Is There Lepton Universality?

Scientific American has an overview of some interesting recent results looking at semileptonic decays of neutral B mesons. Several experiments see what looks like a possible excess of events with tau particles in the final state, which is not allowed in the Standard Model. The Standard Model includes a weak force that has the same couplings to all leptons, so the only differences that we should see between semileptonic decays with electrons, muons or taus are due to phase space and not due to any fundamental interactions. LHCb and Belle both see a fairly large excess of tau events compared to muon events, and earlier results by BaBar also see this kind of effect as well. None of the effects are particularly large, but the combined significance is reported at “3.9 sigma,” which is getting pretty close to the typical threshold for making a discovery of some new effect.

The article also mentions that if this is real, it potentially points toward some exciting new physics like charged Higgs particles (which are predicted in theories like┬ásupersymmetry) or leptoquarks (quark-lepton composite particles). Of course, it could still just be nothing, so we’ll just have to wait to see if it holds up.