Tuesday, December 22, 2020

Apparent Lepton Universality Violations Are Probably Measurement And Statistical Errors

[Rescued from a September 2020 draft]:

In a post entitled "How Big Are The Lepton Universality Violation Tensions With The Standard Model?" I reviewed the evidence that there were lepton universality violations in B meson decays. After reviewing that evidence, I've become convinced that the anomalies observed are some combination of statistical error, measurement error and analysis error. The really key data points are that these anomalies have only appeared in b quark decays, and that:
Lepton universality violations are not seen in W boson decays at the LHC, are not found in tau lepton decays or pion decays (also here), and are not found in anti-B meson and D* meson decays or in Z boson decays. There were still tensions in the data from B meson decays, but the deviations were smaller as of 2019 than they were in 2015 (also here).
The absence of violations in W boson decays is particularly critical. Because this phenomena is only seen "in the semileptonic decays of B mesons involving b → s l+ l− (l ∈ e, µ) neutral current and b → c l ν (l ∈ e/µ, τ ) charged current quark level transitions."

This is because a semi-leptonic decay of a b quark actually has intermediate steps. First, there is a b → c W- step. Then, the W- decays to l- v. This gives you b → c l ν. 

There may next be a decay c → s W+ in which the W+ then decays to l+ v. This gives you b → s l+ l− v v with the neutrino and anti-neutrino produced either being virtual particles or invisible. 

The critical point is that in a semileptonic decay of a b quark, the leptons arise from one or two subsequent leptonic W boson decays. So, if W boson decays don't display violations of lepton universality, then either the leptons observed in semileptonic b quark decays are produced by some means other than an intermediate W boson decay, exclusive to b meson decays, which decays preferentially to lighter leptons, or there is some error in the observation. The latter result is infinitely more likely.

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