Democracy suffers a blow—in particle physics Available to Purchase

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...