Muon g-2 HLbL Developments

Background (mostly, but not entirely, from an October 27, 2022 post and a July 18, 2024 post)

The combined result of the experimental measurements of muon g-2 (all of the numbers that follow are in the conventional -2 and divided by two form times 10^-11) is:

116,592,059 ± 22 

This compares to the leading Standard Model predictions of: 

116,592,019 ± 38 (which is a relative error of 370 parts per billion). This is from A. Boccaletti et al., "High precision calculation of the hadronic vacuum polarisation contribution to the muon anomaly." arXiv:2407.10913 (July 15, 2024).

The gap is only 40 ± 44.9, with the Standard Model prediction still a bit lower than the experimental value.

The QED + EW predicted value is:

116,584,872.53 ± 1.1

About 90% of the combined uncertainty in this  QED + EW value is from the EW component (there may be an error in the standard QED prediction but it is so small that it is immaterial for these purposes).

The difference, which is the experimentally implied hadronic component value (HVP plus HLbL), is:

7186.47 ± 22.02

This has a plus or minus two sigma range of:

7,142.32 to 7,230.51

The hadronic QCD component is the sum of two parts: the hadronic vacuum polarization (HVP) and the hadronic light by light (HLbL) components.

In the Theory Initiative analysis the QCD amount is 6937(44) which is broken out as HVP = 6845(40), which is a 0.6% relative error and HLbL = 98(18), which is a 20% relative error.

The latest LO-HVP calculation component is 7141 ± 33 (a relative error of just 0.46%).

As of November 1, 2024, it was clear that the Theory Initiative calculation of the Standard Model value of the HVP contribution to muon g-2 (which differs from 5.1 sigma from the experimental value) was the flawed one:

Fermilab/HPQCD/MILC lattice QCD results from 2019 strongly favour the CMD-3 cross-section data for e+e−→π+π− over a combination of earlier experimental results for this channel. Further, the resulting total LOHVP contribution obtained is consistent with the result obtained by BMW/DMZ, and supports the scenario in which there is no significant discrepancy between the experimental value for aμ and that expected in the Standard Model.

Similarly, the introduction to a new paper that is the motivation for this post notes that:

estimates based on τ data-driven approaches or lattice QCD calculations significantly reduce the tension between theoretical and experimental values to 2.0σ and 1.5σ, respectively (less than one σ in [44]). The latest CMD-3 measurement of σ(e+e− → π+π−) also points in this direction.

Reference [44] cited in the block quote above is the current state of the art calculation cited above.

The HLbL Contribution

The Theory Initiative HLbL calculation

None of the refinements of the muon g-2 HVP contribution discussed above tweak the Theory Initiative value of the Hadronic Light by Light (HLbL) contribution of 92 ± 18, even though it has the highest relative error of any of the components of the muon g-2 calculation of nearly 20%, because the HLbL is only 1.3% of the total hadronic contribution and still has only half the uncertainty of the HVP contribution.

But, progress has been made on the HLbL component as well, which is now getting more attention as the experimental result's increased precision and the progress on the HVP contribution makes it relevant.

The Chao (April 2021) HLbL Calculation 

On the day that the first new muon g-2 experimental results from Fermilab were released a "new calculation of the hadronic light by light contribution to the muon g-2 calculation was also released on arXiv." This wasn't part of the BMW calculation and increased the HLbL contribution from 92 ± 18 to 106.8 ± 14.7. That paper stated:

We compute the hadronic light-by-light scattering contribution to the muon g−2 from the up, down, and strange-quark sector directly using lattice QCD. Our calculation features evaluations of all possible Wick-contractions of the relevant hadronic four-point function and incorporates several different pion masses, volumes, and lattice-spacings. We obtain a value of aHlblμ = 106.8(14.7) × 10^−11 (adding statistical and systematic errors in quadrature), which is consistent with current phenomenological estimates and a previous lattice determination. It now appears conclusive that the hadronic light-by-light contribution cannot explain the current tension between theory and experiment for the muon g−2.

En-Hung Chao, et al., "Hadronic light-by-light contribution to (g−2)μ from lattice QCD: a complete calculation" arXiv:2104.02632 (April 6, 2021) (the failure of this pre-print to be published, three and a half years later, however, is somewhat concerning, as there is no obvious flaw in the paper from the eyes of an educated layman).

This would increase the Standard Model prediction's value and lower the uncertainty to:

116,592,033.8 ± 36

This would reduce the gap between this combined theoretical prediction and the world average experimental value to 25.2 ± 42.2 (just 0.6 sigma).

