Meson Molecules (aka Deusons) Have Been Discovered, But Not Tetraquarks

Background

Composite particles made out of quark and/or antiquarks and bound together by gluons are called hadrons.

There are three possible color charges for quarks (called, e.g., red, green and blue), and three possible color charges for antiquarks (called, e.g., antired, antigreen and antiblue).  Gluons have two color charges (a quark color and an antiquark color), or a linear combination of such couplings (there are actually only eight rather than nine possible color combinations for gluons).

All hadrons are color neutral by virtue of having one quark with of each of the three colors or each of the three anticolors, or by virtue of having a quark with one of the three colors and an antiquark with the corresponding anticolor.

The need for all composite particles to be color charge neutral has a number of corollaries.  One is that all quarks are "confined" in a hadron (subject to the exception of the top quark which decays essentially the instant it is formed into a lighter down type quark, almost always a bottom quark).  The second is that all hadrons have integer electric charge (mesons can have a charge of -1, 0 or +1, baryons can have a charge of -2, -1, 0, +1 or +2).

Two kinds of hadrons have been observed experimentally.

Two quark particles called mesons are made out of a quark and an antiquark (in which the antiquark has the antiquark color charge corresponding the the color charge of the quark in the meson) bound directly by gluons and have spins of 0 or 1.  Due to the requirement of color charge neutrality, mesons always have an equal number of quarks and antiquarks, mesons always have a baryon number of zero.

 Three quark particles called baryons are made out of three quarks, (one with each of the three color charges, or three antiquarks, one with each of the three antiquark color charges) bound directly by gluons and have spins of 1/2 or 3/2.  Due to the requirement of color charge neutrality, baryons are always composed of entirely of quarks and have a baryon number of 1, or are composite entirely of antiquarks and have a baryon number of -1.

There are a finite number of possible meson and baryon  ground states, although this is complicated a bit by the fact that some neutral hadrons appear only in linear combinations of each other, and by the fact that hadrons can have "excited states" with the same quark content but higher masses that the ground state for a particular kind of meson or baryon. The vast majority of observed composite particles fit neatly into one of these boxes.

All but a couple of hadrons (the stable proton and the metastable neutron with a mean lifetime of 880 seconds in a free state) are ephemeral, with lifetimes of 10^-8 seconds or less.

The stable baryons, protons and neutrons respectively, of course, in turn bind together into atomic nuclei which are held together by a nuclear binding force. This nuclear binding force is derivative of the strong force that holds hadrons together with gluons which use a meson called the pion (the lighest of the mesons which is made only of up and/or down quarks) to carry the force between baryons in the nucleus. Each atom in nature has one electron associated with it for each proton in the nucleus in its unionized state. Atoms, in turn, are often bound to each other in molecules via the electromagnetic forces generated by the protons and electrons in the various atoms. The details of these interactions largely flow from the characteristic way that a particular number of electrons arrange themselves around an atom with the corresponding number of protons.

Quantum chromodynamics (QCD), the part of the Standard Model that explains the strong force that confines quarks together into hadrons, allows for a number of composite particles bound directly by gluons that have not yet been experimentally observed and are hence called "exotic." In principle, QCD allows for the formation of composite particles from any color charge neutral combination of quarks and gluons, subject to limitations that arise because the strong force is a short range force.

Theoretical analysis and experimental searches for exotic composite particles bound by the strong force have focused on three possibilities: Glueballs with zero quarks, tetraquarks made of four quarks, and pentaquarks made of five quarks, in each case, bound to each other directly by gluons and having a neutral color charge.

A more comprehensive review of "unconventional" hadrons can be foundhere.  It adds two more categories to my list.

First, mesons with J^PC quantum numbers* not allowed by the constituent quark model (which is not itself strictly a part of QCD proper). Examples of this include any mesons with spins other than 0 or 1, and electrically neutral mesons with J^PC quantum numbers of 0^--, 0^+-, and 1^-+.

There are by my calculations, in the ground state in the constituent quark model, 5 kinds of possible baryons with just one quark flavor (all spin 3/2), 80 kinds of baryons with two quark flavors (40 possible sets of constituent quarks with spin 1/2 and spin 3/2 versions of each) and 240 kinds of baryons with three quark flavors (60 possible sets of constituent quarks with one spin 3/2 and two spin 1/2 versions of each), for a total of 325 possible baryon ground states and an equal number of antibaryons. Naively, there ought to be 25 spin 0 mesons and 25 spin 1 mesons, for a total of fifty but due to linear combinations, group theory considerations and the like, this may not be precisely right. A more technical analysis is found here. There are a potentially infinite or near infinite set of excited states as well (including all scalar, axial vector and tensor mesons).

Second, experimentally detected resonances whose masses and quantum numbers are not easily fit to either theoretically predicted QCD hadrons or to excited states of those hadrons. For example, the paper identifies experimentally detected resonances of less than 2 GeV of mass that seem to have J^PC quantum numbers of 0⁺⁺ (i.e. true scalar mesons), but only four theoretical candidates (two ground states and two excited states) with those quantum numbers in that mass range in the "constituent quark model."

* Strictly speaking J which is often loosely called "spin" is total angular momentum. In this context, P is parity, and C is charge conjugation parity.

The Results

The good news is that recent wave of high energy physics experiments have observed a number of hadron-like particles that do not fit neatly into one of the ordinary meson and baryon boxes, and that these exotic particles seem to have at a minimum, a four quark content including a bottom quark, a bottom antiquark, a down type antiquark, and an up quark, with masses on the order of 10.6 GeV/c² and a width of 15 MeV (implying a mean lifetime of, for example, about a third of the hypothetical Standard Model Higgs boson).

The bad news is that these observations don't appear to be the long hypothesized "genuine tetraquarks" that theorists have been predicting for years. Instead, they seem to be "meson molecules" in which two ordinary mesons become associated with each other.

During the last three years strong experimental evidence from B and charm factories has been accumulating for the existence of exotic hadronic quarkonia, narrow resonances which cannot be made from a quark and an antiquark. Their masses and decay modes show that they contain a heavy quark-antiquark pair, but their quantum numbers are such that they must also contain a light quark-antiquark pair. The theoretical challenge has been to determine the nature of these resonances. The main possibilities are that they are either "genuine tetraquarks", i.e. two quarks and two antiquarks within one confinement volume, or "hadronic molecules" of two heavy-light mesons. In the last few months there as been more and more evidence in favor of the latter.

From Marek Karliner, "Doubly Heavy Tetraquarks and Baryons" (Pre-Print Submitted January 16, 2014).

The body of the paper proposes the name <b>deuson</b> for a "hadronic molecule" made up of two mesons bound together by something very similar to the forces that bind together protons and neutrons in an atom.

Karliner's paper goes on to "provide fairly precise predictions for masses and quantum numbers of the additional exotic states which are naturally expected in the molecular picture but have yet to be observed." The paper also discusses what experimental signatures we should look for because they would reveal even more exotic hadrons beyond the meson molecule model including "genuine tetraquarks."

Discussion and Analysis

As I commented above, this result is the less exciting of the two possibilities, although it is still pretty remarkable in that it represents the first time that something like the nuclear binding force between protons and neutrons in an atom has been observed in a case of hadrons other than protons and neutrons.

The use of the term "molecule" in this context, while apt in conveying the distinction between a true tetraquark and a state in which two separate mesons are bound to each other in a composite particle of some type, clouds the question of whether the force believed to bind the two mesons together is analogous to the nuclear binding force in atomic nuclei, which is derivative of the QCD strong force, or is actually the electromagnetic force that binds atoms together just as it does in ordinary molecules.

The body of the paper, however, resolves this ambiguity and makes clear that the state observed is really analogous to the nucleus of a deuterium atom, in which a single proton and single neutron are bound by the nuclear binding force transmitted via pions, even though term "molecule" which ordinarily refers to two or more atoms which interact electromagnetically, rather than via the strong force or nuclear binding force. As the paper explains at page 2 (citation omitted, some mathematical symbols translated into words, underlining mine):

The most interesting theoretical question is what are these states?

Their quantum numbers are those of a bb ud tetraquark, but such quantum numbers can also be realized by a system consisting of B* anti- B andB*anti- B* "hadronic molecules" loosely bound by pion exchange. The diff erence between these two possibilities is subtle, because they have the same quantum numbers and therefore in principle they can can mix with each other. The extent of the mixing depends on the overlap between the respective wave functions. By a "tetraquark" I mean a state where all four quarks are within the same "bag" or con finement volume, while by "hadronic molecule" I mean a state where there are two color-singlet heavy-light mesons attracting each other by exchange of pions and possibly other light mesons.

The proximity of the two resonances to the B* -anti- B and B*-anti- B* thresholds strongly suggests a parallel with X(3872), whose mass is almost exactly at the D* -anti- D threshold.

It also provides strong support for the the possibility that these state indeed are deuteron-like "molecules" of two heavy mesons quasi-bound by pion exchange. This is because it is very unlikely that two "genuine" tetraquarks just happened to sit at the respective two-meson thresholds. 

The attraction due to exchange is 3 times weaker in the I=1 channel than in the I=0 channel. Consequently, in the charm system the I=1 state is expected to be well above the D* -anti- D threshold and the I=0 X(3872) is at the threshold. In the bottom system the attraction due to exchange is essentially the same, but the kinetic energy is much smaller by a factor of on about m(B)/m(D) approximately equal to 2.8. Therefore the net binding is much stronger than in the charm system.

