Thursday, January 19, 2017

Lattice QCD Makes Decent Approximations Of Experimental Data And QCD Does Axions

A new preprint compares a wide variety of recent lattice QCD predictions (and post-dictions) of the properties of various mesons and baryons.

The results show a solid array of accurate results. Neither the predictions nor, in many cases, the experimental results, are terribly precise, but the lattice QCD results do consistently make lots of accurate predictions. This tends to disfavor the possibility that there are beyond the Standard Model physics at work in QCD, and to support the hypothesis that while doing the math to determine what the equations of QCD imply in the real world is hard, that the underlying theory is basically sound.

Supercomputers applying Lattice QCD have also made progress in establishing the mass of a hypothetical particle call the axion under a beyond the Standard Model modification introduced to explain the fact that CP violation is non-existent or negligible in strong force interactions. The final result is the axion mass should be between 50 and 1500 * 10-6 eV/c2. Hat tip to Backreaction.

This is on the same order of magnitude as the expected mass of the lightest neutrino mass in a normal mass hierarchy, or perhaps up to about 20-40 times lighter. By comparison the second lightest neutrino mass is not less than about 8000 * 10-6 eV/c2, and the heaviest neutrino mass is not less than about 52,000 * 10-6 eV/c2.

These limits are considerably more narrow than the state of the art as of 2014 as I explained in a blog post at that time:

A new pre-print by Blum, et al., examines observational limits on the axion mass and axion decay constant due to Big Bang Nucleosynthesis, because the role that the axion plays in strong force interactions would impact the proportions of light atoms of different types created in the early universe.
The study concludes that (1) the product of the axion mass and axion decay constant must be approximately 1.8*10^-9 GeV^2, and (2) that in order to solve the strong CP problem and be consistent with astronomy observations, that axion mass must be between 10^-16 eV and 1 eV in mass (with a 10^-12 eV limitation likely due to the hypothesis that the decay constant is less than the Planck mass). The future CASPEr2 experiment could place a lower bound on axion mass of 10^-12 eV to 10^-10 eV and would leave the 1 eV upper bound unchanged. 
Other studies argue that the axion decay constant must be less than 10^9 GeV (due to constraints from observations of supernovae SN1987A) and propose an axion mass on the order of 6 meV (quite close to the muon neutrino mass if one assumes a normal hierarchy and a small electron neutrino mass relative to the muon neutrino-electron neutrino mass difference) or less. Estimates of the axion mass in the case of non-thermal production of axions, which are favored if it is a dark matter particle, are on the order of 10^-4 to 10^-5 eV. There are also order of magnitude estimates of the slight predicted coupling of axions to photons. 
Other studies placing observational limitations on massive bosons as dark matter candidates apply only to bosons much heavier than the axion.
A narrower theoretically possible target, in turn, makes experimental confirmation or rejection of the axion hypothesis much easier. 

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