation for a Higgs boson and the weak theoretical necessity of a Majoron, this interpretation of the LHC results, if they are as rumored, would be disfavored. Majorons would have to have masses in the KeV range to be consistent with the already low in 1996 upper bounds on neutrinoless double beta decay rates, and would be even more constrained by the current experimental bounds of this never experimentally seen form of decay (at least at statistically significant levels reliable enough to constitute a reliable observation) as of 2011. So, this signal cannot be a Majoron either.
Conclusion
The decision tree in evaluating this result, in theory, has multiple steps:
1. Is there a new particle of any kind? If not, examine Higgless models.
2. If there is a particle, is it consistent with the Standard Model Higgs? If not, look for new physics.
And, some people have imagined that it could take a decade to answer the second question.
But, because the Standard Model Higgs boson is so distinctive and simple in its spin zero, charge zero, CP-even properties, and is necessary in both the Standard Model and SUSY models in identical forms, this isn't nearly as daunting a task as it seems. It should be possible to confirm the nature of the newly discovered particle simultaneously with, or very shortly after (a matter of months, not years) a "discovery" class five sigma signal of a Higgs boson is detected, if it is there at all.
Of course, a Higgs boson discovery isn't the immediate end of the road for high energy physics. Particles that no one expected theoretically, most famously, the muon in 1937, which spawned the aphorism "who ordered that?" from 1944 Nobel laureate Isidor Isaac Rabi, have been discovered before through brute force experimentation where nothing in particular was expected in advance, in experiments conducted simply to see what was there in the never before seen world of high energy physics. But, experimental results and theory have given us a much better informed sense of what we should be expecting and what is within the realm of logical possibility, than we had in the 1930s.
If the Standard Model Higgs boson has indeed been found, then confirmation of a hint next week should both reach a five sigma "discovery" threshold and be confirmed to be the predicted Standard Model Higgs boson rather than something else unexpected, within months or perhaps a year of the announcement.
What next?
At that point, assuming that a Higgs boson discovery is confirmed (and the lack of a clear signal in many previous search channels makes that anything but certain), the main tasks left for the LHC will be to confirm or falisfy phenomenological predictions of various supersymmetry theories up to the low to middle single digit TeV scale, and to look for beyond the standard model physics that no one has ordered.
If no BSM physics are observed at the LHC scale, there will still be room to tweak other models that propose new forces or particles or equation terms with just a little more powerful experiment, but all of the fair weather friends of SUSY and String theory will desert it in favor of messier fields like nuclear physics or less mature fields like neutrino physics, leaving only the diehard true believers, if the physics "desert" of the Standard Model at those energy levels in fundamental high energy physics turns out to be a reality. (If supersymmetric physics are found, or something else unexpected is discovered, that will be a whole new adventure.)
The most compelling open question in particle physics if a Higgs boson has been found, but there are no other new physics discoveries in particle physics in the near future, will be the quest to directly observe a particle that can explain the phenomenologically constrained dark matter effects observed by astronomers. This is the only major area in physics where there is experimental data exists that needs to be explained by a theory, and there is not a consensus on what theory fits that data, or experimental confirmation that a particular theory, rather than another one, is the right answer to the problem. (There are many areas of physics where existing theories are unsatisfying or even theoretically unsound at a sufficient level of rigor, or where it is not yet possible to calculate a theoretical prediction accurately, but none of these problems have implied theoretical predictions that are contradicted by experiment.)
No particle fitting the bill of a dark matter candidate has been observed, although many have been ruled out. No force or force carrier that could explain dark matter candidates has been observed directly, although several have been proposed and many of those proposed have been definitively ruled out. Indeed, some direct dark matter searches have seemingly ruled out the existence of particles fitting the parameters that many versions of dark matter explanations of the phenomena observed in astronomy require. Standard Model neutrinos should be too light to fit the bill. No stable Standard Model fundamental particle is heavier than a down quark, and the heaviest electrically neutral Standard Model fundamental particles are the three neutrinos. No stable Standard Model composite particle is heavier than a bound neutron, which is not stable in isolation, there are no stable Standard Model mesons, and the dark matter data from astronomy is apparently inconsistent with hydrogen atoms or other baryonic matter as dark matter. Precision electroweak measurements have ruled out the possibility of weak force interacting particles over almost all of the applicable mass range needed for dark matter models.
Dark matter models consistent with the data that would be best fit by particles that are effectively equivalent to sterile neutrinos with masses in the KeV mass range, but there is no experimental data from particle physics to support the claim that they exist and no direct dark matter searches that have been replicated have found signals that are consistent with each other at any mass range so far. Needless to say, efforts are under way to design better experiments in the appropriate mass ranges to directly search for dark matter, but those experiments won't happen at LHC, which is mostly designed to observe weak force mediated particle decays that don't happen when you have stable particles that don't interact with the weak force by decaying into something familiar. Personally, I expect the answer to be some combination (in approximate order of likelihood) of (1) previously undercounted "dim matter" of the ordinary variety, (2) general relativistic effects attributable to the angular momentum of matter in galaxies and photons and gravitons in transit that simplistic approximations have overlooked, (3) free glueballs (which might be unstable in the vicinity of quarks outside of deep space), (4) neutrino condensates, (5) quantum gravity corrections related to the discreteness or non-locality of space-time or to uncertain principle effects in the graviton propogation equations that manifest at a detectable level only in weak gravitational fields that modify the classical equations of general relativity (perhaps resulting in something like Moffat's gravitational equation modifications), (6) right handed neutrinos that are stable unlike left handed neutrinos that interact with the weak force, (7) composite leptons interacting via weak force bosons) that are only weakly bound to each other, (8) heavy sterile neutrinos, (9) the lighest supersymmetric particles, or (10) something like an electrically neutral up quark that doesn't interact with the weak force either, with the mass predicted by Koide's formula for the up quark, that is somehow stable without confinement or doesn't require nearly so strong a gluon mediated strong force field to bind it into color charge neutral hadrons due to its lack of electroweak interactions.
Every hint of new physics beyond the Standard Model in the last three decades or so (except the discovery that neutrinos have mass) has been a disappointment. Excessive CP violations, particle-antiparticle matter asymmetries, and more have turned out to be false alarms. But, there always seems to be some surprise or another around the corner in a new venue (such as the apparent superluminal neutrino results from OPERA that everybody doubts but nobody has found a way to contradict yet).
Q: "Let's say there is a signal around 125 GeV. What kind of work is then required to show that the signal has a particular spin? The Higgs must be spin 0, but if this was some really odd process that was not in the selection code, and it had a spin different from 0, then one see a real signal that was not the Higgs boson. The analysis of this huge amount of data must presume our understanding of what can happen is almost perfectly complete. Doug Sweetser | 12/08/11 | 14:04 PM"
Answer 1: "'What kind of work is then required to show that the signal has a particular spin?'
A simple way is the measurement of the angular distribution of the Higgs particles. Higgs boson is a scalar, and so we have to expect a spherical angular distribution. Nick (not verified) | 12/08/11 | 14:57 PM"
Answer 2: "If it decays into two photons you are almost done. There is no angular momentum of the two particles in the centre of mass frame so the spin of the original particle is the sum of the spins on the particles it decayed into. A photon has spin +1 or -1 so you already know the source was spin 0 or 2. If they can get the polarization of the two photons they would know which. PhilG (not verified) | 12/08/11 | 18:07 PM."
The rumors have it that much of the evidence for a Higgs boson to be announced on December 13 is from the diphoton channel, so this makes Answer 2 immediately applicable. Also, while neither of the answers above mentions the fact, due to charge conservation, any diphoton signal (as well as any other electrically neutral set of decay products) must also have an electrically neutral source.
The data should be consistent with either a Standard Model Higgs boson or lightest SUSY Higgs boson
Both of these features (spin zero, electromagnetic charge zero), in a particle with 125 GeV rest mass, would be identical to those of the hypothetical Standard Model Higgs boson and unlike any other previously observed particle, although the lightest neutral SUSY Higgs boson of the supersymmetric set of five Higgs bosons would be, so far as I know, indistinguishable from a Standard Model Higgs boson in all respects. SUSY differs from the Standard Model in the Higgs sector by having more of them, not by having a lightest Higgs boson any different from the Standard Model one.
Per Wikipedia: "the Higgs boson has no spin, hence no intrinsic angular momentum. The Higgs boson is also its own antiparticle and is CP-even." The main distinction between CP-even and CP-odd particles observed experimentally is the distinction between the short kaon (CP-even that decays into two pions) and the long kaon (CP-odd that decays into three pions). Presumably, examination of the number of decay products in the presumed Higgs boson signals (which are probably already part of the screening process looking for a Higgs boson signal) would reveal its CP-odd v. CP-even character as soon as a signal is detected.
The already observed and measureable Higgs field vacuum expectation value also supports the existence of a Higgs boson to create it, and as discussed in previous posts at this blog, both much lower mass Higgs bosons, and much higher mass Higgs bosons over a wide mass range have been experimentally ruled out. So, if there is a Higgs boson, to give rise to a Higgs field vev it must be at approximately this mass range, which also conveniently makes the vacuum stable, or at least metastable, up to or almost up to Planck scale energies.
Known Particles Considered As Alternative Signal Sources
All of the Standard Model particles are either spin 1/2 or spin 1, so diphoton signals effectively rule out any Standard Model fundamental particles or higher mass versions of them.
No Standard Model baryons have spin 0 or 2, so they are ruled out.
There are experimentally known peudoscalar mesons with spin 0, but all of them are far too light. All of the possible pseudoscalar mesons in the Standard Model have been observed experimentally, and all of them have a rest mass of under 6 GeV. The most common pseudoscalar mesons have only about 0.1% of the rumored Higgs boson mass. There are no Standard Model mesons with spin 2.
Hypothetical Particles Considered As Alternative Signal Sources
Since we are looking for something never before observed, it is appropriate to consider hypothetical particles other than the Higgs boson to see if those particles could also match the signal that has apparently been seen. The other possibilities, with one exception, are inconsistent with the rumored signal, and in the last case, the hypothetical supersymmetric particle called the sneutrino seems to have been experimentally ruled out in the relevant mass range by other means.
The only predicted hypothetical particle with spin 2 of which I am aware is the graviton (not even supersymmetry predicts any other spin 2 particles), which is also predicted to have a zero rest mass, which would be inconsistent with a 125 GeV rest mass signal. Thus, if the commentator above is correct on conservation of spin, and the particle is a hypothetical particle other than a Higgs boson, a diphoton signal at a 125 GeV rest mass much must still presumably be some other kind of previously unobserved spin 0 particle aka a scalar particle.
There are hypothetical supersymmetric particles with spin 0 in addition to a lightest Higgs boson with a neutral electrical charge called sneutrinos, but they are predicted to have masses in the tens of TeV by most versions of these theories which have a supersymmetric boson mass multiple that is very large. But, those mass estimates are particularly speculative in the case of sneutrinos since we have only a dim idea of the mass of the neutrinos themselves and those masses are many orders of magnitude lower than the masses of other fermions which have bosonic superpartners in SUSY models. It wouldn't be absolutely inconceivable that an electrically neutral, spin zero signal at 125 GeV could be produced by an electron or muon sneutrino, but it would be very surprising. Recent LHC searches for the sneutrino in other channels distinctive to the sneutrino appear to have ruled it out at the 125 GeV mass, although not absolutely definitively. One non-standard supersymmetric theory proposed in a preprint this summer even hypothesizes that the Higgs boson is an sneutrino.