[Note that, in the quoted material referenced, B is a reference to a B meson (which means that it contains a bottom quark or bottom anti-quark), B* to an excited state of a B meson, D to a D meson (which contains a charm quark or charm anti-quark, but not a bottom quark or bottom anti-quark), D* to an excited state of a D meson, and anti-B, anti-B*, anti-D and anti-D* refer respectively to the antiparticles of these mesons. "X(3872)" is a temporary name assigned to an experimentally observed particle with a mass of approximately 3.872 GeV whose quark components are not known.]

What the underlined language is referring to is that in all known mesons and baryons made up of quarks directly bound to each other by gluons, the mass of the composite particle is much greater than the mass of the component quarks and is highly dependent upon the overall spin of the composite particle, with more subtle variations that seem related to other characteristics of the particle like the electric charges involved.  Even mesons or baryons with the same quark flavor content can have very different masses.

In contrast, the difference in mass between sum of the masses of the number of protons and neutrons in a particular atomic nuclei and the mass of the entire atomic nucleus after adjusted for the impact of the nuclear binding force is slight (although measureable - the slight differences in the amount of mass attributable to the nuclear binding force between different atomic nuclei is what is converted into the energy that powers nuclear fission reactors).  It seems unlikely therefore that a true tetraquark would have almost exactly the same mass as the sum of the masses of two mesons with the same combined quark content.

The hypothetical nuclear binding force between the two mesons in these systems is about a third as strong as the force binding quarks directly via gluons within a hadron.

Conjectures Re Exotic Strong Force Composite Particles

Will we find genuine tetraquarks or pentaquarks?

My conjecture is that we will not, or at least that they will be many orders of magnitude more rare than hadronic molecules of mesons and baryons with each other.

While these composite particles aren't naively forbidden by QCD, there is a factor that may make deusons, and analogous particles made of a meson and a baryon bound to each other in a similar way, greatly preferred relative to "genuine" tetraquarks and pentaquarks.

This factor is that in any given QCD color charge neutral tetraquark, it is always possible to divide the four quarks in the "color confinement bag" into two independent systems, each with a quark of a particular color and an antiquark with a corresponding color charge.  These two-quark subsystems can maintain their color charge neutrality independently without any need for interaction between the two subsystems.   So, there is no need for the two subsystems to be confined to each other in order to maintain a color charge neutral particle.

Likewise, in the case of quarks that could form a color charge neutral pentaquark, it is always possible to break the whole into a two quark subsystem and a separate three quark subsystem, without a need for interaction between the two subsystems to maintain a color charge neutral particle.

If the subsystems that need to exchange gluons with each other are effectively independent of each other, can't confine each other, and don't really exchange gluons with the other particles in the system, how can they be said to ever be a "genuine" tetraquark or pentaquark.

In contrast, this is not true in the case of either mesons, or baryons.  There are no possible ways that their quark content could be broken up into color charge neutral subsystems.

Therefore, it seems plausible that "genuine" tetraquarks and pentaquarks either do not exist at all, or are so rare relative to much less energetic hadronic molecule states that they cannot be found in statistically significant numbers at existing experiments.

Does the plausibility of interactions between quarks in different subsystems of tetraquarks or pentaquarks defeat this reasoning?

Tentatively, my answer is no.

There is a subtle caveat to the analysis above, but I am not ultimately convinced that this defeats the argument above against the formation of genuine tetraquarks or pentaquarks (which can be applied with equal force to any composite particle system with even more quarks that can fit within the effective range of the strong force).

Suppose that you have a B-anti-B molecule.  If the quarks in the B meson have red and anti-red color charge, and the quarks in the anti-B meson have blue and anti-blue color charge, then there is really no reason that there would be direct gluon exchange between the two mesons at all.  On the other hand, if the quarks in the B meson have red and anti-red color charge, and the quarks in the anti-B meson have red and anti-red color charge, it would make more sense that the gluon exchanges between the four quarks in the system would become intermingled with each other.

In the single color tetraquark case, it still wouldn't be necessary to keep all four quarks in one bag to create a color charge neutral composite particle to confine the quarks, but one might expect about a third of all such four quark systems to have more cross-subsystem interactions than the other two-third would experience.

This subtle distinction is even more relevant in any possible pentaquark system.   The quarks comprising a color neutral pentaquark can always be decomposed into a meson subsystem and a baryon subsystem. Indeed, there are actually no fewer than one and no more than two ways that this can happen in any given color neutral pentaquark.  But, every decomposition of a color neutral pentaquark will involve a situation where one of the quarks in the baryon subsystem can participate in the same gluon exchanges with one of the quarks in the meson subsystem as the other quark in the meson subsystem.

Thus, in a pentaquark, the non-trival interactions between subsystems which we would naively think would be possible in about one-third of tetraquarks, would always be present.

Ultimately, however, the fact that tetraquarks and pentaquarks never need to have all of their components confined in one system, even if components in one subsystem can have non-trivial interactions with components of another subsystem, probably deprives them of the simultaneous unbreakable strong force bond to all of the other components of the system that gives rise to ordinary mesons and baryons.  Mere coincidence in space and interactions between component quarks, without true confinement, does not appear to give rise to a "genuine" hadron.

Will our discoveries about deusons and tetraquarks have more than theoretical relevance?

We live in a work with six quark flavors.

Only five of these quark flavors, however, have been observed experimentally to hadronize, although I remain unconvinced that mesons and baryons containing top quarks are theoretically impossible, as opposed to merely very rare, because the mean lifetime of a top quark is not too profoundly shorter than the time frame required for hadronization, so one would expect some small subset of top quarks (only a modest finite number of which have ever been synthesized in an observable situation at all) to have actual lifetimes long enough to permit them to briefly hadronize.

But, there are few conditions in Nature, or for that matter, in man-made contexts other than particle accelerators, in which energies are so great that the heavier quark flavors actually come into being or have meaningful physical consequences (apart, perhaps, from their potential impact, for example, on the values of the physical constants for the masses of the Higgs boson and weak force bosons in a theory beyond the Standard Model where these physical constants are not themselves truly fundamental).

A single exotic quark flavor (the strange quark) in addition to the up and down quarks, is more than sufficient to explain pretty much all observed natural phenomena and all man-made systems not specifically designed to produce heavier exotic quarks - with the possible exception of the cosmology of the early universe and the inner workings of quark stars, if such things, as distinct from mere neutron stars, really even exist at all - something for which there is not particularly strong evidence from astronomy at this point.

I certainly can't easily think of engineering applications that rely on the existence of D mesons, B mesons, and baryons that include charm and bottom quarks. These more exotic hadrons take often immense energies to create and are also exceedingly short lived, even compared to more ordinary mesons and exotic baryons, which makes their engineering applications extremely limited.

What about glueballs?

Theory and experiment are neck and neck in the race to discover, or come up with a reason why we cannot discover, glueballs.  Increasing experimental power is likely to provide a definitive resolution of this question.

The reasoning advanced above, with regard to tetraquarks and pentaquarks, also tends to disfavor glueballs with sufficiently large numbers of component gluons.  If my conjecture is sound, then any glueball capable of being decomposed into color neutral subsystems will indeed do so.  So, only simpler glueball systems are really possible (or at least really likely to form at a sufficient frequency to be experimentally observed and described).  It follows that experimental efforts to detect glueballs should continue to focus on these simpler cases.

There are other reasons that could be proposed for either the non-existence of glueballs entirely, or for them to be far more rare given the rules of QCD than one might naively expect without really conducting the right kind of analysis.  But, those conjectures are beyond the scope of this post.

Did a key mutation dramatically expand where humans could live?

The figure above from the PLOS One article shows the proportion of people able to obtain the key biochemical from plants in yellow and the proportion of the Eurasian variant that lacks that gene in blue.

A new study suggests that humans were able to spread out and take over the world because of a new DNA change (or mutation) that popped up in their DNA 85,000 or so years ago. . . . it let them get away with just eating plants for good brain development. . . .

Human brains are sort of like modern electronics – they need rare materials to have them work as well as they should. While an iPhone needs various rare Earth minerals, human brains need lots of something called long chain polyunsaturated fatty acids or LC-PUFAs. Humans are terrible at making these fatty acids on their own and so have to get them from their diet (sort of like vitamin C).  Unfortunately for our ancestors, humans aren’t very good at turning a plant’s fatty acids into the ones they need.  And even though we are good at getting them from animals, we didn’t start hunting in a big way until around 50,000 years ago. Human ancestors probably got these fatty acids from fish and other aquatic animals.  In fact, scientists have long hypothesized that our ancestors’ need for aquatic animals was a big reason humans were such stick-in-the-muds for almost 100,000 years.  Their inability to get the brain food they needed from plants kept them trapped by the water’s edge, unable to spread across the globe. And yet, humans did start to spread in earnest across Africa around 60-80,000 years ago. This is thousands of years before there is any evidence that they did a lot of hunting of land animals. Clearly something changed sometime just before these humans started moving away from the shore.

