A new review of apparent lepton universality violations in b quark decays, which is absent in all other lepton decays, quantifies the evidence in favor of this phenomena and notes some possible explanations. The following excerpt is from the conclusion of the paper.
In recent years, several observations of tree- and loop-level b-hadron decays hint at a possible violation of LU [Lepton Universality]. This article has discussed the relevant measurements as well as their potential implications.
LU plays a peculiar role in the SM [Standard Model] of particle physics, as the same interactions and couplings characterise all three fermion generations. As a consequence, a violation of LU would be an unambiguous sign of the existence of physics beyond the SM. This property has been tested throughout the years by using a variety of different probes: the production and the decay of electroweak gauge bosons, the decay of quarkonia, the leptonic and semileptonic decays of mesons with light quarks (including the c quark), however, no significant signs of deviations from the SM predictions have been observed.
A possible violation of LU is hinted at in two different classes of semileptonic b-quark decays. The measurements are obtained from experiments at the B-factories (BaBar, Belle and soon Belle-II) as well as at the LHC (LHCb). For these processes the SM predictions can be computed using an effective Hamiltonian approach that separates short and long-distance contributions, and require non-perturbative inputs (e.g. form factors) obtained through diverse theoretical methods. Tensions at the level of 4–5 σ are observed in b→ clν [charm quark, negative charged lepton, corresponding anti-neutrino] decays, which are mediated at tree level through a W± boson in the SM, when the branching ratios of decays with l = τ and l = e, µ are compared. Deviations at the level of 3–4 σ are also present in b→ sll [strange quark, lepton and anti-lepton] decays, which are mediated through a loop in the SM, when comparing the branching ratios for l = e and l = µ.
From Simone Bifani, et al., "Review of Lepton Universality tests in B decays" (September 17, 2018).
One factor that makes this anomaly particularly tricky to explain is that absences of deviations of lepton universality in all other contexts makes it particularly difficult to explain the phenomena that is only observed in b quark decays with some sort of general theory, such as a charge lepton CKM matrix that is almost, but not exactly, the matrix that would produce lepton universality.