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.


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).

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