A group of researchers thinks that the answer might lie in a mutation that allowed ancient humans to better use plant fatty acids to make the LC-PUFAs they needed for their brains.  Now humans could eat plants to get enough nutrients and so could migrate and conquer the world. The evidence to support this idea is the fact that Africans have a certain DNA difference that Asians and Europeans do not.  This difference strengthens an enzyme (FADS1) that converts plant fatty acids into the ones humans need for their brains.  In other words, Africans have a mutation that allows them to get at least some of their brain fatty acids from plants. When the researchers looked at the DNA surrounding this enzyme in Africans, they saw fewer DNA differences than expected.  This is a telltale sign that once the mutation appeared, it quickly spread through the population (a “selective sweep” in genetics lingo).  So once a few people were able to use plants effectively, they were off and running and quickly populated the world. A close look at the DNA also allowed these scientists to estimate that the mutation first appeared around 85,000 years ago (which fits the story nicely). . . . The actual number from the paper is 85,000 +/- 84,000 years ago.

The researchers also think that this DNA difference doesn’t come without a cost.  They conclude this from the fact that the mutation is no longer than common in Asians and Europeans.  The idea is that once humans could hunt for land animals, they no longer needed to rely on plants and so the mutation was lost. Of course another possibility is that the humans that headed for Europe and Asia just happened not to have this DNA difference any more.

From here which cites Mathias, et al., Adaptive Evolution of the FADS Gene Cluster within Africa (PLOS One September 19, 2012) (open access).

The Controversial Evolutionary Breakthrough Theories

The hypothesis that a small number of key evolutionary mutation with behavioral impact provided a breakthrough that gave modern humans a distinct evolutionary fitness advantage culminating the dominance of modern humans in the Holocene era, and perhaps not actually occurring until a punctuated evolutionary advance at the dawn of the Upper Paleolithic era is a controversial one.  Critics look at the evidence on brain size and see instead a steady and gradual convergent evolution over the course of hominin evolution in all hominin species.

The term "anatomically modern humans" in human evolution and anthropology reflects the notion that "behaviorally modern humans" may have been the product of an Upper Paleolithic or similar behavioral evolutionary package of genetic mutations that took place tens of thousands of years after modern humans had bodies substantially similar to our own.

Indeed, critics question the assumption that gracile modern humans were really smarter in any meaningful way than Neanderthals.  Judged by standard proxies for intelligence like brain size relative to body size, they may actually have been slightly smarter than modern humans.  And, increasing evidence of modest levels of admixture on many occasions between different varieties of archaic homins and between archaic hominins and modern humans also casts doubt on the notion that "anatomically modern humans" are even truly a biological species, as opposed to a mere subspecies of a diverse single modern hominin species that may extend as far back as, perhaps, Homo Erectus. Compare the anatomical and behavioral diversity of domestic dogs, which are undoubtedly a single species.

Problems With The Narrative

1.  Was large scale terrestrial hunting really that rare until 50,000 years ago?

There are many problems with the narrative quoted above. The notion that modern humans were reliant upon fish as their primary source of meat until 50,000 years ago is certainly news to me.  The paper cites for this proposition: Flinn MV, Geary DC, Ward CV, "Ecological dominance, social competition, and coalitionary arms races: Why humans evolved extraordinary intelligence" (2005). Evolut Human Behav 26: 10–46. This source does assert that hunting of large game increased around this time, but offers no factual support or citations to other sources for this key assertion.  

From context, this vague assertion whose dating isn't very precisely pinned down seems to be a backhanded reference to the earliest evidence of megafauna extinctions, but those can also be explained by theories such as the co-evolution of African megafauna with modern humans that left them less vulnerable to the appearance of modern humans, and to the theory that modern humans were more ecologically dominant in arid plains than they were in jungles preventing mass megafauna extinctions before modern humans arrived in these niche ecological zones.

A recent study from Morocco showing that Mesolithic hunter-gatherer populations there that relied heavily on acorns and shellfish for their diet had high rates of tooth decay atypical of hunter-gather populations elsewhere also suggest that heavy reliance on cereals for food was atypical of pre-Neolithic forager populations.  This disfavors the notion that large scale terrestrial hunting was a late arrival in the modern human hunter-gatherer diet.

Flinn, et al., also argues in their 2005 paper in support of the "evolutionary breakthrough" hypothesis that there are no other extant hominin species, but increasingly, it looks as if as recently as 25,000-28,000 years ago there may have been as many as three to five other extant hominin species (the last of the Neanderthals, up to two archaic hominin species in Africa that admixed with modern humans as recently as 10,000 years ago, and at least one or two species from a greater Denisovan/H. Florensis hominin clade which may have persisted a long time on Flores at least and perhaps in relict populations elsewhere as well).  This conclusion tends to disfavor their overall conceptual scheme.

2.  Why is the adaptation least fixed where it would seem to be most needed?

Access to fish as an important limitation on Out of Africa expansion seems an odd for a wave of migration that was predominantly coastal until well into the Upper Paleolithic, particular when the people who would seem to have the greatest need for the mutation in Eurasia don't have it in great numbers.

What do we know about early modern human hunter-gatherer subsistence?

There is certainly archaeological evidence from 70,000 years ago and before of modern human fishing activity from middens and from evidence of harpoon fishing in Africa.

There is also evidence that modern human Cro-Magnons in Europe tended to have more diverse diets favoring smaller game than the diets of contemporaneous Neanderthals who focused more on bringing down big game like mammoths.

There were what appear to be human driven (or at least correlated) mega-fauna extinctions close in time to the appearance of significant and expanding populations of modern humans in Europe, Siberia, Japan, Australia, Oceania, and the Americas (although this evidence is much less pronounced for more tropical areas in Africa, India and Southeast Asia) which demonstrate that these populations clearly did engage in significant terrestrial hunting.  

Supportable Aspects Of The Narrative

There is good evidence in uniparental genetics for a modern human dispersal within African at a date which is 60,000-80,000 years ago as measured by mutation rate dating in Y-DNA and mtDNA which is also close in time to the estimate time of an Out of Africa dispersal measured by the same kind of mutation rate dating.

Indeed, there is a fair argument that the Out of Africa expansion and the expansion of modern humans within Africa were really pretty much the same phenomena operating in different geographic directions.

The possibility that modern human meat consumption was largely confined to fish for the first half of its evolutionary history or so, also restores some viability to the observations associated with the largely discredited "aquatic ape" hypothesis for explaining anatomical differences between modern humans and our closest primate ancestors.

There Clearly Is a Fitness Based Selection Story To Be Told Related To This Gene

Mathias, et al. (2012) do make an unimpeachable case that the gene that allows chemicals critical to brain development to be obtained from plant as well as animal sources which has reached fixation in African modern humans is indeed the subject of a fitness driven selective sweep sometime at or after the time that modern humans arose and before the Out of Africa event.  And, they also are very convincing in their argument regarding the purpose that these gene serves in brain function and nutritional processing.

There are relatively few such clear examples of a distinct fitness based selection of a particular gene in modern humans, so this story has to be a significant one.  Any gene that was the subject of a fitness based selection that was fitness enhancing enough to reach fixation at some point very likely does tell the tale of an important evolutionary pressure that modern humans faced in their prehistory, although determining the exact nature of that evolutionary pressure is more challenging.  

Large numbers of people lived or died based upon whether or not they had this gene sometime in modern human prehistory in Africa.  Likewise, it is clear that something abated that selective pressure sometime after modern humans left Africa.  But, it is hard to say more than that from the scant archaeological evidence regarding the lives of anatomically modern humans in Africa during the 75,000 to 100,000 or more years prior to their expansion out of Africa.

Issues Related To Dating The African Plant Nutrition Processing Mutation and Its Eurasian Variant

The quoted mutation rate date from the Mathias paper in 2012 which is better expressed as 1,000-169,000 years before present is pretty much meaningless for a species whose oldest skeletal remains have been dated to around 195,000 years ago.  

Even a 195,000 years ago figure for its appearance would be less than 1.5 standard deviations from the mean, something with a p-value of 0.10 or more.  For example, it would not be at all implausible given the date generated from genetic date for this gene to associate the appearance of this gene not with the expansion of modern humans within Africa midway through their evolutionary history, and instead with the package of genetic mutations that make modern humans as a species distinct from earlier archaic hominins.  

But, the fact that it is present in Africans and progressively more rare in Europeans means that it probably arose before modern humans left Africa (around 85,000 years ago based upon autosomal mutation rate dating, but more likely around 120,000 years ago based upon archaeological evidence), and not later, even though the genetic evidence would allow for a more recent date.

In order words, the narrative behind the events that caused this gene to reach fixation in African modern humans is clearly a story that is part of our shared pre-Out of African African heritage.

Similarly, the subsequent mutation displacing it in many Eurasians (which has reached fixation only in the Americas and there probably only due to founder effects) may have arisen before West Eurasian-East Eurasian divide around the same time as Neanderthal admixture, as the advent of the mtDNA M and N clades, and as the divide between the B and CT Y-DNA clades.  The latest mutation rate data put these events at about 85,000 years before present, a date whose absolute calendar year accuracy is dubious but whose systemic error is shared pretty much proportionately by other mutation rate dates, particularly autosomal ones for Y-DNA.

This secondary mutation is clearly part of the early Out of Africa story.

Meta Point and Other Commentary

I covered this story when I first came across it and must have overlooked it at the time that it came out for some reason (I had a lot going on at the time).

Maju covered the story when it broke and shared many of the same skepticisms that I do.  Dienekes also had skeptical commentary when it came out.

Part of his confusion, however, follows from a misunderstanding of a key point when he states: "The same logic that applies in Africa should apply in Eurasia-plus but the fact is that Eurasians retain the ancestral allele and related genetic bloc without obvious damage to the brains." In fact, neither the African nor the Eurasian allelles are ancestral. The Eurasian allelle is a mutation that arises after the African one, which in turn replaced an ancestral allelle that no one carries any longer (a point that presumably could be confirmed now from primate or Neanderthal or Denisovan DNA).