The Zimmerman (October 2024) HLbL Calculation

The most recent total HLbL calculation, from October 2024 reached value of 125.5 ± 11.6, which would reduce the HLbL relative uncertainty to 9% (cutting it in more than half from the Theory Initiative value). This would make the state of the art combined prediction of muon g-2:

116,592,052.5 ± 35

The gap between this combined state of the art calculations of the Standard Model value of muon g-2, and world average experimental value for muon g-2, would be 6.5 ± 41.3 (less than 0.2 sigma).

Other Recent HLbL work

As the introduction to the new paper explains in a nice overview of the HLbL calculation:

The HVP data-driven computation is directly related to the experimental input from σ(e+e− → hadrons) data. HLbL in contrast, requires a decomposition in all possible intermediate states. Recently, a rigorous framework, based on the fundamental principles of unitarity, analyticity, crossing symmetry, and gauge invariance has been developed, providing a clear and precise methodology for defining and evaluating the various low energy contributions to HLbL scattering. The most significant among these are the pseudoscalar-pole (π(0), η and η′) contributions. Nevertheless, subleading pieces, such as the π± and K± box diagrams, along with quark loops, have also been reported, with the proton-box representing an intriguing follow-up calculation. Specifically, a preliminary result obtained from the Heavy Mass Expansion (HME) method —which does not consider the form factors contributions— for a mass of M ≡Mp=938 MeV, yields an approximate mean value of ap−box µ = 9.7 ×10−11. This result is comparable in magnitude to several of the previously discussed contributions, thereby motivating a more realistic and precise analysis that incorporates the main effects of the relevant form factors. In this work, we focus on the proton-box HLbL contribution. We apply the master formula and the perturbative quark loop scalar functions, . . . (which we verified independently), together with a complete analysis of different proton form factors descriptions, which are essential inputs for the numerical integration required in the calculations.

The new paper concludes the proton box contribution to HLbL which was preliminarily estimated at 9.7 is actually 0.182 ± 0.007, which is about 50 times smaller than the preliminary result and immaterial in the total, making the neutral and charged pion, the eta, the eta prime, charged kaon, and quark loop contributions as the primary components of the HLbL contribution to muon g-2.

Another new paper calculates the neutral pion contribution to HLbL which is the single largest component of the HLbL contribution, which accounts for more than half (almost two-thirds) of the HLbL contribution:

We develop a method to compute the pion transition form factors directly at arbitrary photon momenta and use it to determine the 

π0

-pole contribution to the hadronic light-by-light scattering in the anomalous magnetic moment of the muon. The calculation is performed using eight gauge ensembles generated with 2+1 flavor domain wall fermions, incorporating multiple pion masses, lattice spacings, and volumes. By introducing a pion structure function and performing a Gegenbauer expansion, we demonstrate that about 98% of the π0-pole contribution can be extracted in a model-independent manner, thereby ensuring that systematic effects are well controlled. After applying finite-volume corrections, as well as performing chiral and continuum extrapolations, we obtain the final result for the π0-pole contribution to the hadronic light-by-light scatterintg in the muon's anomalous magnetic moment, aπ0−poleμ=59.6(2.2)×10−11, and the π0 decay width, Γπ0→γγ=7.20(35)eV.

Tian Lin, et al., "Lattice QCD calculation of the π(0)-pole contribution to the hadronic light-by-light scattering in the anomalous magnetic moment of the muon" arXiv:2411.06349 (November 10, 2024).

The relative uncertainty in the neutral pion contribution is 3.7%, which is a much larger relative uncertainty than in the EM, weak force, or HVP components, but much smaller than the relative uncertainty in the HLbL calculation as a whole.

This neutral pion contribution calculaton is an incremental improvement in the precision of this estimate, compared to most other recent attempts, and produces in value in the same ballpark as previous attempts (i.e. it is statistically consistent with them):

This also implies that the uncertainty from the charged pion, the eta, the eta prime, charged kaon, and quark loop contributions to HLbL, while small in magnitude (about 29-45 * 10^-11 from all of them combined) have combined uncertainties on the order of 13-17 * 10^-11. This is on the order of 35-45% relative uncertainty, which is far more than any other part of the muon g-2 calculation. 

Future Prospects

As the uncertainty in the HVP calculation falls (and this calculation currently approaches the maximum relative precision possible in QCD), this becomes more material in the overall accuracy of the muon g-2 calculation, and the greater precision will be important as the precision of the experimental measurement continues to improve. QCD calculations definitely can get more precise than the HLbL calculations are today, and especially more precise than the HLbL calculations other than the neutral pion contribution. 

But, it will be quite challenging, and may require a major breakthrough in QCD calculations generally, to get the uncertainty in the muon g-2 calculation to below 33-34 * 10^-11, which would be only about a 3-6% improvement from the best available combination of calculations so far. Therefore, the experimental result will probably be more precise than the QCD calculation for the foreseeable future.

