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

A measurement of the ratio of the branching fractions, R τ/e =B(W→τν)/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 sqrt(s) =13 TeV, collected by the ATLAS experiment at the LHC during Run 2, corresponding to an integrated luminosity of 140 fb-1. The W→τντ (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 R τ/e =0.975±0.012 (stat.)±0.020 (syst.), is consistent with the Standard Model assumption of lepton flavour universality in W-boson decays.