Mixed Results In A Test Of Lepton Universality

A new experimental result finds a local 2.6 sigma deviation from the Standard Model expectation of charged lepton universality in particular kinds of B meson decays. But, after considering look elsewhere effects, it is consistent with the Standard Model. Thus, it tends to disprove the case for charged lepton non-universality from several over measurements that are individually not that notable, but collectively seem to have significance. But, it doesn't do so very strongly.

We present a measurement of angular observables and a test of lepton flavor universality in the B→K∗ℓ+ℓ− decay, where ℓ is either e or μ. The analysis is performed on a data sample corresponding to an integrated luminosity of 711 fb−1 containing 772×106 BB¯ pairs, collected at the Υ(4S) resonance with the Belle detector at the asymmetric-energy e+e− collider KEKB. The result is consistent with Standard Model (SM) expectations, where the largest discrepancy from a SM prediction is observed in the muon modes with a local significance of 2.6σ.

Belle Collaboration, "Lepton-Flavor-Dependent Angular Analysis of B→K∗ℓ+ℓ−" (December 15, 2016).

Other experiments looking for lepton universality violations are reviewed here.