The story I quote above also does a decent job of explaining the purpose of this gene which was a point that Maju had found confusing about the paper and its suggested narrative.

What Other Narratives Could Make Sense?

It is notable that the Eurasian version of the gene has not reached fixation anywhere except the Americas.

Could another narrative make sense of the facts?

Suppose that in Africa, modern humans experienced periodic episodes when hunting did not generate enough meat to make up a meaningful portion of the band's diet for some reason and the band instead relied on gathering plants as in the case of the acorn eaters of Mesolithic Morocco.  During these periods people who lacked the gene allowing them to obtain the right biochemicals from plants would suffer impaired brain development and soon enough selection would remove people without this gene from the population causing the gene to reach fixation in Africa.

This ability to live on plants when their ancestors were obligate meat eaters may even be an important reason why modern humans rather than other archaic hominins ultimately become the dominant hominin species in Africa.

But, then suppose that the Eurasian version of these gene appears in someone just before or just after the Out of Africa migration to Arabia.  Indeed, let us suppose that this gene appears in descendants of a tribal leader who is observant enough to notice that his descendants thrive when they grow up eating enough meat, but fair poorly when they grow up eating predominantly vegetables.

Then, this tribe is hit with an extended patch of bad hunting and has to make a choice: stay in the same general vicinity and subsist predominantly on vegetables or take a big risk by making a major migration into unknown territory in the hope of finding game to kill to get meat so his descendants can grow up healthy.

Facing this choice, the tribal leader takes the risky course unlike all of his ancestors, so that he can save his descendants.  He also has the good fortune to do so at a time when the gamble pays off and there is meat to be found in the great unknown territory that no modern human has hunted in before at the time.  Voila, some portion of the founding population of Eurasia has this mutation and develops norms that encourage further exploration when hunting is weak locally, and hence they conquer the world.

Random Physics Thoughts

This post contains some unsourced random musings on physics, some of which are trivial observations, others of which may illustrate my ignorance or constitute category errors, and none of which necessarily even deserve the status of conjectures, as that implies some confidence that a proposition is true, rather than merely throwing an idea "out there" to put it on paper for future consideration.

* In the Standard Model, baryon number (B) is conserved and lepton number (L) is separately conserved, and there is also on obscure kind of interaction which can't be illustrated in a Feynman diagram, in which only B-L is conserved.

* But, the number of fundamental bosons present in the universe is not conserved.  It is interesting that there is not conservation of fundamental boson number even though there is indirectly something akin to composite boson number via baryon number conservation.  Would mesons, which are baryons made of fermions, be a good place to look for possible violations of baryon number by analogy to non-conservation of fundamental bosons number?

* There is also not a fixed number of fermions (because baryon number assigned positive numbers to particles and negative numbers to antiparticles, and lepton number similarly assigns positive numbers to particles and negative numbers to antiparticles).

* Is there any process in which we can determine that we have assigned the correct charges to what we view as particles and antiparticles respectively? In other words, how do we know, that up type quarks with positive electric charge, down type quarks with negative electric charge, and charged leptons with negative electric charge are matter, while up type quarks with negative electric charge, down type quarks with positive electric charge, and charged leptons with positive electric charge are antimatter?

Obviously, the decision to call one of these groups matter and the other antimatter is purely arbitrary, although the stylistic choice to call the kind of charged fermion that makes up 99.9999999999%ish percent of all charged fermions in the universe matter, and the kind of charged fermion that makes up one part in 10^-10ish of all charged fermions antimatter is the obvious and eminently sensible way to make that arbitrary decision.

But, is there any reason, for example, that up type with positive electric charge quarks could be matter, while down type quarks with negative electric charge could be antimatter?

Suppose that a down quark emits a W- boson and becomes an up quark, and the W- boson then decays to an electron and an electron anti-neutrino in simple beta decay.  If an up type quark with positive electric charge were matter, but a down type quark with negative electric charge were antimatter, then ordinary beta decay via the weak force would violate conservation of baryon number.  So, the matter-antimatter designations of quarks in the Standard Model have to be as they are to be consistent with each other.

What about the other side of the beta decay process, however?  Suppose that negatively charged leptons were antimatter and that positively charged leptons were matter.  In simple beta decay, you could get an anti-electron and an electron neutrino, which would still conserve lepton number, if the matter-antimatter assignments were reserved.  Indeed, separate lepton number conservation implies that matter-antimatter assignments for leptons, in general, are separable from matter-antimatter assignments in the baryon sector.

If the ordinary charged leptons were considered to be antimatter, and the ordinary quarks were considered to be matter, then there would be no matter-antimatter asymmetry in the universe in the charged particle sector.  The baryon number of the universe is equal, to a high degree of precision that could easily be perfect due to conservation of charge in baryogenesis and leptogenesis, to the number of charged leptons in the universe.  In that case, all of the matter-antimatter asymmetry in the universe would arise in the electrically neutral neutrino sector.

* The W+ boson and W- boson are antiparticles of each other.  Is there any principled way to state that one is matter while the other is antimatter, or is that a distinction that does not apply to fundamental bosons at all?

Both W+ bosons and W- bosons can be emitted by ordinary matter quarks.  A W+ boson will frequently decay into a charged lepton with positive electric charge which is ordinarily called antimatter, and a corresponding neutrino (as opposed to a corresponding antineutrino) in order to preserve lepton number.  A W- boson will frequently decay into a charged lepton with negative electric charge which is ordinarily called matter, and a corresponding antineutrino.

Since W and Z boson decays produce equal numbers of matter particles and antimatter particles, and since W and Z bosons can be emitted by both matter particles and antimatter particles, it is probably most appropriate to say that they are neither matter nor antimatter (something also true of photons).  There is an irrational part of me that is tempted to think of the charged lepton that is more important than the neutrino produced in a W boson decay, and hence to link of the W- boson as matter and the W+ boson as antimatter.  But, upon reflection, that view simply must be wrong.

* If there truly only one Z boson particle, or are there Z bosons and anti-Z bosons which we simply lack the capacity to distinguish from each other.  The photon analogy, and the fact that the W+ mass plus W- mass plus Z mass equals Higgs boson mass divided by two formula suggests that there are no anti-Z bosons, because otherwise the formula would be W+ mass plus W- mass plus two times the Z mass.  The absence of right handed neutrinos also suggests that there are no anti-Z bosons.

* How can a neutrino be its own anti-particle, and hence have Majorana mass, if its very existence was based upon its need to conserve lepton number in beta decays by balancing out the lepton number of the charged lepton created in those decays?  This is one of the reasons, rightly or wrongly, that I favor the view that neutrinos have Dirac mass, but not Majorana mass.

* The matter or antimatter status of a quark or charged lepton can be determined simply from its electric charge.  In neutrinos, of course, the only way to distinguish neutrinos from anti-neutrinos observationaly is their parity.  If it has left parity, it is matter.  If it has right parity, it is antimatter.

In order for a theory in which there are right handed neutrinos that are direct counterparts of the Standard Model neutrinos with identical masses in the same generations, a particles matter-antimatter status would have to be a completely hidden variable (testable only by seeing if it interacted with W or Z bosons, I suppose), rather than a property determinable by observation of some other property of the particle.

If, as I have supposed, massive fundamental particles acquire their mass through their weak force interactions, however, since all particle types which have no weak force interactions have no rest mass and all particle types which have weak force interactions have rest mass, then sterile neutrinos, if they are really sterile, ought to be massless.

* Do W bosons, Z bosons and/or gluon have the properties of helicity and polarization that photons do?

* How do quarks know how to produce gluons with the right color charges?  Alternately, what happens, for example, to a blue-antired gluon emitted by a blue quark, if there is no red quark around to receive it?

* Is color charge conserved?  I think that it is.  How would we test for that experimentally?

* Gluons carry color charges including anti-color charges.  Am I right that gluons always carry a color charge and an anti-color charge providing it a net balance of matter affiliated charge and antimatter affiliated charge?  If so, gluons are matter-antimatter neutral but in a rather more dynamic way than other bosons.  Maybe not matter-antimatter neutral so much as simultaneously matter and antimatter objects.

* One often thinks, lazily, of color charge as a thing, a particle of its own, that quarks carry around singly and that gluons carry around in pairs.  But, this is wrong, because if it was right, there would be nine kinds of gluons instead of eight.  This in turn makes one wonder if maybe color charge could be a topological feature in three dimensions of a quark or gluon.

* Would it be possible for one to make a W+ boson couple to an up type quark, or to make a W- boson couple to a down type quark?  Does anything but electromagnetic repulsion prevent this from happening?  Could very strong electromagnetic repulsion at very short ranges always prevent this from happening?

* Some days I'm still not entirely clear on why electrons don't spiral down into protons, although I guess conservation of momentum has something to do with it.

* Does gluon source rest mass in a composite particle behave any differently than Higgs boson source rest mass in a composite particle?  Apparently not, but why?

* Supposedly, a hypothetical proton made up of massless quarks would have a mass of 870 MeV or so, extrapolating from the QCD equations.  But, what if that's wrong.  What if gluons actually amplify the rest mass of individual quarks, but can't create rest mass in the absence of a particle that has Higgs boson source rest mass?  If gluons merely amplify the rest mass of individual quarks, then glueballs would have no rest mass.  But, if gluons really do create rest mass in composite objects via their binding energy without Higgs boson interactions, then glueballs would have mass, as predicted.  At some point, does a failure to observe massive glueballs imply that the accepted theory regarding how mass is generated in QCD is wrong?