The hypothetical spin zero, electrically neutral axion is far too light (about the mass of a neutrino or less) to fit this signal and has been ruled out experimentally by other means. Supersymmetric and extradimensional theories also offer up the saxion aka dilaton as an electrically neutral, spin zero hypothetical superpartner of the axion with an expected relatively low mass, but the fact that the neutral axion has been effectively ruled out experimentally makes the existence of a supersymmetric partner to it implausible as well. Not everyone is convinced that the "strong CP problem" that it was proposed to solve is really a problem with QCD physics at all, and the strong CP problem can be solved, mechanically in the Standard Model equations, simply by setting one of the possible constants in those equations to zero.
A spin zero graviscalar, which is predicted in theories to explain the weakness of gravity by assuming that there are five or more dimesions to time-space, would be expected to have no mass, since gravity is an infinite range force, and also would not be expected to not arise in LHC conditions where gravitational fields are not particularly strong.
The spin zero, charge zero Majoron (predicted to play a part in some versions of a hypothetical seesaw mechanism of neutrino mass generation) would look very much like a Standard Model Higgs boson, but given the strong theoretical motivation for a Higgs boson and the weak theoretical necessity of a Majoron, this interpretation of the LHC results, if they are as rumored, would be disfavored. Majorons would have to have masses in the KeV range to be consistent with the already low in 1996 upper bounds on neutrinoless double beta decay rates, and would be even more constrained by the current experimental bounds of this never experimentally seen form of decay (at least at statistically significant levels reliable enough to constitute a reliable observation) as of 2011. So, this signal cannot be a Majoron either.
Conclusion
The decision tree in evaluating this result, in theory, has multiple steps:
1. Is there a new particle of any kind? If not, examine Higgless models.
2. If there is a particle, is it consistent with the Standard Model Higgs? If not, look for new physics.
And, some people have imagined that it could take a decade to answer the second question.
But, because the Standard Model Higgs boson is so distinctive and simple in its spin zero, charge zero, CP-even properties, and is necessary in both the Standard Model and SUSY models in identical forms, this isn't nearly as daunting a task as it seems. It should be possible to confirm the nature of the newly discovered particle simultaneously with, or very shortly after (a matter of months, not years) a "discovery" class five sigma signal of a Higgs boson is detected, if it is there at all.
Of course, a Higgs boson discovery isn't the immediate end of the road for high energy physics. Particles that no one expected theoretically, most famously, the muon in 1937, which spawned the aphorism "who ordered that?" from 1944 Nobel laureate Isidor Isaac Rabi, have been discovered before through brute force experimentation where nothing in particular was expected in advance, in experiments conducted simply to see what was there in the never before seen world of high energy physics. But, experimental results and theory have given us a much better informed sense of what we should be expecting and what is within the realm of logical possibility, than we had in the 1930s.
If the Standard Model Higgs boson has indeed been found, then confirmation of a hint next week should both reach a five sigma "discovery" threshold and be confirmed to be the predicted Standard Model Higgs boson rather than something else unexpected, within months or perhaps a year of the announcement.
What next?
At that point, assuming that a Higgs boson discovery is confirmed (and the lack of a clear signal in many previous search channels makes that anything but certain), the main tasks left for the LHC will be to confirm or falisfy phenomenological predictions of various supersymmetry theories up to the low to middle single digit TeV scale, and to look for beyond the standard model physics that no one has ordered.
If no BSM physics are observed at the LHC scale, there will still be room to tweak other models that propose new forces or particles or equation terms with just a little more powerful experiment, but all of the fair weather friends of SUSY and String theory will desert it in favor of messier fields like nuclear physics or less mature fields like neutrino physics, leaving only the diehard true believers, if the physics "desert" of the Standard Model at those energy levels in fundamental high energy physics turns out to be a reality. (If supersymmetric physics are found, or something else unexpected is discovered, that will be a whole new adventure.)
The most compelling open question in particle physics if a Higgs boson has been found, but there are no other new physics discoveries in particle physics in the near future, will be the quest to directly observe a particle that can explain the phenomenologically constrained dark matter effects observed by astronomers. This is the only major area in physics where there is experimental data exists that needs to be explained by a theory, and there is not a consensus on what theory fits that data, or experimental confirmation that a particular theory, rather than another one, is the right answer to the problem. (There are many areas of physics where existing theories are unsatisfying or even theoretically unsound at a sufficient level of rigor, or where it is not yet possible to calculate a theoretical prediction accurately, but none of these problems have implied theoretical predictions that are contradicted by experiment.)
No particle fitting the bill of a dark matter candidate has been observed, although many have been ruled out. No force or force carrier that could explain dark matter candidates has been observed directly, although several have been proposed and many of those proposed have been definitively ruled out. Indeed, some direct dark matter searches have seemingly ruled out the existence of particles fitting the parameters that many versions of dark matter explanations of the phenomena observed in astronomy require. Standard Model neutrinos should be too light to fit the bill. No stable Standard Model fundamental particle is heavier than a down quark, and the heaviest electrically neutral Standard Model fundamental particles are the three neutrinos. No stable Standard Model composite particle is heavier than a bound neutron, which is not stable in isolation, there are no stable Standard Model mesons, and the dark matter data from astronomy is apparently inconsistent with hydrogen atoms or other baryonic matter as dark matter. Precision electroweak measurements have ruled out the possibility of weak force interacting particles over almost all of the applicable mass range needed for dark matter models.
Dark matter models consistent with the data that would be best fit by particles that are effectively equivalent to sterile neutrinos with masses in the KeV mass range, but there is no experimental data from particle physics to support the claim that they exist and no direct dark matter searches that have been replicated have found signals that are consistent with each other at any mass range so far. Needless to say, efforts are under way to design better experiments in the appropriate mass ranges to directly search for dark matter, but those experiments won't happen at LHC, which is mostly designed to observe weak force mediated particle decays that don't happen when you have stable particles that don't interact with the weak force by decaying into something familiar. Personally, I expect the answer to be some combination (in approximate order of likelihood) of (1) previously undercounted "dim matter" of the ordinary variety, (2) general relativistic effects attributable to the angular momentum of matter in galaxies and photons and gravitons in transit that simplistic approximations have overlooked, (3) free glueballs (which might be unstable in the vicinity of quarks outside of deep space), (4) neutrino condensates, (5) quantum gravity corrections related to the discreteness or non-locality of space-time or to uncertain principle effects in the graviton propogation equations that manifest at a detectable level only in weak gravitational fields that modify the classical equations of general relativity (perhaps resulting in something like Moffat's gravitational equation modifications), (6) right handed neutrinos that are stable unlike left handed neutrinos that interact with the weak force, (7) composite leptons interacting via weak force bosons) that are only weakly bound to each other, (8) heavy sterile neutrinos, (9) the lighest supersymmetric particles, or (10) something like an electrically neutral up quark that doesn't interact with the weak force either, with the mass predicted by Koide's formula for the up quark, that is somehow stable without confinement or doesn't require nearly so strong a gluon mediated strong force field to bind it into color charge neutral hadrons due to its lack of electroweak interactions.
Every hint of new physics beyond the Standard Model in the last three decades or so (except the discovery that neutrinos have mass) has been a disappointment. Excessive CP violations, particle-antiparticle matter asymmetries, and more have turned out to be false alarms. But, there always seems to be some surprise or another around the corner in a new venue (such as the apparent superluminal neutrino results from OPERA that everybody doubts but nobody has found a way to contradict yet).
Sea Monster From Half A Billion Years Ago Described
About 515 million years ago, an early three foot long arthropod (i.e. bug), from a category ancestral to the various subtypes of arthropods that exist today looked like the picture above, and according to an article in the journal Nature, it was a top level predator roaming the seas. This was determined on the basis of a rare well preserved fossil from Australia. Its compound eyes, which were exceptionally well preserved, were three centimeters long and had a resolution similar to that of a modern dragonfly, one of the sharpest eyed arthropods alive today. It predates many of the distinguishing features used to distinguish taxa such as insects, spiders and crabs, from each other (and predates some distinguishing features that all of them share).
This just goes to show that some of the monsters imagined in science fiction aren't really so unrealistic.
SUSY and the Higgs Boson
Lubos Motl has a guest post from Gordon Kane, the author of a SUSY theorist who stated based with reasoning restated this December 6, 2011 preprint that 127 GeV was the preferred Higgs boson mass in his view in SUSY theories before the recent rumors that suggest that CERN will announce on December 13, 2011 that there is a 4.3 sigma 127 GeV Higgs boson signal in the combined data of the two LHC experiements searching for a Higgs boson. As previously noted at this blog, his predictions put most of the supersymmetric particles at an undetectable with current or near term experiments 30 TeV masses, but leave a few in the sub-TeV mass range, including one Higgs boson similar to the Higgs boson of the Standard Model. The detection or non-detection of the ligher particles in Kane's SUSY landscape is most notable, as they leave open the possibility that there are BSM physics within the range of the LHC and the possibility that SUSY can be falisfied.
Motl expands on the implications of a Higgs boson of this mass in this post. A money quote:
FWIW, I think it is rather likely that the LHC will see no new physics whatsoever beyond the Higgs boson, although it will almost surely refine the values we have for various constants of the Standard Model, may refine the beta functions used to describe the running of the coupling constants of the three Standard Model forces, and may expose one or two bad assumptions that are currently widely used in approximating the predictions of the Standard Model from its current equation set. In my view, the Higgs boson find that is widely anticipated greatly reduces the experimental motivation and theoretical imperative to solve "naturalness" concerns about the Standard Model, related to vacuum instability at high energy levels, with SUSY. Post-Higgs, the biggest piece of uncharted Standard Model territory left is in neutrino physics and it isn't obvious that the main two collaborations at the LHC are doing the kind of work that will answer those questions.
Motl also notes a not very impressive rebuttal to String Theory/M Theory critics that says many nice things about unexpected benefits of String Theory methods outside fundamental physics, and positions String Theory as a theory at least as plausible as any alternative theoretical program, but fails to rebut the key point of the critics which is the absence of an experimental justification for String Theory's dominance as a BSM theory taken seriously in academic faculties and in matters like analysis of big dollar collider results against that theory's predictions.
While I'm add it, I will also note one of those other BSM theories,
Motl expands on the implications of a Higgs boson of this mass in this post. A money quote:
One may say that 125 GeV is exactly the borderline between the "visible SUSY" for very low masses and the "SUSY in the closet" for somewhat heavier Higgs masses.
If the Higgs mass is above 125 GeV, models with a high energy supersymmetry breaking scale become favored. They include many natural E 6 grand unified theories and extensions of the minimal supersymmetric standard model (MSSM) with extra multiplets (when we add a single scalar field, we get the so-called NMSSM, where N stands for "next to"). . . .
Quite generally, these models – much like Kane-led M-theory phenomenology – assume that most of the scalar superpartners (superpartners of known fermions) have masses between 20 and 100 TeV or so. The fermionic superpartners (gauginos and Higgsinos) may be much lighter and some of them are often (predicted to be) accessible at the LHC. The third generation of squarks (and sometimes sleptons) is usually somewhat (or much) lighter than the first two generations, exactly in the opposite way than for the known quarks and leptons.
Some of these models try to promote different philosophies and personal twists but a big part of their message is overlapping. The LHC should see a Higgs most likely in the 120-135 GeV window and there could be new physics, most likely some fermionic superpartners of the known gauge bosons (or the Higgs) that should still be accessible. Many of the details remain unknown. It's not guaranteed that the LHC will see something beyond the 125 GeV Higgs boson but I think it's rather likely and so does my Princeton source.
FWIW, I think it is rather likely that the LHC will see no new physics whatsoever beyond the Higgs boson, although it will almost surely refine the values we have for various constants of the Standard Model, may refine the beta functions used to describe the running of the coupling constants of the three Standard Model forces, and may expose one or two bad assumptions that are currently widely used in approximating the predictions of the Standard Model from its current equation set. In my view, the Higgs boson find that is widely anticipated greatly reduces the experimental motivation and theoretical imperative to solve "naturalness" concerns about the Standard Model, related to vacuum instability at high energy levels, with SUSY. Post-Higgs, the biggest piece of uncharted Standard Model territory left is in neutrino physics and it isn't obvious that the main two collaborations at the LHC are doing the kind of work that will answer those questions.
