Tuesday, September 28, 2021

Quick Miscellaneous Physics Results

Neutrino Physics 

A new paper from the Neutrino-4 experiment makes the case of a sterile neutrino and also estimates the neutrino masses. It also predicts very high neutrino masses compared to other experiments. With an electron neutrino mass of 0.8 eV, a muon neutrino mass of 0.4 eV, a tau neutrino mass of less than 0.6 eV, and a sterile neutrino mass of 2.7 eV. I am highly skeptical of the result, not least because the mass predictions are also out of line with other results. This is a screenshot of the abstract in the paper itself which is used to preserve the fussy formatting:

* Another new review of the sterile neutrino question can be found here (the updated added September 29, 2021):
Two anomalies at nuclear reactors, one related to the absolute antineutrino flux, one related to the antineutrino spectral shape, have drawn special attention to the field of reactor neutrino physics during the past decade. Numerous experimental efforts have been launched to investigate the reliability of flux models and to explore whether sterile neutrino oscillations are at the base of the experimental findings. This review aims to provide an overview on the status of experimental searches at reactors for sterile neutrino oscillations and measurements of the antineutrino spectral shape in mid-2021. 
The individual experimental approaches and results are reviewed. Moreover, global and joint oscillation and spectral shape analyses are discussed. 
Many experiments allow setting constraints on sterile oscillation parameters, but cannot yet cover the entire relevant parameter space. Others find evidence in favour of certain parameter space regions. In contrast, findings on the spectral shape appear to give an overall consistent picture across experiments and allow narrowing down contributions of certain isotopes.

* Neutrino-nucleon collision models still have kinks to be worked out in the low energy, forward muon angle regime where models fail to adequately account for the extent to which events in this part of the parameter space are suppressed. The authors speculate on what might be missing from the models but aren't really sure why the discrepancy arises.

* Neutrino data from experiments and neutrino data from cosmic ray observations are reasonably consistent with each other.

Other Physics

* The charge radius of the proton is measured to be 0.840(4) fm (with conservative rounding assumptions), consistent with prior experimental measurements from muonic hydrogen of 0.840 87(39) fm, and with the better recent measurements using ordinary hydrogen such as a 2019 measurement that found a radius of 0.833(10) fm. 

In 2014, the CODATA average measurement had stated that the charge radius of the proton was  0.8751(61) fm, which has subsequently been determined to be too large due to reliance on older, less accurate experiments with ordinary hydrogen, and less correct theoretical analysis of their results. Correctly theoretically analyzing the old data would have produced a result of 0.844(7) fm.

* Non-perturbative and perturbative QCD models need to be used together to get more precise determinations of the QCD coupling constant. Perturbative QCD methods alone have hit their limits.

* Someone argues for a better way to do renormalization (really a better way to apply existing methods) in QCD.

* Someone makes a more accurate prediction of how many Higgs bosons the LHC should produce at its highest energies. This still has more than a 5% uncertainty, however.

* The Paul Scherrer Institute in Switzerland does mesoscale particle physics experiments at lower energies than the LHC. It has a nice brief review of the relevant Standard Model Physics of the interactions it studies and potential beyond the Standard Model tweaks to it in this regime that it is studying using lower energies but greater precision to study more practically relevant parts of the Standard Model. The abstract of the article is useless, so I quote from the introduction.

These experiments either lead to precise determinations of physical parameters required as input for other experiments (e.g., muon life time, pion mass), or search for physics beyond the Standard Model (BSM). The BSM searches proceed along different frontiers. 

One way to search for new physics is to consider physical observables whose Standard Model (SM) contributions either vanish or are too small to be experimentally accessible. In other words, they are identical to zero for practical purposes. Examples are charged lepton-flavor violating (cLFV) muon decays or a permanent neutron electric dipole moment (EDM). To put constraints on the branching ratios of BSM decays, one has to observe a large number of decays. This is, thus, called a search at the intensity frontier. 

Another way to search for new physics is to consider precision observables and search for deviations from the SM expectations. Prominent examples are the precision QED tests with muonium, as well as the precision laser spectroscopy experiments with muonic atoms. These are, thus, called searches at the precision frontier. The low-energy experiments at PSI are complementary to the experiments at LHC, which sit at the energy frontier.

After a general overview of the theoretical methods applied to describe the processes and bound states in Table 5.1, we will, in turn, consider the muon, the proton, nucleons and nuclei, the free neutron, and the pions.

* The significance of the 151 GeV anomaly at the LHC is overstated.

* Experimental evidence continues to disfavor the existence of a light pseudoscalar Higgs boson "A", which is a generic prediction of models like supersymmetric with multiple Higgs doublets.

* A group of scientists try to explain the charged and neutrino mass hierarchies, muon g-2, electron g-2, leptogenesis, and dark matter with a inverse seesaw model, which is usually only used to attempt to explain neutrino masses and sometimes dark matter. The effort is notable for its breadth, although I very much doubt that it is a correct explanation. A similar model is proposed here.

* Someone proposes a non-SUSY E6 GUT to explain various outstanding physics anomalies consistent with experimental constraints. It is probably wrong.

* Experimental constraints on the proton lifetime (which the Standard Model assumes is stable) are close to ruling out the simplest supersymmetric SU(5) GUT theory.

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