Democracy suffers a blow—in particle physics

Upon learning of the discovery of the muon, I. I. Rabi famously quipped, “Who ordered that?” After all, the muon appeared to be identical to the electron except for its mass. Indeed, in the standard model of particle physics, the charged leptons—electron, muon, and tau—interact in the same way with the model’s gauge bosons, the particles that transmit force. As a consequence of that lepton democracy, the standard model prescribes the relative probabilities, or branching ratios, for a heavy particle to decay into one or another of the charged leptons plus other particles in common. Three years ago the BaBar collaboration at SLAC measured the branching ratios for B-meson decay to produce either a muon or a tau. For two slightly different decays, they found 2σ or greater deviations from the democratic standard-model expectation. Now the LHCb collaboration at CERN has confirmed the BaBar result for one of the decays. In a preprint, the Belle group at KEK in Japan has also announced results that show a similar though less strong deviation from the standard model. The figure (from the Heavy Flavor Averaging Group) shows the branching ratios (R) measured by the groups for the two decays, denoted D and D^∗, along with the standard-model prediction. Taken together, the groups’ measurements have struck a 3.9-σ blow to the principle of lepton democracy. If they hold up, the standard model will have to be modified—perhaps by the addition of a new charged Higgs boson, whose interactions would depend on mass. (R. Aaij et al., LHCb collaboration, Phys. Rev. Lett. 115, 111803, 2015, doi:10.1103/PhysRevLett.115.111803 .)