Motl also notes a not very impressive rebuttal to String Theory/M Theory critics that says many nice things about unexpected benefits of String Theory methods outside fundamental physics, and positions String Theory as a theory at least as plausible as any alternative theoretical program, but fails to rebut the key point of the critics which is the absence of an experimental justification for String Theory's dominance as a BSM theory taken seriously in academic faculties and in matters like analysis of big dollar collider results against that theory's predictions.
While I'm add it, I will also note one of those other BSM theories,
Wednesday, December 7, 2011
More Fun With Koide's Formula
Some efforts to post-dict particle masses based on Koide's formula and related concepts are discussed here with a link in particular to a November 2011 pre-print. I've been following the online discussions leading up to it, which starts at the open access Physics Forums and had industry engineer Carl Brannen as an important contributor, from its inception.
The desirability of a way to determine quark masses other than via direct experimental measurement is quite real, in part, because it illuminates "within the Standard Model" relationship that shed light on deeper relationships between its parameters, and in part, because the accuracy with which we know the ligher quark masses (up, down, strange) is only about one significant digit, and only about two significant digits for intermediate quark masses (charm, bottom).
The uncertainty in the values of the fundamental quark masses is something that injects meaningful uncertainty into QCD background estimates looking for new physics, and since the same values for quark masses are used by almost everyone doing QCD calculations, the bias is systemmic. Experimentally measured QED and electroweak constants are known to far greater precision.
The paper's post-dictions, which include an interesting 3-1 relationship between quarks and leptons that parallels that 3-1 relationship in the weak force frequencies of the two types of particles attributable to the fact that there are three colors of quarks, while only one lepton color, are as follows:
Inputs
me = 0.510998910 MeV ± 0.000000013
mμ = 105.6583668 MeV ± 0.0000038
Mq = 3Ml
q = 3 l
Outputs
m = 1776.96894(7) MeV (Tau)
mt = 173.263947(6) GeV (top)
mb = 4197.57589(15) MeV
mc = 1359.56428(5) MeV (charm)
ms = 92.274758(3) MeV (strange)
md = 5.32 MeV (down)
mu = 0.0356 MeV (up)
The results are within the two sigma error bars of experimental determinations in the case of the tau, top, bottom, charm, strange (80-130 MeV) and down quarks (4.1-5.7 MeV), but has a much lower value than one within the error bars for the up quark mass than the canonical estimate of 1.7-3.1 MeV (mean 2.5 MeV) in a +/- one sigma confidence interval.
The experimental mass values of the heavy quarks cited in the article are:
mt = 172.9 GeV +/- 0.60 +/-.9
mb = 4.19 GeV +0.18−0.06
mc = 1.29 GeV +0.05−0.11 GeV
There have been suggestions by Brannen that one of the neutrinos needs to have a negative sign in the formula to fit the data for that series of masses, and a similar sign adjustment for the up quark (a nice parallel of one reversed sign for the quarks and one for the leptons) would produce a more conventional up quark mass.
Another way to deal with up quark mass is to acknowledge the theoretical issues posed in defining it in terms of observables. There are theoretical virtues to thinking of the fundamental "bare" rest mass of the up quark as zero, which would not be inconsistent with the result achieved from a Koide's mass approximation.
Suffice it to say that I think that this is far more than numerology and that there really is a fundamental physics basis for Koide's formula and its variants that explains the mass matrix.
Taken together with the quark-lepton complementarity principal that relates the parameters of the CKM matrix to those of the PMNS matrix, in a manner that follows a similar ansatz to the extended Koide's formula, there is a mechanism for making very precise phenomenological predictions of Standard Model physical constants that are hard to obtain accurate measurements of experimentally, and for reducing significantly the number of free physical constants in the Standard Model. These precise predictions ought to be possible to confirm continuously over time as experimental data refines the estimated value of these constants.
The predicted low value for the up quark mass, if correct, would also push the total amount of mass in the universe attributable to hadronic glue, as opposed to fundamental particles, to more than 99.6% of the total.
As a footnote, what would be the mass of a b' and t' quarks under the formula used (derived from the top and bottom quark masses and iterated)? It would be approximately 3,555 GeV (i.e. about 3.6 TeV) for the b' and 82,917 GeV (i.e. about 83 TeV) for the t' and 42.96 GeV for the fourth generation charged lepton tau prime (a little less than half of the Z boson mass), barring any sign issues by my back of envelope (literally) calculations. The quark masses implied by this formula for fourth generation quarks would probably be beyond anything that could be discovered at the LHC in the next few years, and the charged lepton mass is experimentally excluded. It would have otherwise been observed in Z boson decays long ago. This, of course, disfavors the notion that there is a fourth generation of fermions at all, something already disfavored by the 45 GeV minimum mass for a fourth generation "fertile" neutrino, which is much, much heavier than the best estimates for the mass of the third generation neutrino without any clear justification or precedent.
The author of some of these papers, Alejandro Rivero, also has the interesting idea, although not stated as clear in his papers as might be hoped, that supersymmetric particles are not fundamental, but are instead composite versions of Standard Model fundamental particles that have the same loop cancellation effects of SUSY particles.
The desirability of a way to determine quark masses other than via direct experimental measurement is quite real, in part, because it illuminates "within the Standard Model" relationship that shed light on deeper relationships between its parameters, and in part, because the accuracy with which we know the ligher quark masses (up, down, strange) is only about one significant digit, and only about two significant digits for intermediate quark masses (charm, bottom).
The uncertainty in the values of the fundamental quark masses is something that injects meaningful uncertainty into QCD background estimates looking for new physics, and since the same values for quark masses are used by almost everyone doing QCD calculations, the bias is systemmic. Experimentally measured QED and electroweak constants are known to far greater precision.
The paper's post-dictions, which include an interesting 3-1 relationship between quarks and leptons that parallels that 3-1 relationship in the weak force frequencies of the two types of particles attributable to the fact that there are three colors of quarks, while only one lepton color, are as follows:
Inputs
me = 0.510998910 MeV ± 0.000000013
mμ = 105.6583668 MeV ± 0.0000038
Mq = 3Ml
q = 3 l
Outputs
m = 1776.96894(7) MeV (Tau)
mt = 173.263947(6) GeV (top)
mb = 4197.57589(15) MeV
mc = 1359.56428(5) MeV (charm)
ms = 92.274758(3) MeV (strange)
md = 5.32 MeV (down)
mu = 0.0356 MeV (up)
The results are within the two sigma error bars of experimental determinations in the case of the tau, top, bottom, charm, strange (80-130 MeV) and down quarks (4.1-5.7 MeV), but has a much lower value than one within the error bars for the up quark mass than the canonical estimate of 1.7-3.1 MeV (mean 2.5 MeV) in a +/- one sigma confidence interval.
The experimental mass values of the heavy quarks cited in the article are:
mt = 172.9 GeV +/- 0.60 +/-.9
mb = 4.19 GeV +0.18−0.06
mc = 1.29 GeV +0.05−0.11 GeV
There have been suggestions by Brannen that one of the neutrinos needs to have a negative sign in the formula to fit the data for that series of masses, and a similar sign adjustment for the up quark (a nice parallel of one reversed sign for the quarks and one for the leptons) would produce a more conventional up quark mass.
Another way to deal with up quark mass is to acknowledge the theoretical issues posed in defining it in terms of observables. There are theoretical virtues to thinking of the fundamental "bare" rest mass of the up quark as zero, which would not be inconsistent with the result achieved from a Koide's mass approximation.
Suffice it to say that I think that this is far more than numerology and that there really is a fundamental physics basis for Koide's formula and its variants that explains the mass matrix.
Taken together with the quark-lepton complementarity principal that relates the parameters of the CKM matrix to those of the PMNS matrix, in a manner that follows a similar ansatz to the extended Koide's formula, there is a mechanism for making very precise phenomenological predictions of Standard Model physical constants that are hard to obtain accurate measurements of experimentally, and for reducing significantly the number of free physical constants in the Standard Model. These precise predictions ought to be possible to confirm continuously over time as experimental data refines the estimated value of these constants.
The predicted low value for the up quark mass, if correct, would also push the total amount of mass in the universe attributable to hadronic glue, as opposed to fundamental particles, to more than 99.6% of the total.
As a footnote, what would be the mass of a b' and t' quarks under the formula used (derived from the top and bottom quark masses and iterated)? It would be approximately 3,555 GeV (i.e. about 3.6 TeV) for the b' and 82,917 GeV (i.e. about 83 TeV) for the t' and 42.96 GeV for the fourth generation charged lepton tau prime (a little less than half of the Z boson mass), barring any sign issues by my back of envelope (literally) calculations. The quark masses implied by this formula for fourth generation quarks would probably be beyond anything that could be discovered at the LHC in the next few years, and the charged lepton mass is experimentally excluded. It would have otherwise been observed in Z boson decays long ago. This, of course, disfavors the notion that there is a fourth generation of fermions at all, something already disfavored by the 45 GeV minimum mass for a fourth generation "fertile" neutrino, which is much, much heavier than the best estimates for the mass of the third generation neutrino without any clear justification or precedent.
The author of some of these papers, Alejandro Rivero, also has the interesting idea, although not stated as clear in his papers as might be hoped, that supersymmetric particles are not fundamental, but are instead composite versions of Standard Model fundamental particles that have the same loop cancellation effects of SUSY particles.
Generic Limits On Fundamental Particle Mass
The muon has a mean lifetime of about 10^-6 seconds. No other quark or charged lepton has a mean lifetime of more than about 10^-8 seconds, and the half life of a top quark is on the order of magnitude of 10^-25 seconds. W boson are slightly less long lived than top quarks at 3*10^-25 s, and Planck time is on the order of 5*10^-44 seconds. Thus, there is a nineteen order of magnitude difference in decay rate between the longest lived second generation particle, and the shortest lived quark. There is a similar nineteen order of magnitude gap between the top quark/W boson decay time and Planck time.
Generally speaking, the heavier a fundamental particle is, the faster it decays. A muon is about 0.1 GeV. A top quark is about 173 GeV. The implication is that it may be that no particle can have a mass much more than 1700 times the top quark mass (about 294 TeV) without having a decay time of less than Planck time.
One could also related the decay rates to particle generation and note that the nineteen order of magnitude gap is between the longest lived second generation particle and the shortest lived third generation particle. This wouldn't rule out a fourth generation of fermions, but would rule out a fifth generation of fermions.
It is also conceivable that the decay rate of the W boson, which is the means by which fundamental particles can decay, is an absolute limit upon particle decay rates, in which case the top quark is the heaviest possible particle.
Given that all know massive particles decay via the weak force, the mass limitation inferred from decay rates are quite model independent. This kind of reasoning should apply, for example to massive technifermions, massive supersymmetric particles, and hypothetical higher generation bosons.
Now, this isn't necessarily a very strong bound. Some of the models consistent with a lightest Higgs boson of 127 GeV estimate that many of the supersymmetric particles would have masses on the order of 30 TeV, although several would have masses of less than 1 TeV, and experimental bounds on supersymmetric particle are similar to those on next generation quarks discussed below (but under 1 TeV).
Another possible bound on fourth generation Standard Model particles is in the CKM and PMNS matrixes; the absence of detectable amplitudes for transitions to more than three generations suggests that fourth generation Standard Model particles would have to be much heavier than a top quark, and that the decay chain would likely be dominated by a single possibility as the top quark decay is dominated by the decays to bottom quarks.
