  The Standard Model of particle physics currently provides our best
description of fundamental particles and their interactions. The theory
predicts that the different charged leptons, the electron, muon and tau, have
identical electroweak interaction strengths. Previous measurements have shown a
wide range of particle decays are consistent with this principle of lepton
universality. This article presents evidence for the breaking of lepton
universality in beauty-quark decays, with a significance of 3.1 standard
deviations, based on proton-proton collision data collected with the LHCb
detector at CERN's Large Hadron Collider. The measurements are of processes in
which a beauty meson transforms into a strange meson with the emission of
either an electron and a positron, or a muon and an antimuon. If confirmed by
future measurements, this violation of lepton universality would imply physics
beyond the Standard Model, such as a new fundamental interaction between quarks
and leptons.