Still, the bottom line, which has been clear since the BMW calculation was published at the time of the first Fermilab muon g-2 measurement, is that there is no muon g-2 anomaly since the predicted value and the measured value are consistent at the 0.2 sigma level. 

This global test of beyond the Standard Model physics at relatively low energies reveals that the Standard Model physics is complete and accurate at sub-parts per million levels, at least at relatively low energies of on the order of low GeVs or less.

The FLAG 2024 Report

FLAG, i.e., the Flavour Averaging Lattice Group, is a collaboration of scientists who generally also have primary employment elsewhere (much like the Particle Data Group) that compiles and analyzes experimental results in lattice QCD and updates these results periodically, to determine the bottom line values of Standard Model fundamental physical constants and also the experimentally measured values of a variety of other physical constants that could in principle be calculated from first principles in the Standard Model.

FLAG is generally more bold in terms of reaching precise values for constants, by being willing to be less methodology and outlier agnostic than the Particle Data Group.

Some select results

* Quark masses (in the MS bar scheme in a 2+1+1 scheme, at pole mass for bottom and charm quarks and a 2 GeV energy scale for the three light quark masses, followed by comparable PDG estimates):

u quark mass: 2.14 ± 0.18 MeV  v. 2.16 ± 0.07 MeV

d quark mass: 4.70 ± 0.05 MeV v. 4.70 ± 0.07 MeV

s quark mass: 93.44 ± 0.68 MeV v. 93.5 ± 0.8 MeV

c quark mass: 1280 ± 12 MeV v. 1273.0 ± 4.6 MeV

b quark mass: 4200 ± 14 MeV v. 4183 ± 7 MeV

* CKM matrix elements from global Standard Model fits:

|V(ud)|: 0.97439(14)

|V(us)|: 0.22483(61)

|V(ub)|: 0.00382(24)

* The strong force coupling constant alpha(s) in a five quark model in the MS bar scheme at the Z boson mass:

0.1183(7)

Combining FLAG and PDG non-lattice estimates:

0.1181(7)

For comparison sake, here are the world average measured values of some other Standard Model Constant values (per the Particle Data Group):

t quark mass direct measurements 172,570 ± 290 MeV (S=1.5)

t quark pole mass cross-section measurements 172,400 ± 700 MeV (S=1.1)

Electron mass 0.051099895000 ± 0.00000000015 MeV

Muon mass 105.6583755 ± 0.0000023 MeV

Tau lepton mass 1,776.93 ± 0.09 MeV

The Koide rule prediction of the tau lepton mass is 1,776.96894(7) MeV.

W boson mass 80,369.2 ± 13.3 MeV

The electroweak fit value of the W boson mass is 80,360.2 ± 9.9 MeV.

Z boson mass 91,188 ± 2 MeV

SM Higgs boson mass 125,200 ± 110 MeV

The full CKM matrix in a global fit per PDG (which differ from the FLAG values above):

The CKM matrix (and the PMNS matrix) can be completely summarized with four parameters (although there is more than one way to choose those four parameters).

Fermi coupling constant (G(F)/(h(bar)*c)^3, a derived version of weak force coupling constant) 0.000011663788(6) GeV^-2

Fine structure constant (i.e. electromagnetic force coupling constant) at a near zero energy scale is 0.0072973525693(11) 

I'll omit Planck's constant, the speed of light, and the physical constants related to neutrinos (the PMNS matrix and the three neutrino masses). I'll also omit the two fundamental physical constants related to gravity (Newton's constant and the cosmological constant).

Citation details are below the fold.

What Is "Quantum Magic"?

What does the term quantum magic, when used in a quantum physics scientific context (for example, in this preprint) mean?

"When you throw something into a black hole, information about it gets scrambled and cannot be recovered," says Goto. "This scrambling is a manifestation of chaos."

The team came across "magic," which is a mathematical measure of how difficult a quantum state is to simulate using an ordinary classical (non-quantum) computer. Their calculations showed that in a chaotic system almost any state will evolve into one that is "maximally magical"—the most difficult to simulate.

This provides the first direct link between the quantum property of magic and the chaotic nature of black holes. "This finding suggests that magic is strongly involved in the emergence of spacetime," says Goto.

From Phys.org discussing Kanato Goto et al, Probing chaos by magic monotones, Physical Review D (2022). DOI: 10.1103/PhysRevD.106.126009

Thus, "quantum magic" is simply an evocative name for a scientifically well-defined, but somewhat technical and obscure concept related to the difficulty of doing quantum physics calculations in a particular situation. 