As I've noted elsewhere, the absence of hadrons made of top quarks also implies, a fortiori, that hadrons made of fourth generation quarks would also not exist. More generally, unless a supersymmetric analog to the strong force had a much more rapid time frame than the Standard Model strong force, supersymmetric hadrons of anything but lowest generation supersymmetric particles would be ruled out, since all supersymmetric particles are heavier than a top quark.
Experimental Bounds on Fourth Generation Particles
Fourth generation quarks (conventionally labeled t' and b') are experimentally excluded for the b' below 199 Gev and for the t' below 256 GeV. These values are not updated for the latest 2011 LHC bounds, which put a lower bound on the b' mass of 385 GeV, and the puts a reasonably expected t' mass values, if there is a t' quark, higher.
If the t' to t quark mass ratio were similar to the t quark to c quark mass ratio, one would expect the t' quark to have a mass of 20 TeV; the b quark to s quark ratio would imply a t' quark mass of 7 Tev; the s quark to u quark mass ratio would imply a t' quark mass of about 3.5 Tev; the tau to muon mass ratio would imply a t' mass of about 3 TeV. The ratio of c quark mass to u quark mass, or of muon to electron mass would suggest an even greater t' mass. Of the b/c, c/s, and s/d mass ratios, none are smaller than a factor of about 3, so one would expect a b' mass of not less than about 522 GeV and much heavier masses on the order of 1 TeV or more would be plausible given expectations for the t' mass.
The lower experimental bound on a tau prime (i.e. fourth generation lepton) mass is about 100 GeV, which is about 55 times the tau mass, and d/s/b type mesons appear to tend to be within an order of magnitude of mass of their same generation charged lepton, so a mass in the high hundreds of GeV would not be particularly unexpected. We know that a Z boson can give rise to a top/antitop pair with a combined mass of 348 GeV which far exceeds the 90 GeV of the Z boson rest mass. But, perhaps at some point there is a limit, and if that limitation is less than the correct mass for fourth generation fermion if there was one, then that limit would prevent fourth generation fermions from arising. The conventional statement of the experimental limitation on fourth generation neutrino mass from precision electroweak measurements is 45 GeV.
Experimental bounds on Z' bosons are in the high hundreds of GeVs.
Generally speaking, the heavier a fundamental particle is, the faster it decays. A muon is about 0.1 GeV. A top quark is about 173 GeV. The implication is that it may be that no particle can have a mass much more than 1700 times the top quark mass (about 294 TeV) without having a decay time of less than Planck time.
One could also related the decay rates to particle generation and note that the nineteen order of magnitude gap is between the longest lived second generation particle and the shortest lived third generation particle. This wouldn't rule out a fourth generation of fermions, but would rule out a fifth generation of fermions.
It is also conceivable that the decay rate of the W boson, which is the means by which fundamental particles can decay, is an absolute limit upon particle decay rates, in which case the top quark is the heaviest possible particle.
Given that all know massive particles decay via the weak force, the mass limitation inferred from decay rates are quite model independent. This kind of reasoning should apply, for example to massive technifermions, massive supersymmetric particles, and hypothetical higher generation bosons.
Now, this isn't necessarily a very strong bound. Some of the models consistent with a lightest Higgs boson of 127 GeV estimate that many of the supersymmetric particles would have masses on the order of 30 TeV, although several would have masses of less than 1 TeV, and experimental bounds on supersymmetric particle are similar to those on next generation quarks discussed below (but under 1 TeV).
Another possible bound on fourth generation Standard Model particles is in the CKM and PMNS matrixes; the absence of detectable amplitudes for transitions to more than three generations suggests that fourth generation Standard Model particles would have to be much heavier than a top quark, and that the decay chain would likely be dominated by a single possibility as the top quark decay is dominated by the decays to bottom quarks.
As I've noted elsewhere, the absence of hadrons made of top quarks also implies, a fortiori, that hadrons made of fourth generation quarks would also not exist. More generally, unless a supersymmetric analog to the strong force had a much more rapid time frame than the Standard Model strong force, supersymmetric hadrons of anything but lowest generation supersymmetric particles would be ruled out, since all supersymmetric particles are heavier than a top quark.
Experimental Bounds on Fourth Generation Particles
Fourth generation quarks (conventionally labeled t' and b') are experimentally excluded for the b' below 199 Gev and for the t' below 256 GeV. These values are not updated for the latest 2011 LHC bounds, which put a lower bound on the b' mass of 385 GeV, and the puts a reasonably expected t' mass values, if there is a t' quark, higher.
If the t' to t quark mass ratio were similar to the t quark to c quark mass ratio, one would expect the t' quark to have a mass of 20 TeV; the b quark to s quark ratio would imply a t' quark mass of 7 Tev; the s quark to u quark mass ratio would imply a t' quark mass of about 3.5 Tev; the tau to muon mass ratio would imply a t' mass of about 3 TeV. The ratio of c quark mass to u quark mass, or of muon to electron mass would suggest an even greater t' mass. Of the b/c, c/s, and s/d mass ratios, none are smaller than a factor of about 3, so one would expect a b' mass of not less than about 522 GeV and much heavier masses on the order of 1 TeV or more would be plausible given expectations for the t' mass.
The lower experimental bound on a tau prime (i.e. fourth generation lepton) mass is about 100 GeV, which is about 55 times the tau mass, and d/s/b type mesons appear to tend to be within an order of magnitude of mass of their same generation charged lepton, so a mass in the high hundreds of GeV would not be particularly unexpected. We know that a Z boson can give rise to a top/antitop pair with a combined mass of 348 GeV which far exceeds the 90 GeV of the Z boson rest mass. But, perhaps at some point there is a limit, and if that limitation is less than the correct mass for fourth generation fermion if there was one, then that limit would prevent fourth generation fermions from arising. The conventional statement of the experimental limitation on fourth generation neutrino mass from precision electroweak measurements is 45 GeV.
Experimental bounds on Z' bosons are in the high hundreds of GeVs.
Monday, December 5, 2011
Implications of a 126 GeV Higgs Boson For Various BSM Physics
A 126 GeV Higgs Boson, which is rumored to have been seen (at a 4.3 sigma, not quite "discovery" standard), has major implications for beyond the Standard Model physics. It destroys the motivation for technicolor or a composite Higgs boson. It tends to disfavor a fourth generation of fundamental particles.
It undermines, without dealing a death blow to, supersymmetry (i.e. SUSY), and by implication, string theory, both because those models favor lighter Higgs bosons and because the justification for their existence is reduced if the Standard Model is stable at high energy levels.
It disfavors any introduction of new fundamental forces with any phenomenological impact.
It seems to be relatively neutral in its impact of extensions of the Standard Model with right handed or otherwise "sterile" neutrinos. It doesn't really have any strong implications for the notion of quark-lepton complementarity.
It leaves a void in the realm of fundamental dark matter candidates, which tends to make heavy stertile neutrino models and composite dark matter candidates attractive.
It doesn't necessarily have much impact on loop quantum gravity inquiries, or on the possibility of a spin-2 graviton stripped from a string theory context.
UPDATE: An exhaustive compilation of 81 recent published Higgs boson mass predictions as of June 2010, including 23 supersymmetric ones, shows what a parlor game this has become. Sixty-six of the predictions would be falsified by a 126 GeV Higgs boson, and many of the rest would be correct only by virtue of wide error bars. One of the many upper bounds would be invalidated, but only by a few GeV which would probably be within the error bars of the announcement a week from today. Fifteen predictions were consistent with this mass, although many of those predictions were consistent only by virtue of their wide error bars and were not particularly close to a 126 GeV predicted value.
In the supersymmetric theories, one of the effects of finding a "lightest Higgs boson mass" (generically, SUSY models tend to have more than one Higgs boson), is that it fixes one of two key constants from which supersymmetric particle masses are derived in these models, dramatically constraining the mass spectrum of the other particles and as a result, making falsification of the models from failure to find other supersymmetric particles in the expected mass ranges much easier.
The Minimal Supersymmetric Standard Model (MSSM), originally proposed in 1981 as the first supersymmetric model, predicted the exitence of new particles in the 100 GeV to 1000 GeV range, and is already a stretch considering experimental bounds already in existence on the existence of such particles and its need for a light Higgs boson mass, but less minimal SUSY theories aren't ruled out at this point. In addition to providing high lower bounds on the masses of the lightest supersymmetric particles, the LHC has also largely ruled out "R-party conserving" models of supersymmetry.
At the very least, the LHC has established that all supersymmetric particles are heavier than than all non-supersymmetric particles by a considerable margin.
It undermines, without dealing a death blow to, supersymmetry (i.e. SUSY), and by implication, string theory, both because those models favor lighter Higgs bosons and because the justification for their existence is reduced if the Standard Model is stable at high energy levels.
It disfavors any introduction of new fundamental forces with any phenomenological impact.
It seems to be relatively neutral in its impact of extensions of the Standard Model with right handed or otherwise "sterile" neutrinos. It doesn't really have any strong implications for the notion of quark-lepton complementarity.
It leaves a void in the realm of fundamental dark matter candidates, which tends to make heavy stertile neutrino models and composite dark matter candidates attractive.
It doesn't necessarily have much impact on loop quantum gravity inquiries, or on the possibility of a spin-2 graviton stripped from a string theory context.
UPDATE: An exhaustive compilation of 81 recent published Higgs boson mass predictions as of June 2010, including 23 supersymmetric ones, shows what a parlor game this has become. Sixty-six of the predictions would be falsified by a 126 GeV Higgs boson, and many of the rest would be correct only by virtue of wide error bars. One of the many upper bounds would be invalidated, but only by a few GeV which would probably be within the error bars of the announcement a week from today. Fifteen predictions were consistent with this mass, although many of those predictions were consistent only by virtue of their wide error bars and were not particularly close to a 126 GeV predicted value.
In the supersymmetric theories, one of the effects of finding a "lightest Higgs boson mass" (generically, SUSY models tend to have more than one Higgs boson), is that it fixes one of two key constants from which supersymmetric particle masses are derived in these models, dramatically constraining the mass spectrum of the other particles and as a result, making falsification of the models from failure to find other supersymmetric particles in the expected mass ranges much easier.
The Minimal Supersymmetric Standard Model (MSSM), originally proposed in 1981 as the first supersymmetric model, predicted the exitence of new particles in the 100 GeV to 1000 GeV range, and is already a stretch considering experimental bounds already in existence on the existence of such particles and its need for a light Higgs boson mass, but less minimal SUSY theories aren't ruled out at this point. In addition to providing high lower bounds on the masses of the lightest supersymmetric particles, the LHC has also largely ruled out "R-party conserving" models of supersymmetry.
At the very least, the LHC has established that all supersymmetric particles are heavier than than all non-supersymmetric particles by a considerable margin.
Higgs Rumors Getting More Specific and More Credible
The latest Higgs rumor (via Not Even Wrong), as of Saturday, regarding a December 13, 2011 announcement of LHC results is as follows:
December 3, 2011:
A 4.3 sigma signal supporting the last particle predicted by the Standard Model (and required for it to work properly) would be a triumph for the Standard Model and will eliminate the need for a large class of theories developed to address the possibility that a light Higgs boson wouldn't be found.
This result, if the rumor is true, also leaves open the very serious possibility that this will be the very last bit of new particle physics discovered experimentally in the foreseeable future. No new non-composite particles or new fundamental forces are needed to make the Standard Model work up to all energy levels that can be probed directly for the foreseeable future.
The implications of a Higgs boson at that mass are explored in this post, which notes that:
The abstract for the referenced paper states:
They sum up their findings in the conclusion of the article itself:
The Planck scale is 2.4*10^18 GeV. The Fermi scale aka the electroweak scale is ca. 246 GeV (aka the Higgs vacuum expectation value).
As another treatment of some of these issues from 2007 explains, when there is a Higgs boson mass in a range that includes 126 GeV:
In other words, the experimentally favored result appears to dovetail neatly with a Higgs boson mass predicted on the basis that it would eliminate the need for any new physics all the way up to the Planck scale.