* All fundamental bosons have some mass-energy, even those with no rest mass like photons and gluons. Each fundamental fermions of the same type (twelve possibilities) and the three kinds of massive fundamental bosons has an exact rest mass associated with it that always stays the exactly the same for all particles of that type (subject to running masses with energy scale).  Given matter-energy conservation, therefore, the only possible source of energy that a fermion can call upon to emit bosons like photons and gluons is kinetic energy or potential energy from its position in fields.  What if an electron is perfectly at rest with no kinetic energy?  How can it emit photons?  How can it have electric charge if it doesn't emit photons, however?  Does it follow that an electron or other charged particle is incapable of being perfectly at rest with no kinetic energy?  Do hot magnets have different amounts of electro-magnetic charge than cold ones as a result?

* I strongly suspect that the possible 1 eV-ish sterile neutrino predicted from reactor anomalies is eventually determined not to exist.

Is The Higgs Boson A Walking Technicolor Glueball?

I've quoted at some length below from the new pre-print on the subject of glueballs in lattice QCD (i.e. one of the main ways of making calculations in the Standard Model theory of the strong nuclear force) by Biagio Lucini.  This is because this fairly readable paper does a good job of discussing the practical limitations involved in the study of glueballs and QCD generally, both on the theoretical calculation side, and on the experimental side.

Lucini also provides a few interesting insights into ways that walking technicolor theory might remain relevant.  In particular, he raises the possibility that the observed Higgs boson could actually be a composite glueball in this beyond the Standard Model theory.

After the quoted material, I make a few observations of my own.

Abstract

Recent numerical calculations of the glueball spectrum in QCD, in SU(N) Yang-Mills theory in the large-N limit and in candidate theories of strongly interacting dynamics beyond the standard model (in which the lowest-lying scalar plays the role of the Higgs boson) are reviewed and their implications for our theoretical understanding of glueballs in QCD-like theories and in strongly coupled gauge theories with a (near-) conformal dynamics are discussed.

Kimchee and Other National Food History

A recent post at Language Log discussed a South Korean effort to change the ideograms used to represent Kimchee, the Korean national dish, in Chinese.

One of my comments, and follow up to it (some by people much more knowledgeable about the subject than I), discussed the history of this dish and other famous national dishes.  I recap the pertinent posts below:

1. John said,

   January 3, 2014 @ 1:06 am
   韓 was also the name of a Chinese kingdom towards the end of the BC years.

2. ohwilleke said,

   January 3, 2014 @ 2:58 am
   As a follow up to John's comment, it is interesting to note that Korea's national dish, Kimchee, is a post-Columbian invention. The spice-hot spice in Kimchee comes from a pepper that is native to Meso-America and probably arrived in Korea via trade sometime in the 17th century CE.
   Korea is not the only country with relatively late arrived signature foods. Kumra the sweet potato of the Maori in New Zealand is a pre-Columbian arrival from South America probably via Easter Island ca. 500-1000 CE. Ireland's famous potatoes are a post-Columbian New World plant, and all of the famous European chocolates are likewise post-Columbian New World transfers. The tea to which the English are addicted was imported by the Dutch India company from Southeast Asia starting around the 16th century. The coffee that Americans started drinking in droves in protest of the English tea trade, of course, originates in Ethiopia. And, the bananas that we envision African primates eating arrived in Africa via Austronesian seafarers probably from Borneo ca. 500 CE. Many of the staple foods of Southern India arrived from the African Sahel where they are native ca. 2500 BCE.

3. Sohbet said,

   January 3, 2014 @ 7:40 am
   Kimchee has found a place in the Hawaiian barbecue restaurants that are ubiquitous on the US West Coast, but perhaps not so much elsewhere. And of course in the Korean-Mexican fusion cuisine that is diffusing out of LA. In a few decades it will be as American as sushi.

4. Victor Mair said,

5. January 3, 2014 @ 11:23 am

6. From Eugene Anderson, renowned Chinese food maven:

7. Good grief, people are obsessed with this. Certainly the Koreans are obsessed with kimchi.
   Ohwilleke is only sort of right, however. Pickled cabbage and so forth is well attested as far back as anything complicated is in east Asia. Chiles just got added to an already-perfected thing. They probably substituted for more expensive stuff, possibly long pepper and/or brown pepper, as they did in most of east and southeast Asia (cf change of meaning of "lada" from long pepper to chile in Bahasa Malaysia/Indonesia).
   There is a traditional pickle in the Auvergne in France that is just like kimchi. No possibility of contact, so far as I know. Just independent invention. Pickled cabbage of one sort or another is all over Eurasia. It uses lactic acid fermentation, which is interesting in itself….

8. Bendrix said,

   January 3, 2014 @ 1:58 pm
   Victor Mair, I think what you're saying about the tradition of pickled vegetables could be said of lots of foods, like noodles, dumplings, breads, etc. But I don't think anyone would argue that makes a won ton any less Chinese.

9. Victor Mair said,

   January 3, 2014 @ 4:10 pm
   @Bendrix
   Quite the contrary, "won ton" is very much Chinese, and I think that even Julie Lee would approve of this name.
   The Mandarin pronunciation is húntún / húntun 馄饨 / 餛飩, but the Cantonese pronunciation, from which we borrowed the English form, is wan4 tan1.
   Along the right side of this article (http://en.wikipedia.org/wiki/Wonton), you can see some of the many different ways of writing the name of this stuffed pasta item, which I've sometimes heard referred to lamely as "dumplings" or "ravioli", but "won ton" has definitely become a solid borrowing in English, so everyone can feel confident in using it.
   Húntún / wan4 tan1 is a very old Chinese word that is cognate with hùndùn 混沌 ("chaos"), which John Lagerwey long ago when we were graduate students together at Harvard used to refer to cleverly as "Humpty Dumpty".
   http://en.wikipedia.org/wiki/Hundun
   See especially the section on Daoist texts. But compare the translation of Zhuang Zi, chapter 7, section 7 (the last section of that chapter) by Mair in his Wandering on the Way, which may be found here (on p. xxxix) and on p. 71 of the same book (available here):
   =======
   The emperor of the Southern Sea was Lickety, the emperor of the Northern Sea was Split, and the emperor of the Center was Wonton. Lickety and Split often met each other in the land of Wonton, and Wonton treated them very well. Wanting to repay Wonton’s kindness, Lickety and Split said, “All people have seven holes for seeing, hearing, eating, and breathing. Wonton alone lacks them. Let’s try boring some holes for him.” So every day they bored one hole, and on the seventh day Wonton died.
   =======
   See p. 16 of the following for a brief paragraph on this subject:
   Mair, Victor H. 1994. "Introduction and Notes for a Complete Translation of the Chuang Tzu." Sino-Platonic Papers 48. (pdf available free here:http://www.sino-platonic.org/)
   For decades I've wanted to write a paper on the cognation of the disyllabic words / morphemes húntún / húntun 馄饨 / 餛飩 ("won ton") and hùndùn 混沌 ("chaos"), together with a study of their origin, which I strongly suspect (am virtually certain) is not Sinitic, but I haven't gotten around to it yet. One of these days I will, since I already have lots of notes assembled for that purpose.

10. Chris Waugh said,

    January 3, 2014 @ 6:35 pm
    @ohwilleke: The sweet potato is kumara, you were missing an a. And it was my understanding that the modern day kumara arrived after Eurpean contact and the pre-European kumara was considerably smaller. Nevertheless, it's a good example of how awesome the ancient Polynesian navigators were, ranging right across the Pacific as far as South America, then back as far southwest as New Zealand. And yes, it's a comparatively late-arriving "signature food" – later, even, than your 500 – 1000 CE. But then again, human settlement of New Zealand was really quite late, happening in the 13th century according to Michael King's "Penguin History of New Zealand".

11. Bendrix said,

    January 3, 2014 @ 8:01 pm
    Victor Mair,
    No, I agree completely that won tons are Chinese. The impression I got from your earlier posts was that you were implying the Koreans' cultural claim to kimchee is iffy because pickled vegetable dishes exist throughout Asia. But I was saying the same could be said of many similar foods – that does not mean, however, that a country's claim to its food culture is invalid.

12. Ferrer said,

    January 5, 2014 @ 8:39 am
    Interesting to note that speaking of lactic acid fermentation of cabbage many dishes have been mentioned, even some from Auvergne in France, and yet nobody has written of Sauerkraut or choucrout. True, Sauerkraut is not spicy, but also highly iconic for Germans and Alsatian French. Actually, Germans are sometimes called Krauts by some foreigners across the Channel.