This emergent descriptive property would naively just be a degree of difficulty factor useful for nothing more than estimating how much work will be involved in doing the calculations. It seems, surprisingly however, to have real world physical relevance in a manner somewhat analogous to the usual concept of "entropy" which was originally developed in the physics associated with thermodynamics. Indeed, as the preprint below operationalizes the concept, it is actually a very particularly defined kind of entropy (as distinct from the ordinary plain vanilla kind that is implied when the term entropy is used without any qualifiers or specifiers).

The preprint linked above and its abstract are as follows:

We consider the quantum magic in systems of dense neutrinos undergoing coherent flavor transformations, relevant for supernova and neutron-star binary mergers. Mapping the three-flavor-neutrino system to qutrits, the evolution of quantum magic is explored in the single scattering angle limit for a selection of initial tensor-product pure states for N(ν)≤8 neutrinos. 

For |ν(e)⟩^⊗N(ν) initial states, the magic, as measured by the α=2 stabilizer Renyi entropy M(2), is found to decrease with radial distance from the neutrino sphere, reaching a value that lies below the maximum for tensor-product qutrit states. 

Further, the asymptotic magic per neutrino, M(2)/N(ν), decreases with increasing N(ν). In contrast, the magic evolving from states containing all three flavors reaches values only possible with entanglement, with the asymptotic M(2)/N(ν) increasing with N(ν). 

These results highlight the connection between the complexity in simulating quantum physical systems and the parameters of the Standard Model.

Ivan Chernyshev, Caroline E. P. Robin, Martin J. Savage, "Quantum Magic and Computational Complexity in the Neutrino Sector" arXiv:2411.04203 (November 6, 2024).

The term "qutrit" (a.k.a. or quantum trit) mentioned in the preprint's abstract, in turn, "is a unit of quantum information that is realized by a 3-level quantum system, that may be in a superposition of three mutually orthogonal quantum states."

The downside of the term "quantum magic", however, is that the phrase is also used in a variety of contexts where its narrow quantum physics definition is not the intended meaning.

My Cues For Identifying Unhelpful Physics Papers (And Interesting Ones)

I skim the preprints at arXiv in astrophysics, general relativity, HEP-experiment, HEP-Lattice, and HEP-phenomenology almost every single day (going back to look at days I missed when I can't) and have done so for years.

There are lots of papers published, that in my personal, subjective, educated layman's view, are barking up the wrong tree in a line of research that I find to be unlikely to reflect reality and in that sense dubious. 

I mean this without denigrating the fact that these papers are mostly written by legitimate professional PhD physics researchers and represent predominately accurate and good faith efforts to apply their knowledge to unsolved problems in physics. These parts are often good enough if one makes the assumptions associated with that subfield, but I simply don't think that these subfields and the assumptions or conjectures behind them are correct.

Some of this is driven purely by my own informed intuition, even though I can't rigorously establish that these lines of inquiry are wrong. Others have been long disfavored by observational evidence that the authors choose to ignore or overlook for some reason. And, capricious as I am, sometimes something else about a paper that would generally seem unhelpful cause it to capture my attention anyway, usually in some aspect of the paper unrelated to the unhelpful parts.

I'll call these "unhelpful" physics papers. Also, to be clear, there are plenty of papers that are not in the "unhelpful" category, but simply aren't in an area of physics that I'm particularly interested in. 

For example, astronomy searches for exoplanets or analysis of how exoplanets form (that don't implicate new physics) are perfectly legitimate "helpful" papers, but I simply don't care much about the search for exoplanets or the process by which they form, since my main area of interest is the question to improve our understanding of the laws of physics themselves. Similarly, I am rarely interested in papers that recount new astronomy data without any meaningful analysis of it (it may be important, but I'm not skilled enough to analyze it without professional astronomer assistance). Likewise, HEP-experiment papers that discover and characterized new hadrons whose existence and properties are predicted by the Standard Model are "helpful" but just not all that interesting given my core interests.

Another class of papers which I largely ignore, although they are also not "unhelpful" in this sense, are papers discussing proposed new physics experiments that haven't reached the construction phase. Similarly, I don't pay much attention to highly technical instrumentation upgrade papers (in astronomy as well as HEP).

I identify "interesting papers" mostly by process of elimination, although some classes of papers, like measurements of Standard Model physical constants, analyses of the nature of scalar and axial vector mesons, modified gravity explanations of dark matter and dark energy phenomena, and mirror gravity cosmology papers are presumptively interesting.

One class of papers that hovers between the unhelpful and uninteresting category is the class of papers focusing on gravity and modified gravity theories in the very strong field regime (e.g. near black holes and neutron stars). I suspect that old fashioned classical GR does a very good job in this context, and in general, I'm just not all that interested in black holes.

What are some of the cues I use to determine that a preprint or paper is "unhelpful." Some papers have multiple "unhelpful" flags.