The Large Hadron Collider experiments are reaching the tens of TeV energy scale. If there is a Standard Model Higgs boson at 126 GeV, then new physics associated with vacuum instability don't crop up until the millions of TeV energy scale in the Standard Model, and it doesn't take much to tweak the appearance of new physics to even higher energy levels, or even to conclude that this is the critical value at which any need for Beyond the Standard Model physics is eliminated.
We might be able to see the pattern that drives the entire Standard Model and produces all of its parameters in a far more constrained manner, once we finally determine what all of its parameters and equations are, but this could well be the moment when we determine that we have the complete set of rules for the universe except in the circumstances where General Relativity and the Standard Model are inconsistent (e.g. the truly point-like nature of fundamental particles).
December 3, 2011:
ATLAS has a 3 sigma excess at 126 GeV while CMS has a smaller excess at 126 GeV, perhaps 2 sigma, both in diphoton channels. These are close enough to combine to give a 3.5 sigma. That would be enough to claim an “observation” but is well short of “discovery”. There will be interest in whether other channels such as ZZ or WW add anything to the result. By the end of 2012 they will have up to four times the data which is enough to multiply the significance by two if the signal holds up. ( I am assuming that the results to be shown on the 13th will use the full 5/fb collected this year. It could be less.) . . . [updated later to] 3.5 sigma in ATLAS and 2.5 sigma in CMS which amounts to about 4.3 sigma combined for the 10/fb. This is about right for the expected significance at this mass.
A 4.3 sigma signal supporting the last particle predicted by the Standard Model (and required for it to work properly) would be a triumph for the Standard Model and will eliminate the need for a large class of theories developed to address the possibility that a light Higgs boson wouldn't be found.
This result, if the rumor is true, also leaves open the very serious possibility that this will be the very last bit of new particle physics discovered experimentally in the foreseeable future. No new non-composite particles or new fundamental forces are needed to make the Standard Model work up to all energy levels that can be probed directly for the foreseeable future.
The implications of a Higgs boson at that mass are explored in this post, which notes that:
Certainly the central topic of the debate will be the stability of the vacuum and whether it implies new physics, and if so, at what scale? . . . It has been known for about twenty years that for a low Higgs mass relative to the top quark mass, the quartic Higgs self-coupling runs at high energy towards lower values. At some point it would turn negative indicating that the vacuum is unstable. In other words the universe could in theory spontaneously explode at some point releasing huge amounts of energy as it fell into a more stable lower energy vacuum state. This catastrophe would spread across the universe at the speed of light in an unstoppable wave of heat that would destroy everything in its path. Happily the universe has survived a very long time without such mishaps so this can’t be part of reality, or can it? As it turns out a Higgs mass of 125 GeV is quite a borderline case. . . . It is also possible that some amount of vacuum instability could really be present. If there is meta-stability the vacuum could remain in its normal state. There would be the possibility of disaster at any moment but the half-life for the decay of the vacuum would have to be more than about the 13 billion years that it has survived so far. . . . At 126 GeV the vacuum might remain stable up to Plank energies (see e.g. Shaposhnikov and Wetterich [2010]). If this is the case then there is nothing to worry about, but depending on the precise values of the standard model parameters, instability could also set in at energies around a million TeV. This is well above anything we can explore at the LHC but such energies are found in the more extreme parts of the universe and nothing bad has happened. The most likely explanation would be that some new unknown physics changes the running of the coupling to avert it from going negative. . . . if the mass of the Higgs turns out to be 120 GeV despite present rumours to the contrary then the stability problem would be a big deal. This would be a big boost for SUSY models that stabilize the vacuum and mostly prefer the light Higgs mass. If on the other hand the Higgs mass was found at 130 GeV or more, then the stability problem would be no issue. 125 GeV leaves us in the uncertain region where more research and better measurements of the top mass will be required.
The abstract for the referenced paper states:
There are indications that gravity is asymptotically safe. The Standard Model (SM) plus gravity could be valid up to arbitrarily high energies. Supposing that this is indeed the case and assuming that there are no intermediate energy scales between the Fermi and Planck scales we address the question of whether the mass of the Higgs boson m_H can be predicted.
For a positive gravity induced anomalous dimension A_lambda is greater than zero the running of the quartic scalar self interaction lambda at scales beyond the Planck mass is determined by a fixed point at zero. This results in m_H=m_{rm min}=126 GeV, with only a few GeV uncertainty.
This prediction is independent of the details of the short distance running and holds for a wide class of extensions of the SM as well. For A_lambda is less than zero one finds m_H in the interval m_{\rm min}< m_H < m_{\rm max}=174 GeV, now sensitive to A_lambda and other properties of the short distance running. The case A_lambda is greater than zero is favored by explicit computations existing in the literature.
They sum up their findings in the conclusion of the article itself:
Detecting the Higgs scalar with mass around 126 GeV at the LHC could give a strong hint for the absence of new physics influencing the running of the SM couplings between the Fermi and Planck/unification scales.
The Planck scale is 2.4*10^18 GeV. The Fermi scale aka the electroweak scale is ca. 246 GeV (aka the Higgs vacuum expectation value).
As another treatment of some of these issues from 2007 explains, when there is a Higgs boson mass in a range that includes 126 GeV:
[T]he SM can be extrapolated, in a fully consistent way, to extremely high-energy scales with a “big desert” picture up to MGUT or even MPl. The presence of right-handed neutrinos at some intermediate scale does not significantly change this conclusion, unless their Majorana masses are very large and we insist to extrapolate the theory at scales even beyond MGUT
In other words, the experimentally favored result appears to dovetail neatly with a Higgs boson mass predicted on the basis that it would eliminate the need for any new physics all the way up to the Planck scale.
The Large Hadron Collider experiments are reaching the tens of TeV energy scale. If there is a Standard Model Higgs boson at 126 GeV, then new physics associated with vacuum instability don't crop up until the millions of TeV energy scale in the Standard Model, and it doesn't take much to tweak the appearance of new physics to even higher energy levels, or even to conclude that this is the critical value at which any need for Beyond the Standard Model physics is eliminated.
We might be able to see the pattern that drives the entire Standard Model and produces all of its parameters in a far more constrained manner, once we finally determine what all of its parameters and equations are, but this could well be the moment when we determine that we have the complete set of rules for the universe except in the circumstances where General Relativity and the Standard Model are inconsistent (e.g. the truly point-like nature of fundamental particles).
Friday, December 2, 2011
Doubt Cast On Another Excess CP Violation In D Mesons
It is starting to look like a high sigma CP violation (relative to the Standard Model expectation) in the decay of mesons with charm quarks in them, call D mesons, at LHC is really a product of an insufficiently accurate QCD calculation that doesn't include high enough order terms in an infinite series approximation and just moderately higher than expected values within the range of normal statistical variation. It could be something if the theoretical expectation calculations aren't too far off, but there is a good chance that it isn't. The literature on the QCD expectation in inconsistent and prior collider experiments show a CP violation of a similar order of magnitude in a comparable experiment that is a bit lower than the one seen in this experiment (the older experiment has a bigger margin of error also consistent with zero).
Prior coverage of the results at this blog can be found here.
Prior coverage of the results at this blog can be found here.
Coming Higgs Attractions
The next major official announcement about the LHC Higgs boson search will be made on December 13, but rumors are already flying and a semi-official internal memorandum proclaims that the result will be inconclusive (i.e. presumably neither a five sigma gold standard detection, nor a 95% confidence interval exclusion of the Standard Model Higgs boson at any mass).
So far, I haven't found the rumors (e.g. a 2-3 sigma signal at 125 GeV) very persuasive, given the data that has been released to date (which seemed to show signal strength getting weaker at masses greater than 119 GeV) and the lack of any more specificity in the rumors to coroborate them (e.g. the lack of the decay channel where the signal appears or any description of the nature of the source of the information). Frankly, the rumors so far sound like nothing more than informed guesses.
I also recall discussions earlier this fall about a proposal for CERN to describe a mere 95% confidence interval exclusion, as opposed to a five sigma exclusion, as "inconclusive" rather than saying that the Higgs boson which is the reason for their existence has been ruled out (as CERN has done in the past when there is a 95% confidence interval exclusion in a certain mass range), and so there could be some redefined semantics going into the internal memorandum about the results being inconclusive. There was never a public announcement from CERN regarding it position on this proposal concerning how it should describe a 95% exclusion over all relevant mass ranges.
Of course, given the sophistication of the physics blogosphere and science journalist corps that has been watching this experiment with an eagle's eye for years, and with special intensity since the LHC run began, the spin that CERN chooses to put on a finding won't really matter.
If there is a 95% confidence interval exclusion of a light Standard Model Higgs boson at all remaining mass ranges, then, no matter what CERN calls it, the result will be touted far and wide within minutes by everyone who has been watching the issue, as a proclaimation that the Standard Model Higg boson is dead, as are the lighest Higgs bosons of all of the most popular SUSY models. Speculation on Higgsless and heavy Higgs bosons will begin in earnest. SUSY theorists will begin to experience intense pangs of existential doubt about how they've spent the last many years of their lives that may escalate into panic.
If there is neither a 95% confidence interval exclusion, nor a signal of a Higgs boson at some mass at something very close to three sigma, then the results really will be considered inconclusive, with the sigma cutoff at which an observer thinks the results mean anything mostly dependent upon that person's theoretical inclinations in advance of the announcement. The lower the significance of the signal, the more interest there will be in the very bottom of the remaining light Higgs boson mass range (115 GeV to whatever the latest exclusion range rules out), since the absence of a signal there has the least meaning.
Gibbs also gets credit for explaining what different possible Higgs boson masses would mean to physicists:
There are no mass ranges much over 140 GeV that haven't already been pretty definitively ruled out by the LHC that are consistent with the theoretical limits on the mass range of the Standard Model's Higgs boson, under some quite reasonable assumptions. Also, notably, "metastable" as used in the link above simply means that the Standard Model breaks down at some energy level less than 10^16 GeV. A modest reduction in the mass of a Higgs boson stable mass of 160 GeV +/-, that is still well in excess of the 1 TeV stable Higgs boson mass of 71 GeV, still leaves lots of "wiggle room" in between that can make any break down of the Standard Model at high energy levels practically inaccessible for the foreseeable future without actually reaching Planck energy.
A 2001 study that looked at the vacuum metastability considerations that Gibbs mentions in the block quote above concluded that:
A Standard Model Higgs boson, if one is discovered, will probably fit squarely in the mass range predicted in 2001, and indeed realistically, within one half of a standard deviation of the mean predicted value from a decade ago based on theoretical considerations.
An absense of a Standard Model Higgs boson anywhere in this mass range would favor composite Higgs models or technicolor models relative to SUSY and the Standard Model, although I wouldn't but bets on any of them at this point. There are also many models that are extremely similar to the Standard Model but have a suppressive Higgs boson signal at the appropriate mass.
I also have considerable doubt about the extent to which the vacuum stability problems are just a result of a particular Higgs boson mass, as opposed to renormalization mathematics that are somehow inaccurate or an inaccurate assumption about the extent to which fundamental particles are point-like or that space-time is continuous or an inaccurate beta function for the running of the coupling constants in the three Standard Model forces or the omission of very heavy fourth generation extremely ephemeral fermions. As another possibility, perhaps the unstable energy range is phenomenologically impossible to reach due to some sort of general relativity effect that has not been properly considered.
Anyway, stay tuned! We'll have the official announcement with lots of the devils in the details in a couple of weeks and will probably have more accurate rumors even sooner.
