  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
comparing processes involving electrons and muons have not revealed any
significant violation of this assumption, recent studies 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.