* Papers that discuss a see-saw mechanism for neutrino mass. See, e.g., here.

* Papers that analyze Majorana mass for neutrinos theoretically. See, e.g., here.

* Papers that propose "right handed" and/or "sterile" neutrinos. See, e.g., here and here.

* Papers that propose new GeV scale or more massive dark matter candidates. See, e.g., here and here.

* Papers that think we need new post-Big Bang physics to explain the baryon asymmetry of the universe. See, e.g., here.

* Supersymmetry models. See, e.g., here.

* String theory models. See, e.g., here.

* Lie Group based GUT and TOE models.

* Papers proposing new Higgs bosons in addition to the Standard Model Higgs boson (although papers ruling them out in some new parameter space deserve a passing glance). See, e.g., here.

* Papers exploring cosmological inflation models.

* Papers that propose leptoquarks. See, e.g., here.

* Papers discussing theories that give rise to proton decay.

* Papers that analyze "naturalness". See, e.g., here.

* Papers that propose non-sphaleron violations of baryon number or lepton number conservation (e.g. flavor changing neutral currents).

* Papers about quark stars and/or boson stars.

* Papers about macroscopic wormholes.

* Papers about magnetic monopoles.

* Papers about tachyons.

* Papers proposing more than five dimensions.

* Papers proposing tiny "rolled up" dimensions.

* Papers about Kaluza-Klein gravitons.

* Papers about Proca models.

* Papers about QCD Peccei-Quinn axions and/or the "strong CP problem". See, e.g., here.

* Papers proposing violations of the combined CPT symmetry.

* Papers proposing faster than light communication or information transfer.

* Papers about cosmological bounce models.

* Papers about "tired light" models.

* Papers that attempt to explain a discrepancy between experimental measurements of muon g-2 and the Standard Model value of it with new physics.

* Papers that take the new CDF W-Boson mass anomaly seriously. See, e.g., here.

* Papers that employ anthropic reasoning.

* Papers proposing theories where different generations of a particle have different properties (such as couplings) that are not coded in the particle's mass or its CKM/PMNS matrix couplings.

* Papers proposing fourth generation fundamental fermions. See, e.g., here.

I only blog a small percentage of the articles that I flag as interesting and bookmark (by subject area). Sometimes, they are too incremental or too technical or similar to papers on similar subjects that I've blogged before, and sometimes, I'm just not moved to write about them.

Quote Of The Day

We live in the wrong kind of world to be described by string theory. No physicist has ever won a big prize for string theory. I can tell you with absolute certainty that it is not the real world that we live in. So we need to start over.

- Lenny Suskind (a famous theoretical physicist) via Peter Woit's blog.

Archaic Hominins That Were Contemporaneous With Modern Humans

There were at least eight named non-H. sapiens archaic hominin species that existed in time periods overlapping those of  modern humans, H. sapiens, who evolved around 300,000 years ago. All were in the "Old World". There is no credible evidence that archaic hominin ever made it to the Americas, Australia, New Zealand, or most of the other islands of Oceania beyond island southeast Asia.

One was H. erectus that went extinct around 100,000 years ago after more than two million years roaming the Old World, first in Africa and later in Asia. There were also two hobbit-like species in island Southeast Asia, three East Eurasian sister clades to modern humans, the small but not hobbit-like H. naledi in Southern Africa, and Neanderthals in West Eurasia, although some of these species may actually be regional variants of the same species. 

All of these archaic hominin species had gone extinct (with the possible extended duration of a small isolated relict population or two, perhaps H. floresiensis) by about 29,000 years ago, leaving modern humans as the sole survivors (although some modern humans have Neanderthal ancestry (up to about 2%) from admixture events starting around 100,000 years ago, and some modern humans have both Neanderthal and also up to about 6% Denisovan ancestry, although absolute percentages are harder to estimate than percentages relative to other individuals with the same kind of introgression) from admixture events around the time that modern humans reached Southeast Asia. Incidentally, the introgression went both ways, with Neanderthals having up to 6% modern human introgressed ancestry, possibly due to events as much as 250,000 years ago. These sister clades capable of producing hybrid offspring may have been regionally specialized into climate niches.

Notably the remains of the Red Deer Cave People of China from 14,000 years ago (a few thousand years before the start of the Holocene era) are genetically modern humans and are not archaic hominins despite some of their seemingly archaic features. See also here.

I am also inclined to think that they may yet be a small relict population of small archaic hominins in a remote Indonesian jungle on the island of Sumatra and perhaps Flores as well, where these cryptids, called Orang Pendek, locally, have been attested but not definitively confirmed to still exist. I discuss this further at this post.