So far, I haven't found the rumors (e.g. a 2-3 sigma signal at 125 GeV) very persuasive, given the data that has been released to date (which seemed to show signal strength getting weaker at masses greater than 119 GeV) and the lack of any more specificity in the rumors to coroborate them (e.g. the lack of the decay channel where the signal appears or any description of the nature of the source of the information). Frankly, the rumors so far sound like nothing more than informed guesses.
I also recall discussions earlier this fall about a proposal for CERN to describe a mere 95% confidence interval exclusion, as opposed to a five sigma exclusion, as "inconclusive" rather than saying that the Higgs boson which is the reason for their existence has been ruled out (as CERN has done in the past when there is a 95% confidence interval exclusion in a certain mass range), and so there could be some redefined semantics going into the internal memorandum about the results being inconclusive. There was never a public announcement from CERN regarding it position on this proposal concerning how it should describe a 95% exclusion over all relevant mass ranges.
Of course, given the sophistication of the physics blogosphere and science journalist corps that has been watching this experiment with an eagle's eye for years, and with special intensity since the LHC run began, the spin that CERN chooses to put on a finding won't really matter.
If there is a 95% confidence interval exclusion of a light Standard Model Higgs boson at all remaining mass ranges, then, no matter what CERN calls it, the result will be touted far and wide within minutes by everyone who has been watching the issue, as a proclaimation that the Standard Model Higg boson is dead, as are the lighest Higgs bosons of all of the most popular SUSY models. Speculation on Higgsless and heavy Higgs bosons will begin in earnest. SUSY theorists will begin to experience intense pangs of existential doubt about how they've spent the last many years of their lives that may escalate into panic.
If there is neither a 95% confidence interval exclusion, nor a signal of a Higgs boson at some mass at something very close to three sigma, then the results really will be considered inconclusive, with the sigma cutoff at which an observer thinks the results mean anything mostly dependent upon that person's theoretical inclinations in advance of the announcement. The lower the significance of the signal, the more interest there will be in the very bottom of the remaining light Higgs boson mass range (115 GeV to whatever the latest exclusion range rules out), since the absence of a signal there has the least meaning.
Gibbs also gets credit for explaining what different possible Higgs boson masses would mean to physicists:
[A]t [a Higgs boson mass of] 125 GeV . . . the standard model has problems with vacuum stability that are likely to require supersymmetry or something similar to stabilize. If on the other hand the Higgs were at 140 GeV we would be left with a simple but unsatisfying model that could exist without modification up to energies well beyond anything we can explore in man-made experiments. In other words we might never be able to detect anything new.
There are no mass ranges much over 140 GeV that haven't already been pretty definitively ruled out by the LHC that are consistent with the theoretical limits on the mass range of the Standard Model's Higgs boson, under some quite reasonable assumptions. Also, notably, "metastable" as used in the link above simply means that the Standard Model breaks down at some energy level less than 10^16 GeV. A modest reduction in the mass of a Higgs boson stable mass of 160 GeV +/-, that is still well in excess of the 1 TeV stable Higgs boson mass of 71 GeV, still leaves lots of "wiggle room" in between that can make any break down of the Standard Model at high energy levels practically inaccessible for the foreseeable future without actually reaching Planck energy.
A 2001 study that looked at the vacuum metastability considerations that Gibbs mentions in the block quote above concluded that:
[I]mposing the condition that the standard model effective Higgs potential should have two approximately degenerate vacua, such that the vacuum we live in is just barely metastable: the one in which we live has a vacuum expectation value of 246 GeV and the other one should have a vacuum expectation value of the order of the Planck scale. Alone borderline metastability gives, using the experimental top quark mass 173.1±4.6 GeV, the Higgs boson mass prediction 121.8±11 GeV [i.e. 110.8 GeV to 132.8 GeV]. The requirement that the second minimum be at the Planck scale already gave the prediction 173±4 GeV for the top quark mass according to our 1995 paper.
A Standard Model Higgs boson, if one is discovered, will probably fit squarely in the mass range predicted in 2001, and indeed realistically, within one half of a standard deviation of the mean predicted value from a decade ago based on theoretical considerations.
An absense of a Standard Model Higgs boson anywhere in this mass range would favor composite Higgs models or technicolor models relative to SUSY and the Standard Model, although I wouldn't but bets on any of them at this point. There are also many models that are extremely similar to the Standard Model but have a suppressive Higgs boson signal at the appropriate mass.
I also have considerable doubt about the extent to which the vacuum stability problems are just a result of a particular Higgs boson mass, as opposed to renormalization mathematics that are somehow inaccurate or an inaccurate assumption about the extent to which fundamental particles are point-like or that space-time is continuous or an inaccurate beta function for the running of the coupling constants in the three Standard Model forces or the omission of very heavy fourth generation extremely ephemeral fermions. As another possibility, perhaps the unstable energy range is phenomenologically impossible to reach due to some sort of general relativity effect that has not been properly considered.
Anyway, stay tuned! We'll have the official announcement with lots of the devils in the details in a couple of weeks and will probably have more accurate rumors even sooner.
Wednesday, November 30, 2011
Ritual Human Sacrifice In Pagan Iceland
The is archaeological evidence in Iceland that is suggestive of ritual human sacrifices in the period prior to its conversion to Christianity ca. 1000 CE.
Tuesday, November 29, 2011
Is Dali Man Really A 209,000 Year Old Modern Human? No.
There are only a few non-African purportedly modern human skeletal remains prior to 100,000 years ago, when they are first modern human remains are seen in the Levant. These are the Dali Man remains "discovered by Shuntang Liu in 1978 in Dali County in the Shaanxi Province of China" which are claimed to be 209,000 years old. Images of these remains can be found here.
Shaanxi Province (via Wikipedia)
The Dali Man find is described in the Wikipedia link above as follows:
The Jinniushan cranium found in China in 1985 which is 250,000-260,000 years old and similarly shows strong Homo Erectus traits.
Dali Man and the Jinniushan cranium rival in age some of the oldest anatomically modern human remains from Africa. But, my own take is that Dali Man is simply a highly evolved descendant of Homo Erectus, arguably deserving a new species name, perhaps Homo Denisovia, if he is of the same species as the Denisovians, which seems likely given his proximity in time and geography. He may have evolved some Homo Sapien-like traits in parallel to modern humans, but the more plausible reading of the totality of the facts is that neither Dali Man nor Jinniushan man are in any way genetically descended from modern humans.
Then, there is also "Maba Man, a 120 to 140,000 year old fragmentary skull from Guangdong in China shows the same general contours of the forehead." As explained in the image link above:
Maba Man too seems more likely to be a late evolutionary stage of Asian Homo Erectus than any form of Homo Sapien, hybrid or otherwise. The Narmada cranium from central India dated to 236,000 years ago, to which these finds are often compared is likewise probably a Homo Erectus, although from a West Eurasian, rather than an East Asian subspecies.
Indeed, Maba Man shows continuity of the Homo Erectus species in Asia pretty much right up until modern humans arrive, admix with a species that is genetically kid to the Denisovian remains in Siberia, which then goes extinct upon contact with modern humans, just like the Neanderthals and many species of megafauna.
Megafauna Extinctions In Japan
As an aside, it is interesting to note that Japan's major megafauna extinction took place around 12,000 years ago, even though it was inhabited by the modern human Jomon people from perhaps as much as 35,000 years ago. In other places, like North America and Australia, megafauna extinctions followed much more quickly after the arrival of modern humans and it isn't clear why Japan was different.
UPDATE: John Hawks discusses how anthropology resolves these kinds of issues opening with a quote arising from the study of Asian Homo Erectus.
Shaanxi Province (via Wikipedia)
The Dali Man find is described in the Wikipedia link above as follows:
The Dali cranium is interesting to modern anthropologists as it is possibly an ideal specimen of an archaic Homo sapiens. It has a mixture of traits from both Homo erectus and Homo sapiens. The details of the face and skull are however distinct from European Neandertals and earlier European hominids like the finds from Petralona and Atapuerca.
Skull vault
The skull is low and long, though the posterior end of the skull is rounded, unlike the contemporary broad-based H. erectus or top-wide skull of modern humans. It does however bear a prominent sagital crest, a trait found in H. erectus but in few modern humans. The brain appears to have been sitting mainly behind the face, giving an extremely low forehead. The cranial capacity is estimated to around 1,120 cc, at the lower end of the modern human range, and upper end of the H. erectus range. The base of the cranium is less robust than in H. erectus. The posterior margin lacks the heavy neck muscle attachment seen in that group. Unlike the distinct tubular form seen in H. erectus, the tympanic plate is thin and foreshortened, a condition similar to that of modern humans.
Unlike H. erectus skulls, the Dali skulls lack the "pinched" look between the face and the cranial vault.
Face
The face is topped by massive brow ridges. The ridges curve over each eye, unlike the straight bar-like ridges seen at the Peking man material from Zhoukoudian. The curvature is more similar structurally to the brow ridges in archaic humans from Europe and Africa. The cheek bones are delicate, and the nasal bone flattened, again a curious combination of traits. During fossilization, the upper jaw has been fractured and dislocated upwards, giving the cranium the appearance of having a very short face. If reconstructed, the face would be probably be similar in overall dimensions to that of the Jinniushan man skull.
The Jinniushan cranium found in China in 1985 which is 250,000-260,000 years old and similarly shows strong Homo Erectus traits.
Dali Man and the Jinniushan cranium rival in age some of the oldest anatomically modern human remains from Africa. But, my own take is that Dali Man is simply a highly evolved descendant of Homo Erectus, arguably deserving a new species name, perhaps Homo Denisovia, if he is of the same species as the Denisovians, which seems likely given his proximity in time and geography. He may have evolved some Homo Sapien-like traits in parallel to modern humans, but the more plausible reading of the totality of the facts is that neither Dali Man nor Jinniushan man are in any way genetically descended from modern humans.
Then, there is also "Maba Man, a 120 to 140,000 year old fragmentary skull from Guangdong in China shows the same general contours of the forehead." As explained in the image link above:
The Maba cranium, dated to approximately 120,000 years ago was discovered in 1958 in the southern Chinese province of Guangdong. . . . It was initially thought to be an Asian Neandertal but does not in fact show any of the derived features of Neandertals as known from Europe and the Near East. The Maba skull is similar to other more complete finds . . . subsequantly found in China, differing only in minor ways, such as the size and shape of the orbits and nasal bones. Maba is also somewhat reminiscent of the recently discovered Narmada skull from India.
Maba Man too seems more likely to be a late evolutionary stage of Asian Homo Erectus than any form of Homo Sapien, hybrid or otherwise. The Narmada cranium from central India dated to 236,000 years ago, to which these finds are often compared is likewise probably a Homo Erectus, although from a West Eurasian, rather than an East Asian subspecies.
Indeed, Maba Man shows continuity of the Homo Erectus species in Asia pretty much right up until modern humans arrive, admix with a species that is genetically kid to the Denisovian remains in Siberia, which then goes extinct upon contact with modern humans, just like the Neanderthals and many species of megafauna.
Megafauna Extinctions In Japan
As an aside, it is interesting to note that Japan's major megafauna extinction took place around 12,000 years ago, even though it was inhabited by the modern human Jomon people from perhaps as much as 35,000 years ago. In other places, like North America and Australia, megafauna extinctions followed much more quickly after the arrival of modern humans and it isn't clear why Japan was different.
UPDATE: John Hawks discusses how anthropology resolves these kinds of issues opening with a quote arising from the study of Asian Homo Erectus.
Monday, November 28, 2011
Extended Particle Models
Between preon models, that assume that the so called fundamental particles are really composites of smaller pieces, and point particle models that are inconsistent with general relativity, are extended particle models, that have non-point-like particles that occupy a non-point-like volume, although the line between the two is fine one.
