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, R<inf>τ/e</inf> = 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 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 → τν<inf>τ</inf> (with τ → eν<inf>e</inf>ν<inf>τ</inf>) and W → eν<inf>e</inf> 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<inf>τ/e</inf> = 0.975 ± 0.012 (stat.) ± 0.020 (syst.), is consistent with the Standard Model assumption of lepton flavour universality in W-boson decays. © 2025 Elsevier B.V., All rights reserved.