13. Jongseong Park said,

    January 5, 2014 @ 1:32 pm
    @ohwilleke: As a follow up to John's comment, it is interesting to note that Korea's national dish, Kimchee, is a post-Columbian invention. The spice-hot spice in Kimchee comes from a pepper that is native to Meso-America and probably arrived in Korea via trade sometime in the 17th century CE.
    Chili pepper probably came to Korea slightly earlier, in the late 16th century. The Japanese who invaded Korea in 1592 and 1597 supposedly found chili pepper growing in Korea. The island of Kyushu in Japan had acquired chili pepper from Portuguese traders earlier, and thence it was introduced to Korea through trade. But chili pepper remained unknown to other parts of Japan, so ironically it was from Korea that it was introduced to the rest of Japan.
    Chili pepper was not actually used in Korean cuisine until much later on, around the 18th century. Before that, it was mainly considered an ornamental plant. Even when it began to be used in Korean cuisine, it was used sparingly because it was an expensive spice. Powdered chili pepper became an essential ingredient in kimchi only after the end of Japanese colonial rule in 1945.
    The most common type of kimchi today is made of napa cabbage, but this only became a popular ingredient of kimchi in the late 19th century. It was not easy to grow cabbages in Korea and it was only when napa cabbage was introduced from North China probably around the middle of the 19th century that it began to be used in kimchi. Recipes from the early 19th century mention several different types of kimchi, but none made from cabbages.
    So the kimchi we know today is a product of globalization, quite different from the kimchi of a hundred years ago, let alone that of a thousand years ago. The two essential ingredients, chili pepper and napa cabbage, are both relatively recent imports to Korea.

14. Milan said,

    January 6, 2014 @ 4:01 pm
    @ohwillike: It's not unusual for a national dish, and indeed whole national cuisines, to be rather young compared to the history of a people. The potato for example, an integral part of virtually every German meal, wasn't accepted in Germany until the late 18th century, when Frederick the Great promoted its cultivation in order to fight famines.

Kumara probably left South American a century or more before it arrived in New Zealand.

Of course, not all signature foods are recent.  For example, curry has been eaten in the Indus River Valley since the pre-Indo-Aryan Harappan era.

Also, vaguely on point, the pre-historic hunter-gatherer population of Morocco had very high rates of tooth decay, probably due to a very carb-heavy diet of sweet acorns and snails.  The acorns were likely boiled and also likely made into flours, a good reminder of the fact that flour is much, much older than grain domestication.

Y-DNA Haplogroup I Predominant in European Hunter-Gatherers

We have long known that mtDNA haplogroup U (especially U5) was predominant in European hunter-gatherers.  We now have a substantial set of ancient Y-DNA from early European hunter-gathers that establishes that Y-DNA haplogroup I (particularly I2) was predominant in those same European hunter-gatherers.

The data is, of course, incomplete.  It isn't entirely clear how true this is of European hunter-gatherers in Southwest Europe, for example.  But, the fact that all of the half a dozen Mesolithic European hunter-gatherers from the two sites for which data is available have Y-DNA haplogroup I is quite powerful evidence (none of the samples exclude I2, but not all are that specific).

Today, Y-DNA haplogroup I accounts for a little less than 20% of Europeans with a much higher percentage of Scandinavians (among the last population of Europe to adopt farming) and in the Balkans. I1 which is not present in the ancient samples currently more common than I2.  I1 appears to have expanded much later, perhaps around the time of the Nordic Bronze Age or maybe even a later phase of the Nordic Bronze Age.  Thus, less than 10% of European men are in the I2 Y-DNA clade.   Its mtDNA counterpart, mtDNA haplogroup U, is found in about 11% of Europeans (the non-U5 clades are mostly outside of Europe or rare).  These estimates aren't perfect since some clades of each haplogroup may have been late arrivals, and there may be other haplogroups in ancient DNA of that period which has not yet been discovered.

But, it is a fair guess that the Paleolithic European hunter-gatherer population in which both Y-DNA I and mtDNA U were predominant is the source of something on the order of one-ninth or one-tenth of the modern European gene pool.  Likewise, it is fair to say that modern Uralic and Nordic European populations in far Northern Europe in modern Europe, which have high concentrations of both haplogroups, are probably closest to these ancestral populations within Europe.  The corollary of this observation is that the contribution of this Paleolithic hunter-gatherer population in other parts of Europe may be even more shallow.

Y-DNA haplogroup R1 is the dominant Y-DNA haplogroup in almost all of the rest of Europe (with more R1b in the West and more R1a in the East, generally speaking); similarly mtDNA haplogroup H is dominant in the modern European gene pool.

The Eurogenes blog recaps a lot of the key figures relating to overall autosomal data from the recent paper.  Sardinians, unsurprisingly, are one of the closest matches to Early European Farmers.  Early European Farmers are supposed by the authors to about about 44 +/- 10% "basal European" farmer stock and a remainder assimilated indigenous European foragers.  But, Sardinians exceptionally low level of Ancient Northeast Eurasian (ANE) admixture could actually reflect a higher percentage of this basal European component, as almost all other Europeans are in a much narrower ANE admixture band that is very low across the board.  Thus, while a "three component" mixture (Western Hunter Gatherer, Ancient Northeast Eurasian, and Early European Farmer with a possible additional Scandinavian Hunter Gatherer component) is proposed, for most Europeans a simple two component (Western Hunter Gatherer-Early European Farmer) admixture does a good job of explaining the European autosomal data.

Archaeology and modern population genetics tends to support a Mesolithic expansion of a maritime hunter-gatherer population associated with mtDNA haplogroup V, possibly from a Franco-Cantabrian refugium, whose impact stretched from Northwest Africa to Arctic Scandinavia in coastal areas along the Atlantic Ocean and Baltic Sea coast as a minor contributing population to the more dominant mtDNA U haplogroup population that was present in Europe before the Last Glacial Maximum ca. 20,000 years ago, retreated to European refugia as Northern Europe was completely depopulated at that time, and then repopulated Europe as soon as the glaciers retreated.

We don't know what Y-DNA haplogroups were present together with mtDNA haplogroup V peoples.  The ancient DNA from Mal'ta boy and the location of modern Y-DNA R1, R1a and R2 bearers all support the notion that R1b had origins far to the East of its current distribution.  But when, given the unreliability of mutation rate dating for Y-DNA?

Eurogenes also notes that the Ancient Northeast Eurasian component appears to be a relatively recent arrival admixing just 5,500 to 4,000 years ago - a match strong suggestive of the expansion of the Uralic languages (such as Finnish, Estonian and Hungarian).  The following observation is particularly notable:

Nevertheless, if not for the ANE, we'd simply have a two-way mixture model between indigenous European foragers and migrant Near Eastern farmers, at least for most Europeans anyway. Moreover, the seemingly late arrival of ANE in much of Europe is fascinating, because it's yet another smoking gun for a major genetic upheaval across the continent during the Copper Age (aka. Late Neolithic/Early Bronze Age). Interestingly, archeological data suggest that this was also the period which saw the introduction of new social organization and perhaps Indo-European languages across most of the continent. None of this was lost on the authors of the paper, but it appears they'd rather be cautious pending more ancient genomic data, because they chose not to explicitly mention the Indo-Europeans.

He then quotes the study itself (from which I've omitted end note citations for easier reading):

The absence of Y-haplogroup R1b in our two sample locations is striking given that it is, at present, the major west European lineage. Importantly, however, it has not yet been found in ancient European contexts prior to a Bell Beaker burial from Germany (2,800-2,000BC), while the related R1a lineage has a first known occurrence in a Corded Ware burial also from Germany (2,600BC). This casts doubt on early suggestions associating these haplogroups with Paleolithic Europeans, and is more consistent with their Neolithic entry into Europe at least in the case of R1b. More research is needed to document the time and place of their earliest occurrence in Europe.Interestingly, the Mal’ta boy belonged to haplogroup R* and we tentatively suggest that some haplogroup R bearers may be responsible for the wider dissemination of Ancient North Eurasian ancestry into Europe, as their haplogroup Q relatives may have plausibly done into the Americas.

Regular readers of this blog know that my working hypothesis, although not an established fact, is that R1b does expand in Europe via the Bell Beaker culture and its successors, while Indo-Europeans exemplified by the Corded Ware culture were probably the source of R1a in Europe.  Further, my working hypothesis is that the Bell Beaker culture was Vasconic (i.e. Basque-esque) rather than Indo-European linguistically, although it and its successor cultures were a technological and military peer of the Corded Ware Indo-Europeans for about a thousand years or so (basically until Bronze Age collapse ca. 1200 BCE).  Eurogenes has a post largely in accord with this hypothesis.  See also here and here and here and here and here.

My working hypothesis is that Bell Beaker culture has origins geographically near that of some early Indo-European culture (somewhere east of France) and absorbed many of the key technological innovations associated with the Indo-Europeans, but retained their own language and culture and then rapidly expanded in population genetic terms in parallel to the Indo-Europeans.  Thus, when Indo-European Urnfield and Celtic peoples arrive in Western Europe on the eve of and after Bronze Age collapse, the already technologically advanced Western Europeans adopt the language and much of the culture of the conquering Indo-Europeans, but don't have a huge demographic impact when they arrive.

This working hypothesis is not without its flaws.  There is not strong archaeological evidence for substantial population replacement in the Bell Beaker evidence.  But, R1b is present at very high levels in the Basque whom all agree were pre-Indo-European in Western Europe, and it is absent in European hunter-gatherer ancient DNA to date and in all available early Neolithic ancient Y-DNA from Europe.  There aren't a lot of other good candidates to bring about this change in the right time and place.

Of course, it looks likely that a flood a new early and middle Neolithic ancient DNA is likely to be analyzed in the next several years and this may resolve the question of the source of R1a and R1b in Europe more definitively.  If my working hypothesis is correct, there will be little or no R1b in Southwest Europe prior to the arrival of the Bell Beaker culture there ca. 3000 BCE, with earlier Neolithic ancient Y-DNA in Southwestern Europe being mostly Y-DNA haplogroup G2 and other early Neolithic Y-DNA clades, and R1b would also be absent from Mesolithic (aka Epipaleolithic) ancient Y-DNA in Southwest Europe.