A recent example of such a model is Chih-Hsun Lin, Jurgen Ulbricht, Jian Wu, Jiawei Zhao, "Experimental and Theoretical Evidence for Extended Particle Models" (2010). The abstract, in part, for the 147 page paper with 41 figures reads as follows:
Some of the same authors have explored the same ideas in papers here (2009), here (2003), here (2001), and here (1999).
The 2009 paper is much shorter and has a nice survey of the research in the field, with this particular approach related closely to non-linear electrodynamics coupled to gravity (NED-GR) theories and the Born-Infeld Lagrangian, discussed, for example, here (2010) and here (2009). Most of the literature related to this focuses on modeling atypical hypothetical types of black holes.
Ultimately, the matter goes to fundamental issues of quantum gravity discused, for example, in this paper (2000) that systematically explores different possible couplings of the electromagnetic field and gravity.
A recent example of such a model is Chih-Hsun Lin, Jurgen Ulbricht, Jian Wu, Jiawei Zhao, "Experimental and Theoretical Evidence for Extended Particle Models" (2010). The abstract, in part, for the 147 page paper with 41 figures reads as follows:
We review the experimental searches on those interactions where the fundamental particles could exhibit a non point-like behavior. In particular we focus on the QED reaction measuring the differential cross sections for the process $ \EEGG $ at energies from sqrt{s} =55 GeV to 207 GeV using the data collected with the VENUS, TOPAZ, ALEPH, DELPHI L3 and OPAL from 1989 to 2003.
The global fit to the data is 5 standard deviations away from the standard model expectation for the hypothesis of an excited state of the electron, corresponding to the cut-off scale Lambda =12.5 TeV.
Assuming that this cut-off scale restricts the characteristic size of QED interaction to 15.7x10^{-18} cm, we perform an effort to assign in a semi-mechanical way all available properties of fundamental particles to a hypothetical classical object. Such object can be modeled as a classical gyroscope consisted of a non rotating inner massive kernel surrounded by an outer rotating massive layer equipped with charged sorted in a way to match the charge contents for different interactions. The model size of an electron agrees with 1.86x10^{-17} cm with the experiment. The introduction of a particle like structure related to gravity allows to estimate the inner mass kernel of an electron to 1.7x10^{-19} cm and the mass of a scaler to 154 GeV. The extension of the model to electrical charged particle-like structure in nonlinear electrodynamics coupled to General Relativity confirms the model in the global geometrical structure of mass and field distribution.
Some of the same authors have explored the same ideas in papers here (2009), here (2003), here (2001), and here (1999).
The 2009 paper is much shorter and has a nice survey of the research in the field, with this particular approach related closely to non-linear electrodynamics coupled to gravity (NED-GR) theories and the Born-Infeld Lagrangian, discussed, for example, here (2010) and here (2009). Most of the literature related to this focuses on modeling atypical hypothetical types of black holes.
Ultimately, the matter goes to fundamental issues of quantum gravity discused, for example, in this paper (2000) that systematically explores different possible couplings of the electromagnetic field and gravity.
Ancient DNA Suggests Multiple Neolithic Population Waves In Iberia
We successfully extracted and amplified mitochondrial DNA from 13 human specimens, found at three archaeological sites dated back to the Cardial culture in the Early Neolithic (Can Sadurnà and Chaves) and to the Late Early Neolithic (Sant Pau del Camp). We found that haplogroups with a low frequency in modern populations—N* and X1—are found at higher frequencies in our Early Neolithic population (∼31%). Genetic differentiation between Early and Middle Neolithic populations was significant (FST∼0.13, P less than 10−5), suggesting that genetic drift played an important role at this time.
To improve our understanding of the Neolithic demographic processes, we used a Bayesian coalescence-based simulation approach to identify the most likely of three demographic scenarios that might explain the genetic data. The three scenarios were chosen to reflect archaeological knowledge and previous genetic studies using similar inferential approaches. We found that models that ignore population structure, as previously used in aDNA studies, are unlikely to explain the data. Our results are compatible with a pioneer colonization of northeastern Iberia at the Early Neolithic characterized by the arrival of small genetically distinctive groups, showing cultural and genetic connections with the Near East.
Via Dienekes.
Based on ancient DNA, this study makes three notable claims:
(1) The Cardial Pottery culture had a strong demic component; it cannot be explained by cultural diffusion in the absence of migration.
(2) There was a demographically distinct wave of migration to Iberia after the initial early Neolithic Cardial Pottery wave and before the Bronze Age, i.e. in the Middle Neolithic.
(3) There is a lack of genetic continuity between the early Cardial Pottery culture and modern Iberians (or for that matter, modern Europeans). A model in which modern Iberians are descendants of the first farmers and herders, or of hunter-gatherers who learned to farm and herd from their neighbors and trade partners via cultural diffusion, is wrong.
Analysis
Maju reports in the comments to the linked post that the full haplogroup breakdown in this sample was "3H, 2N*, 1U5, 1K and 1X1" (and implictly that five samples didn't produce definitive haplogroup identifications) and that "another paper (published in Spanish language only, from 2009 but unknown) on very early Neolithic DNA from Navarre [found mtDNA haplogroups] . . . 3H*, 2H3, 1U, 1K, 1I and 1 HV."
Without considering subtypes and lumping the two samples one gets from a total sixteen data point sample from two contemporaneous but independent sources:
7H, 2N, 2U, 2K, 1I, 1 HV and 1 X1.
The pre-Neolithic origins of the mtDNA U samples in each group is relatively uncontroversial. Maju argues, and Dienkes disputes, that H is pre-Neolithic.
The notion that one can make a p=0.00001 claim about anything with a simple size of thirteen or less is doubtful. Small samples are inherently prone to contain flukes. The margin of error at the 95% confidence level in a sample of this size for a trait upon which there is an even split in the total population is +/- 27 percentage points (i.e. a 50% results is consistent with an actual percentage in the population of 23%-77%).
Also, unlike an ordinary sample populations, ancient DNA samples from a single region are very likely not independent of each other; there is a good chance that some samples are from members of the same family that were buried together because they were members of the same family and that other burial sites from the same community from other families are absent from the sample because they weren't well preserved.
Thus, the fact that a group of haplogroups that are rare today made up 31% of a non-random sample then, could easily be consistent with a frequency in the overall early Neolithic population of well under 10%.
You can be certain that each of the haplogroups in the sample was present at the time. You can be reasonably confident that groupings of haplogroups that are common, if there is any sound reason for the grouping, were not rare at the time. You can say that it is possible that haplogroups that don't show up at all might not have been present at the time and can be more confident in that conclusion if that haplogroup suddenly appears in reasonable frequency at a more shallow time depth and as the total sample size at the older time depth grows. You can't say a whole lot else.
The overlap of mtDNA haplogroups H, U and K at reasonably similar frequencies in two early Neolithic Iberian ancient DNA samples that probably are independent of each other does suggest that there is a good chance that the percentages of mtDNA H, U and K in these samples are not wildly atypical and that N*, I, HV and X1 were all present in the larger early Neolithic Iberian population, probably at more than vanishingly small frequencies.
While the early Neolithic ancient Iberian mtDNA isn't really consistent with modern distributions, since the odds of having that many quite rare haplogroups in a draw from a modern Iberian population wouldn't be that great, the overlap between modern Iberian mtDNA and early Neolithic Iberian mtDNA isn't huge. An identical sized sample from modern Iberians would probably have an overlap of more than 65% in broad haplogroup type with this early Neolithic sample, and finer distinctions could simple represent mutations within a population in continuity with the modern one. I suspect that in this case, the mtDNA is less volatile than Y-DNA over the last 7,500 years or so.
We think we know that the mtDNA X1 v. X2 split dates to a time prior to the arrival of proto-Native Americans in the Americas, thousands of year before the Neolithic revolution. The Druze of the Levant are only modern population of which I am aware that has significant proportions of both X1 and X2. This suggests that the Druze are either descendants of the ancestral X* population, or are an admixed population that had sources in a population with significant X1 percentages and a population with significant X2 percentages in the place where Druze ethnogenesis took place, which by tradition would be in the mountains of Turkey or Iran.
But, tracing an mtDNA lineage to the Near East doesn't necessarily tell you very much about its time depth. It is undisputed that all modern humans in Europe arrived from a source in common with the Near East in the last 40,000 years or so and that there was a major reduction in modern human populations of Europe at the last global maximum around 20,000 years ago. There is little evidence to tell us how much of a demographic influx, if any, Europe experienced between the last global maximum and the early Neolithic, but it wouldn't be controversial to argue that there was at least some population influx into Europe from the Near East in that time period. Mutation rate based dating also provides very little insight into whether a particular haplogroup had Epipaleolithic or early Neolithic origins.
The ancient DNA evidence for anything other than a couple of subhaplogroups of mtDNA haplogroup U (e.g. U4 and U5) prior to the early Neolithic in Europe is very thin, and as Dienekes notes in the comments to the linked post, there is reason to be skeptical of the only unpublished study that has made that claim (as far as I know, ancient DNA samples from North Africa putting other haplogroups commonly found in Europe there as far back as 12,000 years ago are less controversial). Certainly, non-U mtDNA haplogroups were very rare outside Southern Europe for most of the Upper Paleolithic era. But, it is harder to rule out the possibility that there may have been an influx of people more genetically similar to modern Southern Europeans in Southern Europe probably via a mostly coastal route, perhaps 14,000 years ago or so, a relatively late and geographically confined part of the Upper Paleolithic, but also many thousands of years before farmers or herders from the Near East arrived on the scene.
In an ancient mtDNA sample from pre-Roman Iron Age Iberia, (apparently N=17), "[t]he most frequent haplogroup is H (52.9%), followed by U (17.6%), J (11.8%), and pre-HV, K and T at the same frequency (5.9%). No samples were found to correspond to other haplogroups that are widely present in the Iberian peninsula populations (Table 7), such as V, X, I or W. The North African U6 subhaplogroup and Sub-Saharan African L lineages are also absent from the ancient Iberians analyzed so far[.]" More Iberian ancient DNA links can be found here.
The overlap between this sample and the early Neolithic sample is even stronger. The H, U and K percentages in the Iron Age sample are well within sampling error ranges of either of the early Neolithic samples (or both combined), and those three percentages make up 76.4% of the Iron Age sample. This would suggest that the matriline contribution to the Iberian gene pool in the metal ages was pretty modest, despite suggestive evidence that the Y-DNA contribution was greater. Given the proven antiquity of HV in Iberia, the likelihood that pre-HV is also not a relatively recent arrival also seems pretty strong, suggesting an early Neolithic to Iron Age mtDNA shift in Iberia of less than 20% (subject to a significant margin of error).
There was also more mtDNA continuity from ancient mtDNA of the early Neolithic era to the present in Iberia than in Central Europe where the ancient DNA of LBK Neolithic peoples is in stark contrast to that of both modern Central Europeans and Central European hunter-gatherers.
The mtDNA case for a repopulation of Europe after the Last Glacial Maximum from a Southwest European refugia with population expansion taking hold around 14,000 years ago, which is calibrated to a great extent by mtDNA mutation rate dating (which while subject to serious doubts and error bars is certainly more reliable than Y-DNA mutation rate dating) from the modern distributions of haplogroups H and V and their subtypes can be seen here.
Ultimately, the question of how much of Iberia (and Europe's) mtDNA is Epipaleolithic and how much is Neolithic or later, mostly boils down to your assessment of the time when mtDNA H (and probably HV) arrived. If these haplogroups are Epipaleolithic, then 80% or so of Europe's matrilines are ancient; if these haplogroups are Neolithic, then 80% or so of Europe's matrilines were replaced in the Neolithic revolution or afterwards.
The ancient DNA makes clear that H has been in Iberia at least since the early Neolithic, so if it is Neolithic or later, it was there from the start. But, it doesn't tell us if 5500 BCE or 14000 BCE is closer to the time when it arrived.