Homo floresiensis (discovered in 2003) are commonly known as "hobbits" and have been found on the island of Flores. Their phylogeny is disputed, but I find the theory that they are an asian branch of H. habilis to be most convincing. H. luzonesis (discovered in 2007) is similar and contemporaneous, but found further east in the Philippines and is supported by a less complete archaeological record. Both of these diminutive species are found in association with late Pleistocene tools and "oriental fauna".

Personally, being more of a lumper than a splitter, I'm inclined to see H. floresiensis and H. luzonesis as sub-species variations of the same species ("race" within that species to use some outdated terminology), and likewise to see H. longi, H. juluensis, and Denisovans as sub-species variations of the Denisovan species. The Hualongdong archaic hominin fossils, might be another hybrid individual or could be a hybrid individual, perhaps a Neanderthal-Denisovan hybrid individual (something that has precedent in a Denisovan cave DNA sample). 

Academic anthropologists, in contrast, tend to be splitters, in part, because it is cool and career advancing to discover and name your own archaic species, in part because the data is so fragmentary that grouping different fragmentary remains in a clade presumes relationships between the remains that aren't solidly proven, and in part, because it is easy to underestimate how much morphological diversity is possible within a single species if populations of it exposed to different environmental conditions.

H. longi a.ka. "dragon man" dates to an earlier time period (still contemporaneous with modern humans in Africa) in China and Manchuria, was discovered in 1933, and has been hypothesized to be a sister clade to Neanderthals, Denisovans, and modern humans, and a descendant of the pre-modern human hominin species H. antecessor due in part to basal archaic features in the skull. 

H. juluensis (literally "big heads") is contemporaneous H. longi, and beyond that time frame into the time frame of H. floresiensis and was discovered from 1976-1979 in China and Tibet. The authors assign this specimen along with Xiahe and Penghu fossils, to the Denisovan species (a sister clade to Neanderthals and modern humans) based upon comparisons of their fossil teeth and rough geographic proximity. H. juluensis is found in association with early Paleolithic tools and remains of Paleoarctic fauna. But they have larger brain cases than H. longi. A previous suggestions of the link between H. longi and the Denisovan species are discussed here and here at this blog. At least one Denisovan tooth has been found in Laos dated to 131,000 years ago.

The article also discusses the Hualongdong archaic hominin fossils that "date to the late Middle Pleistocene (~300,000 years BP) and display a mosaic of characteristics that cannot be easily fitted into any one lineage," although they are closer to H. longi and H. juluensis. This individual could be a hybrid between these two subspecies, with H. erectus, or with a Neanderthal who was far east of his usual range.

Prior to 2021, H. longi and H. juluensis tended to be classified as H. erectus (remains of which start to appear at a much greater time depth in Asia) or as archaic modern humans.

The Narmada and Maba partial skulls, especially the latter, are suggestively associated with Neanderthals by the article.

These Asian archaic species also overlap in time with the Southern African archaic hominin clade H. naledi which is a sister clade to the modern human ancestors and to the common ancestor of modern humans, Neanderthals, and Denisovans, but is not actually among our ancestors. As I explained at the link, this species "is basically a story from The Silmarillion of hominin evolution. It is entertaining, especially for hard core human evolution fans, but it doesn't really advance the plot."

A small number of papers reported genetic evidence in modern Africans of admixture with an archaic hominin "ghost species" in Africa, but subsequent papers have explained this "ghost species" signal as a methodological artifact that merely arises from population structure in early modern human Africans (see also here). But there may have been relict archaic hominins that did not admix with modern humans in Africa that were also contemporaneous with modern humans, at least, early on.

The question of whether behaviorally modern humans started showing advanced behavior around 70,000-50,000 years ago (at the dawn of the Upper Paleolithic era and close in time to the Out of Africa event for modern humans), was associated with an evolutionary leap in their brains is an open and unresolved question. See also here (addressing the question of what made modern humans genetically distinct from archaic hominins).

A greater degree of Late Quaternary hominin morphological variability is present in eastern Asia than previously assumed. Indeed, a number of distinct populations are present, some that now have new specific names: Homo floresiensis; H. luzonensis; H. longi; H. juluensis. With this piece, we describe the various groupings based on the current hominin fossil record of eastern Asia.

Christopher J. Bae & Xiujie Wu, "Making sense of eastern Asian Late Quaternary hominin variability" 15 Nature Communications 9479 (November 2, 2024) (opoen access) (hat tip to neo).

Another Observational Problem For LambdaCDM

Every time you turn around there is another conflict between the ΛCDM Standard Model of Cosmology and observational evidence from astronomy.

A promising way to test the physics of the accelerated expansion of the Universe is by studying the growth rate of matter fluctuations, which can be parameterized by the matter energy density parameter to the power γ, the so-called growth index. It is well-known that the ΛCDM cosmology predicts γ = 0.55. However, using observational data, Ref. Nguyen:2023 measured a much higher γ = 0.633 + 0.025 − 0.024, excluding the ΛCDM value within 3.7σ.

