Test of lepton flavour universality in W-boson decays into electrons and τ -leptons using pp collisions at √s = 13 TeV with the ATLAS detector

A measurement of the ratio of the branching fractions, Rτ /e = B(W →τ ν)/B(W → eν), is performed using a sample of W bosons originating from top-quark decays to final states containing τ -leptons or electrons. This measurement uses pp collisions at
√s = 13 TeV, collected by the ATLAS experiment at the Large Hadron Collider during Run 2, corresponding to an integrated luminosity of 140 fb−1. The W → τ ντ (with τ → eνeντ ) and W → eνe decays are distinguished using the differences in the impact parameter distributions and transverse momentum spectra of the electrons. The measured ratio of branching fractions Rτ /e = 0.975 ± 0.012 (stat.) ± 0.020 (syst.) , is consistent with the Standard Model assumption of lepton flavour universality in W -boson decays.