The mtDNA mutation dating of mtDNA haplogroup H subhaplogroups, however, in the Iberian refugia link above, however, along with the older Upper Paleolithic ancient mtDNA does, however, make a pretty strong case that H's predominance in Europe is at the very least, a post-LGM event, and probably many thousands of years later than that.
It also bears a mention in passing that no modern humans have ever been found to have Neanderthal mtDNA. (Query if populations with high levels of mtDNA haplogroup U5 have elevated Neanderthal DNA percentages.)
Sunday, November 27, 2011
Venezuelan Maternal Ancestry Mostly Native American
We analyzed two admixed populations that have experienced different demographic histories, namely, Caracas (n = 131) and Pueblo Llano (n = 219). The native American component of admixed Venezuelans accounted for 80% (46% haplogroup [hg] A2, 7% hg B2, 21% hg C1, and 6% hg D1) of all mtDNAs; while the sub-Saharan and European contributions made up ∼10% each, indicating that Trans-Atlantic immigrants have only partially erased the native American nature of Venezuelans. A Bayesian-based model allowed the different contributions of European countries to admixed Venezuelans to be disentangled (Spain: ∼38.4%, Portugal: ∼35.5%, Italy: ∼27.0%), in good agreement with the documented history.
From here (hat tip to Dienekes).
Thus, approxmiately 80% of the matrilineal ancestry of urban Venezuelans was Native American, 10% was African (presumably descendants of women brought to the New World in the slave trade), and 3.8% is Spanish, 3.6% is Portugese, and 2.7% is Italian.
One would expect that the patrilineal ancestry of Venezuelans (and also autosomal ancestry which is derived from all ancestors, rather than merely the patriline and matriline ancestors) would have a much larger Southern European component. Also, by implication, the percentage of urban Venezuelans who have exclusively European ancestry is less than (and probably significantly less than) 10%.
The Native American component, consistent with other studies of Native American mtDNA in North American and in Latin America, shows less genetic diversity than many North American Native American populations. It remains unclear how much of that difference is due to serial founder effects and how much is due to the impact of possible subsequent waves of migration to the Americas that did not penetrate to Central and South America in significant numbers.
It is also reasonable to expect, given historical patterns of colonization in Venezuela, that the Native American component of rural populations in Venezuela is probably larger than the population in two of its largest cities.
This data point also supports the frequent reality that a paternal superstrate population can give rise to language shift even in the absence of a large maternal genetic contribution.
Wednesday, November 23, 2011
New Evidence Favors SE Asian or South Chinese Dog Domestication
Dogs were almost surely the first domesticated animal. But, there is dispute over where dogs were domesticated with different studies based on different evidence reaching different conclusions. New evidence suggests that the lion's share of modern dogs, based on Y-DNA and mtDNA diversity, suggests that they have roots in dogs domesticated in Southeast Asia or South China.
Dead Sea Scroll Writers Wore White Linen Clothes
New data points on the kind of clothing favored by the writers of the Dead Sea Scrolls (white linen, rather than the more common wool and colorful clothes of the era) support the already leading theory that the more recent of the scrolls in the group were written by members of the ancient Jewish religious group called the Essenes. The absence of the equipment necessary to make linen at the site (which was almost exclusively women's work at the time) also suggests that the Dead Sea Scrolls may have been stored at a male dominated religious center, rather than a place that had a whole community of complete families.
Tuesday, November 22, 2011
Soybeans Domesticated 3500 BCE
Examination of charred soybean remains in Northern China, Japan and South Korea has pushed their date of domestication back from 0 CE (with a connection to the Zhou dynasty), the best estimate of the date of domestication until now, to the much earlier 3500 BCE at a location in Central China, early on in the East Asian Neolithic. Earlier studies of the East Asian Neolithic had found millet that old in these regions, but not soybeans, which are less easily preserved.
Soybeans (Glycine max) have been found in the wild in a predomesticated small bean version in the region from 7000 BCE.
Soybeans (Glycine max) have been found in the wild in a predomesticated small bean version in the region from 7000 BCE.
Monday, November 21, 2011
Instatons Liquid Vacuum Models In QCD
Let us briefly summarize the basic picture of the QCD vacuum that has emerged from the instanton liquid model. The main point is that the gauge fields are very inhomogeneous. Strong, polarized fields are concentrated in small regions of space-time. Quark fields, on the other hand, cannot be localized inside instantons. In order to have a finite tunneling probability, quarks have to be exchanged between instantons and anti-instantons. This difference leads to significant differences between gluonic and fermionic correlation functions. Gluonic correlators are short-range and the mass scale for glueballs is significantly larger than that for mesons. In addition to that, the fact that the gluonic fields are strongly polarized leads to large spin splittings in both glueballs and light mesons.
From this 1997 review.
Basically, the strong force is concentrated in globs within the vacuum of about 0.35 femtometers (fm) that interact with each other and take up a bit less than 2% of the space-time in a hadron. Quarks hop from one of these globs to the next in quantum tunnelling interactions (i.e. they can move from point A to point B in a field even though they lack the energy to cross field potentials that exist between those two points) at predictable rates.
This model remains relevant and accurate in predicting a wide variety of experimental and lattice QCD results such as the conditions under which quark-gluon plasmas form, and provide a way to mediate between raw QCD equations for interactions between individual quarks and gluons and the phenomenological models that fit the experimental results.
Friday, November 18, 2011
Julien Riel-Salvatore on Neanderthal Assimilation To Modern Humans
The Denver Post is hyping a new finding by hometown Neanderthal expert Julien Riel-Salvatore (author of the blog "A Very Remote Period Indeed" in the sidebar), which shows how Neanderthals could have been wiped out as a species simply by admixture with modern humans who had superior population densities.
I have two main criticisms of Riel-Salvatore's take on the matter.
First, the absence of any Neanderthal Y-DNA or mtDNA despite the fact that 1-4% of autosomal DNA in modern non-Africans has a Neanderthal source does not support the kind of admixture dilution model that he is proposing. You need a very specific kind of admixture that takes into account factors like differences between hybrid fertility and intraspecies fertility to get the observed result, and you also need to take into account post-Neanderthal replacements of much of the modern human population of Europe to make the geographic uniformity of the Neanderthal percentages fit what is observed. Also, there is pretty solid evidence to indicate that there were no significantly mixed Neanderthal-modern human communities; tribes from the diifference kinds of hominin mostly kept their distance from each other. There is not a single case of a Neanderthal skeleton and a modern human skeleton buried in the same graveyard in the same culturally continous time frame, although there is evidence of modern humans using caves previously inhabited in prior eras by Neanderthals.
Second, there is a quite strong case to be made that Neanderthals, while more sophisticated than Homo Erectus, for example, were not the intellectual equals of modern humans. Their material culture remained stagnant for 200,000 years, while modern human technologies evolved continuously and at a much higher rate. Some of the technological advances attributed to them during the contact period with modern humans (e.g. the Uluzzian) have been determined to very likely have been modern human tool cultures, and in the cases where there was change in Neanderthal tool culture, the change was probably in lower quality imitation of modern humans and didn't happen until contact took place. Their food sources were much less diverse (they were pretty much exclusively apex predators while modern humans had more fish, although Neanderthals did some non-harpoon fishing, more small game, and more fruit and vegetable gathering). Their geographic range was much narrower (they didn't make into Southeast or East Asia, nor did they make it to Africa, and they had a more limited presence in the far North and Siberia although there are some traces of them in parts of these places). The quantity and quality of Neanderthal artistic expression and symbolic activity was a pale shadow of that of modern humans (they had simple burials but no elaborate paintings or sculptures). The range of materials that Neanderthals used in tools was smaller (e.g. they used far less processed bone and twine). They had less of an ecological impact on megafauna than modern humans, suggesting that they were less successful hunters. Their social groupings were smaller from a quite early point.
You may not have to make assumptions about Neanderthals being stupid or less flexible than modern humans to explain their demise. But, despite having relatively large brains, and genes that would have given them more language abilities than some of their hominin predecessors, there is ample evidence from the material culture that they left behind that they were not as adaptable as modern humans. And, if one can determine that from independent evidence, it isn't unreasonable to include that in models of how Neanderthals went extinct.
"Neanderthals were not inferior to modern humans," said Julien Riel-Salvatore, assistant professor of anthropology at UC Denver and co-author of the study published in Human Ecology.
"They were just as adaptable and in many ways simply victims of their own success," he said. . . .
Riel-Salvatore said the new computer model takes the added step of showing how Neanderthals and homo sapiens roamed farther afield than assumed.
The computer models ran for 1,500 generations, following estimated samples of humans and Neanderthals roving across the two continents.
"You set some baseline parameters and see how these 'agents' interact," said Riel- Salvatore, who co-wrote the study with C. Michael Barton of Arizona State University. One parameter included researchers' knowledge that the last ice age forced Neanderthals and humans to widen their search for food.
They interacted, and because there were far more humans, over time that species diluted Neanderthals until they appear as just a fraction of our DNA.
Evidence of Neanderthal prowess as mates follows a March study co-authored by CU-Boulder, showing Neanderthals were experts at keeping fires burning continually. That skill, key to flourishing, had previously been thought beyond their reach.
"It shows you don't need to depend on the old cliches to explain the disappearance of Neanderthals," Riel-Salvatore said. "You don't need to make assumptions about them being stupid or less flexible."
I have two main criticisms of Riel-Salvatore's take on the matter.
First, the absence of any Neanderthal Y-DNA or mtDNA despite the fact that 1-4% of autosomal DNA in modern non-Africans has a Neanderthal source does not support the kind of admixture dilution model that he is proposing. You need a very specific kind of admixture that takes into account factors like differences between hybrid fertility and intraspecies fertility to get the observed result, and you also need to take into account post-Neanderthal replacements of much of the modern human population of Europe to make the geographic uniformity of the Neanderthal percentages fit what is observed. Also, there is pretty solid evidence to indicate that there were no significantly mixed Neanderthal-modern human communities; tribes from the diifference kinds of hominin mostly kept their distance from each other. There is not a single case of a Neanderthal skeleton and a modern human skeleton buried in the same graveyard in the same culturally continous time frame, although there is evidence of modern humans using caves previously inhabited in prior eras by Neanderthals.
Second, there is a quite strong case to be made that Neanderthals, while more sophisticated than Homo Erectus, for example, were not the intellectual equals of modern humans. Their material culture remained stagnant for 200,000 years, while modern human technologies evolved continuously and at a much higher rate. Some of the technological advances attributed to them during the contact period with modern humans (e.g. the Uluzzian) have been determined to very likely have been modern human tool cultures, and in the cases where there was change in Neanderthal tool culture, the change was probably in lower quality imitation of modern humans and didn't happen until contact took place. Their food sources were much less diverse (they were pretty much exclusively apex predators while modern humans had more fish, although Neanderthals did some non-harpoon fishing, more small game, and more fruit and vegetable gathering). Their geographic range was much narrower (they didn't make into Southeast or East Asia, nor did they make it to Africa, and they had a more limited presence in the far North and Siberia although there are some traces of them in parts of these places). The quantity and quality of Neanderthal artistic expression and symbolic activity was a pale shadow of that of modern humans (they had simple burials but no elaborate paintings or sculptures). The range of materials that Neanderthals used in tools was smaller (e.g. they used far less processed bone and twine). They had less of an ecological impact on megafauna than modern humans, suggesting that they were less successful hunters. Their social groupings were smaller from a quite early point.
You may not have to make assumptions about Neanderthals being stupid or less flexible than modern humans to explain their demise. But, despite having relatively large brains, and genes that would have given them more language abilities than some of their hominin predecessors, there is ample evidence from the material culture that they left behind that they were not as adaptable as modern humans. And, if one can determine that from independent evidence, it isn't unreasonable to include that in models of how Neanderthals went extinct.
Subscribe to:
Posts (Atom)