Every once and a while it is worth mentioning a basically unexceptional paper that tells us nothing that we didn't expect from the Standard Model, to demonstrate how incredibly precise its predictions can be. In this case, the Standard Model predicts to within one standard deviation of certainty the experimentally measured frequency of a decay of a light meson which is predicted to occur in only 4 out of a billion decays of a meson of that type.
The η meson (pronounced "eta me son") is a JPC = 0−+ (i.e. electromagnetically neutral pseudoscalar) composite particle with a mass of 547.9 MeV, made up of an admixture of up, down, and strange quarks, usually described as follows:
The four muon decay of the η meson → μ+μ−μ+μ− is theoretically predicted to have a branching fraction of (3.98 ± 0.15) × 10^−9 in the Standard Model.
A new preprint from the CMS collaboration reports the experimental detection of this rare decay at more than 5 sigma significance, with a branching fraction of "(5.0 ± 0.8 (stat) ± 0.7 (syst) ± 0.7 β(2μ) ) × 10^−9, where the last term is the uncertainty in the normalization channel branching fraction."
A one sigma difference between the two is 1.28 x 10^-9 (99% due to experimental measurement uncertainty which is 1.27 x 10^-9), i.e. 1.28 part per billion, while the difference between the measured and theoretical values is 1.02 x 10^-9. So, the Standard Model prediction in this tiny branching fraction is consistent with the experimental measurement at the 0.8 sigma level.
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