On the CKM Matrix

The Cabibbo-Kobayashi-Maskawa (CKM) matrix is a three by three unitary matrix, each element of which contains the square root of the probability that the up-type quark for that row will be converted into a down-type quark of that column in the event that it emits a W+ boson.

Seven of the nine elements can be explained by an utterly transparent theory.

The ratio of the probability that up-type quarks will transition into strange quarks to the probability that up-type quarks will transition into down quarks is always within the margin of experimental error of the ratio of the mass of the strange quark to the mass of the down quark, for up quarks, for charm quarks and for top quarks.

Likewise, the ratio of the probability than an up quark will transition into a bottom quark to the probability than an up quark will transition into a strange quark is within the margin of experimental error of the ratio of the mass of the bottom quark to the mass of the strange quark.

Alas, this very simple relationship between the quark masses and the CKM transition probabilities breaks down in two instances. The ratio of the probability that a top quark will transition into a bottom quark is about fourteen times greater than the ratio of the probability that a top quark will transition into a strange quark (the ratio is about 611-1 and should be about 44-1).

Likewise the ratio of the probability that a charm quark will transition into a bottom quark is about 588 times the probability that it will transition into a strange quark, about 13 times as great as the 44-1 ratio that would be expected in a simple formula.

The top to bottom transition probability, and the charm to bottom transition probability, are both about what one would expect if the bottom quark had a mass of 51-63 GeV (notably, more than 1/2 of the Z boson mass but less than 1/2 of the Higgs boson mass), rather than the canonical 4.2 GeV.

These discrepancies are too great to address with various combinations of the running of quark masses with energy scales.

One almost imagines that there are really two bottom quarks, one with a mass of 4.2 GeV that couples to up quarks, but not to charm and top quarks, and another with a mass of 55 GeV that couples to charm and top quarks, but not to up quarks, that are otherwise identical, although this is ridiculous and would have been discovered long ago if it were true, one would suppose.

While this transparent theory about CKM matrix elements clearly doesn't work unmodified, it is worth noting how well it fits seven of the nine elements in manner that suggests that the CKM matrix elements flow very directly from the quark masses alone, before looking for an elusive fix to this flawed theory.

The Latest CMS Update On The Higgs Boson (And Other New Experimental Data)

The CMS experiment at the Large Hadron Collider has confirmed, in yet another set of measurements released at year end, that the particle discovered is consistent in all respects measured with the properties of a 125.6 GeV mass Standard Model Higgs boson.

The properties of a Higgs boson candidate are measured in the H to ZZ to 4l decay channel, with l=e,mu, using data from pp collisions corresponding to an integrated luminosity of 5.1 inverse femtobarns at center-of-mass energy of sqrt(s)=7 TeV and 19.7 inverse femtobarns at sqrt(s)=8 TeV, recorded with the CMS detector at the LHC. The new boson is observed as a narrow resonance with a local significance of 6.8 standard deviations, a measured mass of 125.6+/-0.4 (stat.) +/-0.2 (syst.) GeV, and a total width less than 3.4 GeV at a 95% confidence level. The production cross section of the new boson times the branching fraction to four leptons is measured to be 0.93 +0.26/-0.23 (stat.) +0.13/-0.09 (syst.) times that predicted by the standard model. Its spin-parity properties are found to be consistent with the expectations for the standard model Higgs boson. The hypotheses of a pseudoscalar and all tested spin-one boson hypotheses are excluded at a 99% confidence level or higher. All tested spin-two boson hypotheses are excluded at a 95% confidence level or higher.

"Measurement of the properties of a Higgs boson in the four-lepton final state", CMS Collaboration (Submitted on 18 Dec 2013).

Collapsing the multiple error estimates into a single error estimate by taking the square root of the sum of the error sources in quadrature, this means that:

* The mass of the Higgs boson is 125.6 +/- 0.45 GeV.

This is essentially unchanged from previous estimates.  This is based on the aggregation of estimates from 4 electron decays (126.2 +1.5/-1.8 GeV N=4), 2 electron and 2 muon decays (126.3 +0.9/-0.7 GeV N=13), and 4 muon decays (125.1 +0.6/-0.9 GeV N=8).

Based upon the conjecture that two times the Higgs boson mass equals the sum of two times the W boson mass plus the Z boson mass, and using global fit values of the W and Z boson masses, my personal expectation is that the true Higgs boson mass is 125.97 GeV, which is at the high end of the experimentally permitted value, but is certaintly not ruled out at this point.

While an uncertainty of 0.3% is impressive for a particle discovered less than two years ago, the uncertainty in the Higgs boson mass measurement and the top quark mass measurement (which also has a less than 1% precision) are the dominant sources of uncertainty in efforts to understand global properties of the mass matrix of the Standard Model fundamental particles and couplings such as: (1) the question of whether the Higgs boson Yukawa vector is unitary, (2) the question of the extent of not quite fermion-boson mass inequality in the Standard Model, (3) the possibility of hidden relationships between the Higgs boson mass and other Standard Model constants, and (4) the detection of indirect evidence of particles that are missing from the set of the Standard Model, if any.

CMS experimenters realistically hope to bring the uncertainty in this measurement down to 0.1 GeV or so by the time that the LHC experiment is concluded, a narrowing of the 1 sigma mass range of the Higgs boson by 77%.

* The width of the Higgs boson is less than 3.4 GeV at the 95% confidence level.

This is also no surprise. The Standard Model expectation is about 4 MeV which is about 0.13% of the directly measured two sigma limit, and the measured result is perfectly consistent with the Standard Model expectation.

This large discrepancy is entirely due to the inability of the experimental apparatus at the LHC to resolve a direct measurement of the Higgs boson width more precisely.

The indirect and model dependent way to measure the Higgs boson total width is to estimate the extent to which particular measured decays in decay paths expected in the Standard Model deviate from the Standard Model expectation, and to use some form of weighted average of the measurable decay paths to estimate the percentage by which the total decay width of the Higgs boson deviates from the Standard Model expectation. Using this approach, the experimental measurements match the theoretically predicted values to within 10%-20% or so of the theoretically predicted values, implying a total decay width that probably deviates by less than 1 MeV from the theoretically predicted value.

By way of comparison, the width of the W boson is about 2 GeV, the width of the Z boson is about 2.5 GeV, and the width of the top quark is a few percent less than 2 GeV. The expected width of the Higgs boson implies that it has a mean lifetime that is about 666 times a long as the W boson, although this is still shorter than, for example, the tau lepton's mean lifetime which is about 10⁸ to 10⁹ times as long as the W boson.

Generally, heavier particles have shorter lifetimes than lighter particles, but the Higgs boson's expected mean lifetime is much longer than the lighter W and Z bosons and is also much longer than that of the only modestly heavier top quark.

* The measured four lepton Higgs boson decay branching fraction, relative to a Standard Model expectation value of 1.0, is 0.93+ 0.29/-0.24.

This is within about 1/4 of a Standard Deviation from the expected value.  The sample sizes used to develop these estimates are small enough that there is a great deal of statistical uncertainty in the measurements. The analysis centered on just 25 four lepton events attributable to Higgs boson decays (based upon the inferred mass of the source particle) that were culled from just 470 four lepton observations (over a much larger inferred mass range), with both the 25 and 470 event sets further broken down into three subcategories of events.

In the long run, the hardest part of the process of confirming that the experimentally observed particle matches the Standard Model Higgs boson is the task of determining if any of its branching fractions differ significantly from the theoretically expected values.  For each of the half dozen of so most important Higgs boson branching fractions measured so far, there is no statistically significant difference from the theoretically expected value.

The branching fractions of the Higgs boson are highly sensitive to the existence of beyond the Standard Model massive fundamental particles.  So, the closer the Higgs boson branching fractions are to their expected Standard Model values, the less likely it is that there are undiscovered massive fundamental particles of an electroweak scale mass.  Experimental uncertainty in mass measurements makes it hard to rule out very light particles on this basis, and there isn't enough mass-energy in even a very energetic Higgs boson for its decays to produce particles much much heavier than a Higgs boson (e.g. at the 10 TeV mass scale) or even for the existence of those particles to have very much impact on observed Higgs boson branching fractions.

* The confidence that the Higgs boson is a spin-0 scalar boson (which is the Standard Model expectation) rather than a pseduo-scalar boson is at least three sigma, as is the confidence that it is not a spin-1 boson. The confidence that the Higgs boson is not a spin-2 boson is at least two sigma.

Realistically, the important distinction is that it is not pseudo-scalar, which is the most subtle difference from the Standard Model expectation.  This is ruled out at the 99.9% level.

One very plausible possibility, in theories with multiple Higgs bosons (including all supersymmetry theories) is that there were two Higgs bosons of identical or almost identical mass, one of which was scalar (typically called H or h), and one of which was pseudo-scalar (typically called A), that combined, the H and A bosons (or h and A bosons) act like a Standard Model Higgs boson on average. A three sigma or better conclusion that the particle observed is scalar rather than pseudo-scalar largely rules out this scenario.

The spin-1 models are actually ruled out at the 99.97% level.  The exclusions aren't quite so powerful for the spin-2 models, but there is very little theoretical motivation for the Higgs boson to be spin-2 and they are still strongly disfavored relative to the spin-0 model.

Further, it is important to observe that the CMS spin and parity determinations are confirmed by the ATLAS experiment's published results:

The pseudoscalar hypothesis is excluded by CMS and ATLAS experiments at a 95% CL or higher. ATLAS has also excluded at 99% CL the hypotheses of vector, pseudovector, and graviton-like spin-two bosons, under certain assumptions on their production mechanisms.

