The BESIII experiment reports the first case of strong experimental evidence for lepton flavor universality violation (i.e. electrons, muons and tau leptons having properties other than mass that differ from each other), outside semi-leptonic decays of B mesons.
But, its claim of a strong departure from lepton flavor universality is, in fact, a rookie class misinterpretation of the reported data which the introductory text seems to acknowledge, which is, in fact, not inconsistent with lepton flavor universality at the two sigma level.
Instead of studying semi-leptonic B meson decays, this study looks at the decays of excited J/Psi mesons ( a spin-1 (i.e. vector) charmonium meson (i.e. it has a charm quark and an anticharm quark as valence quarks), with a ground state mass of 3097 MeV and a mean lifetime in the ground state of 7.1(2)*10-21 seconds.
In contrast, the B mesons showing apparent lepton universality violations have a b quark and a non-b antiquark (or a non-b quark and a b antiquark) as valence quarks, ground state rest masses of 5297-5415 MeV. Pseudoscalar B mesons have a mean lifetime on the order of 1.5(1)*10-12 seconds.
The introduction to the paper provides important context:
On the one hand, lepton flavor universality (LFU) is expected to be obeyed in SM. In recent years, however, indications for violation of LFU have been reported in semileptonic decays of the kind b → s ℓ+ℓ −. In 2014, LHCb measured the ratio of branching fractions RK = B(B+ → K+µ +µ −)/B(B+ → K+e +e −), and found a deviation from the SM prediction by 2.6σ. The measurements have continuously been updated by LHCb and Belle. Very recently, LHCb reported their latest result with full Run I and Run II data, which deviates from SM prediction by more than 3σ. . . . It is therefore urgent to investigate the validity of LFU in other experiments.
J/ψ → ℓ +ℓ −, where ℓ may be either e or µ, are two such precisely measured channels, and their measured branching fractions are consistent with Quantum Electrodynamics (QED) calculations.
Other purely leptonic decays, which have never been studied experimentally, are J/ψ → ℓ + 1 ℓ − 1 ℓ + 2 ℓ − 2 ℓ, where ℓ1 = ℓ2 = e, ℓ1 = ℓ2 = µ or ℓ1 = e and ℓ2 = µ. For the first two cases, there is no special order for the four leptons.
Recently, the branching fractions of J/ψ → ℓ + 1 ℓ − 1 ℓ + 2 ℓ − 2 ℓ decays were calculated at the lowest order in nonrelativistic Quantum Chromodynamics (NRQCD) factorization in the SM. Given the collinear enhancement when the lepton mass tends to zero, the predicted branching fraction of J/ψ → e+e−e+e- is (5.288 ± 0.028) × 10−5, significantly greater than that of J/ψ → e+e−µ+µ- ((3.763 ± 0.020) × 10^−5) and two orders of magnitude greater than that of J/ψ → µ+µ−µ+µ- ((0.0974 ± 0.0005) × 10^−5).
Therefore, the ratio Beeee: Beeµµ: Bµµµµ provides a good opportunity to verify the validity of LFU.
(I omit a discussion of the muon g-2 anomaly as a motivation for new lepton coupling particles, which I personally think is due to a flawed theoretical prediction which is contradicted by a methodologically more sound theoretical prediction that matches the experimental result).
The paper and its abstract are as follows: