# LHCb on the trail of lepton nonuniversality

The LHCb experiment at CERN has made the first measurement at a hadron collider of decays of B mesons that have already shown possible deviations from the predictions of the Standard Model in earlier studies at an electron-positron collider.

Decays of hadrons containing the b quark that produce a tau lepton and its neutrino provide a sensitive probe of extensions to the Standard Model that involve an enhanced coupling between the b quark and these third-generation leptons. Such an effect would indicate a breakdown of lepton universality according to which the charged leptons – electron (e), muon (μ) and tau (τ) – have exactly the same basic couplings.

LHCb has measured the branching fraction ratio R(D*): B0 → D*+τ- ντ/ B0 → D*+μ- νμ. The result 0.336±0.027±0.030 is larger than the Standard Model expectation of 0.252±0.003 by 2.1 standard deviation (σ). The BaBar experiment at SLAC has already found a similar deviation from the Standard Model prediction.

See the paper here