This confirmation makes the CMS conclusions more powerful.

A non-Standard Model scalar Higgs boson model with the same spin and parity as the Standard Model Higgs boson, but that does not participate in electroweak symmetry breaking and has higher dimensional operator terms than the Standard Model Higgs boson, is the least powerfully excluded of the models at CMS (unsurprisingly since it differs so subtly from the Standard Model Higgs boson), and this model has also apparently not yet been tested at ATLAS. This is excluded at a 93% confidence level.

Keep in mind that these strong exclusions have been possible despite the fact that so far CMS is using only 25 data points to reach these conclusions.  As the sample sizes increase, the extent to which non-Standard Model spin and parity models can be excluded is likely to increase a great deal until the samples are about 20-40 times as large as they are now, at which point the marginal benefits of larger sample sizes starts to taper off.

Other Results Today:

* Higgs data and SUSY Fits

Another new paper by John Ellis evaluates the impact of new Higgs boson data (although probably not today's results) on SUSY parameter space using a broader set of experimental input data, but a less generic set of models, than the recent LHC specific analysis by Matt Strassler, et al. that was conducted in a fairly model independent manner.

As Ellis explains, other than the discovery of a Standard Model-like Higgs boson, the two LHC experiments, ATLAS and CMS "have found no trace of any other new physics, in particular no sign of supersymmetry."  Not every supersymmetry model has been excluded by these negative results.  But, the failure of a theory that has been around since 1964 to have even a single clear experimental confirmation in the subsequent 49 years isn't very impressive.

Some of the notable charts in his paper show:

(1) the very small non-exclusion range for SUSY theories with a lightest supersymmetric particle lighter than a top quark and a stop (SUSY top quark partner) of less than 800 GeV,

(2) the strong exclusion of an LSP of less than 400 GeV together with a gluino of less than 1200 GeV, and

(3) best fits of two simplified SUSY models to a wide range of collider and astronomy data involve characteristic masses for supersymmetric fermions and bosons in the single digit TeV range (as the heavy mass scale best fit), or alternately with fermion masses on the order of 1 TeV and boson masses on the order of half that amount (as the light mass scale best fit).

The best fit of the parameter space of mSUGRA is very tightly confined with a characteristic fermion mass around 1,400 +/- 200 GeV, a characteristic boson mass of around 1,000 +/ 100 GeV and the tan beta = ca. 42 (implying a heavy scalar Higgs boson of about 5,250 GeV).  Dark matter considerations nudge the parameters to favor a point around 1600 GeV characteristic fermion mass (e.g. applicable to gluinos) and 900 GeV characteristic boson mass (e.g. applicable to stops).

Ellis finds best fits for SUSY dark matter at the high end of the range from 10 GeV to 100 GeV with exceedingly low cross-sections of interaction with other matter on the order of 10^-45 to 10^-48 (well below that of neutrinos, for example).  This is a poor fit to astronomy data which discriminate between cold dark matter and warm dark matter, however, which favor dark matter particles which are each roughly a million times lighter than those best fit values.  Neither the CSMSSM nor the NUHM1 models he examines have any meaningful capacity to accommodate dark matter candidates with masses consistent with warm dark matter models.

It is increasingly hard to stomach theories that put new, heavy SUSY particles just around the corner from discovery at the electroweak scale without giving rise to even the faintest real experimental indicator of their existence at energy scales that we can measure.  SUSY was invented to tame electroweak symmetry breaking, yet seems to have virtually no phenomenological manifestations at that energy scale.

* Why 125.6 GeV?

Is it significant that the Higgs boson mass is very nearly the one that maximizes the rate at which a Higgs boson decays to photons?

It is remarkable that the measured Higgs boson mass is so close to the value which maximizes the Higgs decay rate to photons as predicted by the Standard Model. In this letter we explore the consequences to assume that an ∼126 GeV Higgs boson mass is not accidental, but fixed by some fundamental principle that enforces it to maximize its decay rate into photons. We provide evidence that only a very narrow slice of the parameters space of the Standard Model, which contains their measured values, could lead to a maximal Higgs boson with that mass. If the principle actually holds, several Standard Model features get fixed, as the number of fermion families, quark colors, and the CP nature of the new boson, for example. We also ilustrate how such principle can place strong bounds on new physics scenarios as a Higgs dark portal model, for example.

"Is There a Hidden Principle in the Higgs Boson Decay to Photons?", Alexandre Alves, E. Ramirez Barreto, A. G. Dias (Submitted on 18 Dec 2013).

The paper notes that the combined CMS-ATLAS Higgs boson mass is 125.66+/-0.34 GeV. The photon decay maximizing value is estimated to be 126.16 +/- 0.35 GeV. It notes that consistent with that principal, any "scalar dark matter" would have to have a mass of 55-63 GeV and a coupling to the Higgs boson of less than 0.01. The paper also observes that peak gluon-gluon decays, and peak photon-Z boson decays are also quite close to the measured Higgs boson mass.

This analysis a similar flavor to the the observation that the Higgs boson masses sits squarely in the metastable region between vacuum instability in the universe and a stable vacuum, at a point where the expected lifetime of the universe's vacuum stability is on the order of the age of the universe but it is not fully stable over an infinite time scale, which was the basis of one of the most successful advanced predictions of the Higgs boson mass. It also further prompts the question - why should a Higgs boson mass that maximizes photon production also imply a metastable vacuum?

* FCNC's in Top Quarks.

The proportion of top quark decays showing a flavor changing neutral current in the Standard Model is expected to be 10^-12 to 10^-17.  In some BSM models, it is expected to be as much as 10^-3.  The latest experiments establish that FCNC branching fractions from top quarks are less than 10^-2, and ultimately, if there are none will be able to rule out FCNC branching fractions at the 10^-4 to 10^-5 level, ruling out some BSM models on that basis.  But, this is not a measurement likely to rule out whole classes of models any time soon.  Suppressing FCNC's in top quarks by a factor of 100 from a naively expected value in a BSM theory simply isn't all that hard to do.

Thus, the current results aren't very interesting because nobody has theories that predict FCNC's in top quark decays at rates that could be detected so far at the LHC.  But, we are very close to the threshold where that will start to happen.

* Testing QCD Predictions.

Almost every collision at the LHC tests some aspect of QCD (i.e. the Standard Model theory of the Strong Force which is known as quantum chromodynamics). A recent paper reviews the results.

The four best available independent estimates of the strong force coupling constant to date, normalized to the energy scale of the Z boson mass (one from each of the LHC experiments, one from Fermilab, and one from another experiment) are consistent with the world average value of 0.1184(7). But, uncertainty in theoretical QCD modeling is the dominant source of uncertainty in each of the estimates.

Equally important, energy scales from hundreds of MeVs to 1 TeV have been studied and there has been no deviation from the expected running behavior of the strong force coupling constant with energy scale from the Standard Model expectation for this coupling constant's beta function.

This conclusion, if continued to higher energy scales, is a clear way to discriminate between the Standard Model and SUSY alternatives, because supersymmetry theories, generically, have a running of the strong force coupling constant that is very different from that of the Standard Model, which makes gauge coupling unification (which does not naively appear in the Standard Model) possible. In the case of the running of the strong force coupling constant, the differences are quite stark and should be discernable even at energy scales of a few TeVs which are potentially within the reach of the LHC.

In principle, there are material differences between SM and SUSY predictions for the strong force coupling constant even at energy scales of just 1-2 TeV that have already been probed, but getting sufficient precision in this subset of QCD measurements and theoretical calculations at the extreme fringe of the existing data set is challenging and has thus far not been achieved.

More generally, while the experimental results across the board in countless experiments at colliders to date and the LHC in particular match the theoretical predictions of QCD, this is less impressive a feat than it is in the precision electroweak measurements.  This is because the theoretical predictions of QCD have so much uncertainty, because doing the math involved in making a QCD prediction is so hard.

For example, even though we have huge data sets measuring the properties of hadrons containing up, down, strange and charm quarks, with the properties of the composite mesons and baryons often measured with extreme precision, turning those inputs into fundamental QCD theoretical inputs is extremely challenging because the calculations that have to be used to reverse engineer first and second generation quark properties from the hadrons that they form are very difficult to pin down correctly.  The proton and neutron's mass, for example, are known to many significant digits of accuracy, but the mass of the up and down quarks that combine to form protons and neutrons are known only to 25% and 10% accuracy respectively, and theoretical estimates of nucleon masses from QCD first principles have only about a 1% precision.  The theoretical values match the experimental ones, but the theoretical predictions aren't very specific at all.

This was the situation when Richard Feynmann wrote his book "QED" in 1988 and while progress has been made in the intervening 25 years, it remains the basic problem facing QCD physicists today.

This is why theoretical developments in how to do QCD calculations more efficiently, like the Amplituhedron or new Monte Carlo methods for doing QCD calculations hold so much promise for the future of high energy physics.  The former works by discovering hidden symmetries in the calculations that makes it possible to ignore redundant calculations, while the latter uses a statistical sampling of the vast number of sub-calculations that go into the final result to make a reasonable estimate of it.  Should one or more of these results work, the experimental power of almost every collider experiment ever conducted will be greatly enhanced, because the amount of uncertainty in the background estimates will be greatly reduced, making the signals of the phenomena that experimenters are trying to observe much more clear.