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

Abstract

A measurement of the ratio of the branching fractions, Rτ/e = B(W RIGHTWARDS ARROWτν)/B(W RIGHTWARDS ARROW eν), is performed using a sample of W bosons originating from top-quarkdecays to final states containing τ -leptons or electrons. This measurement uses pp collisions atSQUARE ROOTs = 13TeV, collected by the ATLAS experiment at the Large Hadron Collider during Run 2,corresponding to an integrated luminosity of 140 fb-1. The W RIGHTWARDS ARROW τντ (with τ RIGHTWARDS ARROW eνeντ ) andW RIGHTWARDS ARROW eνe decays are distinguished using the differences in the impact parameter distributionsand transverse momentum spectra of the electrons. The measured ratio of branching fractionsRτ/e = 0.975+-0.012 (stat.)+-0.020 (syst.), is consistent with the Standard Model assumptionof lepton flavour universality in W-boson decays.