From here.

The linked study casts doubt on the hypothesis that dark energy can explain this discrepancy.

An Underground Particle Physics Lab in Idaho Springs, Colorado

Honestly, I probably wouldn't be blogging this article if it weren't discussing a new major physics experiment facility less than an hour away from my home. It is even closer to the University of Colorado at Boulder, and the Colorado School of Mines, both of which have high quality advanced physics programs staffed by elite physics PhD professors. CU-Boulder has been the home institution of quite a few Nobel prize winners.

The problem is that cosmic rays that include muons, and other surface bound sources of background noise, prevent scientists from conducting precision studies of low-energy muon production in ordinary university laboratories on the surface.

The basic strategy at this laboratory is to remove that background noise by putting the detection instruments deep in a mine originally dug for gold mining (the high school in Idaho Spring's somewhat titillating mascot as a result of this legacy of gold mining is the "gold diggers"). Initial calibrating tests show that the cosmic muon flux deep in the mine that was detected matches what the designers of the experiment expected to a reasonable degree. 

This laboratory setting provides a relatively inexpensive space to do research without background noise from cosmic and Earth-bound sources of contaminating charged particles, which is important for high precision measurements that could be done in a university laboratory but for their background interference. It reduces background noise 700-fold despite providing shielding only equivalent to a depth of about 415 meters.

There are only five underground laboratories of this kind in North America and only about a dozen in the entire world, not including CURIE, and they are in high demand.

We present the characterization of cosmogenic muon backgrounds for the Colorado Underground Research Institute (CURIE), located in the Edgar Experimental Mine (EEM) in Idaho Springs, Colorado. 

The CURIE facility at the EEM offers a versatile shallow underground environment, with accessible horizontal tunnel access and stable rock formations ideal for low-background physics experiments. We have measured the total underground muon flux in two locations, Site 0 and Site 1, yielding values of ϕ = 0.246 ± 0.020 sys. ± 0.012 stat. and 0.239 ± 0.025 sys. ± 0.010 stat. μ/m2/s, respectively. 

We have utilized GEANT4 and PROPOSAL Monte Carlo simulations with Daemonflux and MUTE to model the muon flux at both sites, as well as an additional future location. We find good agreement between measurement and simulations, demonstrating the first instance of this computational framework being successfully used for depths < 1 km.w.e. The measured underground flux corresponds to a factor of 700 reduction compared to the sea level flux. 

Additionally, we present a new depth-intensity relationship to normalize the mountain overburden to an equivalent flat depth, enabling direct comparison with other underground facilities. We report an average equivalent vertical depth of 0.415 ± 0.027 km.w.e. Based on our measurements, this work highlights the facility's capability for hosting low-background experiments, addressing the demand for shallow underground research spaces.

Dakota K. Keblbeck, Eric Mayotte, Uwe Greife, Kyle G. Leach, Wouter Van De Pontseele, "Cosmogenic Muon Background Characterization for the Colorado Underground Research Institute (CURIE)" arXiv:2411.01626 (November 3, 2024).

More Evidence That There Is No Muon g-2 Anomaly

Once again, it has become clear that the "data driven" means of estimating the Standard Model prediction for the anomalous magnetic moment of the muon was flawed and the the experimental result is actually consistent with the Standard Model prediction in a very global tests of the completeness and correctness of the Standard Model at high precision that strongly disfavors a variety of new physics models.

An accurate calculation of the leading-order hadronic vacuum polarisation (LOHVP) contribution to the anomalous magnetic moment of the muon (aμ) is key to determining whether a discrepancy, suggesting new physics, exists between the Standard Model and experimental results. 

This calculation can be expressed as an integral over Euclidean time of a current-current correlator G(t), where G(t) can be calculated using lattice QCD or, with dispersion relations, from experimental data for e+e−→hadrons. The BMW/DMZ collaboration recently presented a hybrid approach in which G(t) is calculated using lattice QCD for most of the contributing t range, but using experimental data for the largest t (lowest energy) region. Here we study the advantages of varying the position t=t1 separating lattice QCD from data-driven contributions. The total LOHVP contribution should be independent of t1, providing both a test of the experimental input and the robustness of the hybrid approach. 

We use this criterion and a correlated fit to show that Fermilab/HPQCD/MILC lattice QCD results from 2019 strongly favour the CMD-3 cross-section data for e+e−→π+π− over a combination of earlier experimental results for this channel. 

Further, the resulting total LOHVP contribution obtained is consistent with the result obtained by BMW/DMZ, and supports the scenario in which there is no significant discrepancy between the experimental value for aμ and that expected in the Standard Model. 

