  One of the key assumptions of the Standard Model of fundamental particles is
that the interactions of the charged leptons, namely electrons, muons, and
taus, differ only because of their different masses. While precision tests have
not revealed any definite violation of this assumption, recent studies of B
meson decays involving the higher-mass tau lepton have resulted in observations
that challenge lepton universality at the level of four standard deviations. A
confirmation of these results would point to new particles or interactions, and
could have profound implications for our understanding of particle physics.
