Precise test of lepton flavour universality in $W$-boson decays into muons and electrons in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

ATLAS Collaboration

The ratio of branching ratios of the $W$ boson to muons and electrons, $R_W^{\mu /e}=B(W\to \mu \nu )/B(W\to e\nu )$, has been measured using 140 fb${}^{-1}$ of $pp$ collision da…