We then discuss how improved lattice results in this hybrid approach could provide a more accurate total LOHVP across a wider range of t1 values with an uncertainty that is smaller than that from either lattice QCD or data-driven approaches on their own.

C. T. H. Davies, et al., "Utility of a hybrid approach to the hadronic vacuum polarisation contribution to the muon anomalous magnetic moment" arXiv:2410.23832 (October 31, 2024).

A Novel And Intriguing Explanation For The CKM And PMNS Matrixes

I've never seen anyone reach this remarkable insight before and it is indeed very tantalizing. This is huge if true. The authors note in the body text that: 

To our knowledge, this is the first time the differing CKM and PMNS structures have arisen from a common mechanism that does not invoke symmetries or symmetry breaking.

The paper and its abstract are as follows: 

The Cabibbo-Kobayashi-Maskawa (CKM) matrix, which controls flavor mixing between the three generations of quark fermions, is a key input to the Standard Model of particle physics. In this paper, we identify a surprising connection between quantum entanglement and the degree of quark mixing. Focusing on a specific limit of 2→2 quark scattering mediated by electroweak bosons, we find that the quantum entanglement generated by scattering is minimized when the CKM matrix is almost (but not exactly) diagonal, in qualitative agreement with observation. 

With the discovery of neutrino masses and mixings, additional angles are needed to parametrize the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix in the lepton sector. Applying the same logic, we find that quantum entanglement is minimized when the PMNS matrix features two large angles and a smaller one, again in qualitative agreement with observation, plus a hint for suppressed CP violation. 

We speculate on the (unlikely but tantalizing) possibility that minimization of quantum entanglement might be a fundamental principle that determines particle physics input parameters.

Jesse Thaler, Sokratis Trifinopoulos, "Flavor Patterns of Fundamental Particles from Quantum Entanglement?" arXiv:2410.23343 (October 30, 2024).

The paper's literature review does note one prior paper making a similar analysis:

Entanglement is a core phenomenon in quantum mechanics, where measurement outcomes are correlated beyond classical expectations. In particle physics, entanglement is so ubiquitous that we often take it for granted, but every neutral pion decay to two photons (π0 → γγ) is effectively a mini Einstein–Podolsky–Rosen experiment. In the context of SM scattering processes, though, the study and quantification of entanglement in its own right has only begun relatively recently [12–26]. In terms of predicting particle properties from entanglement, the first paper we are aware of is Ref. [27] which showed that maximizing helicity entanglement yields a reason but every neutral pion decay to two photons (π0 → γγ) is effectively a mini Einstein–Podolsky–Rosen experiment. In the context of SM scattering processes, though, the study and quantification of entanglement in its own right has only begun relatively recently [12–26]. In terms of predicting particle properties from entanglement, the first paper we are aware of is Ref. [27] which showed that maximizing helicity entanglement yields a reasonable prediction for the Weinberg angle θ(W), which controls the mixing between electroweak bosons.

References 12-26 are from 2012 to 2024.

Ref. [27] is A. Cervera-Lierta, J. I. Latorre, J. Rojo and L. Rottoli, Maximal Entanglement in High Energy Physics, SciPost Phys. 3 (2017) 036, [1703.02989].

Footnote 6 of the main paper is also of interest and addresses the fact that using only a one-loop calculation the get a value of 6º for a parameter whose measured value is 13º:

We happened to notice that in the limit where we neglect photon exchange, the exact valueθmin C =13◦ is recovered. However, we do not have a good reason on quantum field theoretic grounds to neglect the photon contribution. Because of the shallow entanglement minimum in Fig.2a, a 10% increase in the charged current process over the neutral-current one would be enough to accomplish this shift, which is roughly of the expected size for higher-order corrections.

A somewhat similar prior analysis that is not cited is Alexandre Alves, Alex G. Dias, Roberto da Silva, "Maximum Entropy Principle and the Higgs boson mass" (2015) (cited 42 times) whose abstract states:

A successful connection between Higgs boson decays and the Maximum Entropy Principle is presented. Based on the information theory inference approach we determine the Higgs boson mass as M(H) = 125.04 ± 0.25 GeV, a value fully compatible to the LHC measurement. 

This is straightforwardly obtained by taking the Higgs boson branching ratios as the target probability distributions of the inference, without any extra assumptions beyond the Standard Model. Yet, the principle can be a powerful tool in the construction of any model affecting the Higgs sector. We give, as an example, the case where the Higgs boson has an extra invisible decay channel within a Higgs portal model.

I would argue that all three of the papers linked in this post are not just "numerology" papers, as they suggest a plausible physical mechanism or theoretical principal by which the values of the SM physical constants in question can be determined.