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

TL;DR

This paper measures the ratio of W boson decays into tau neutrinos versus electron neutrinos using 13 TeV proton-proton collision data from ATLAS, confirming lepton flavor universality within experimental uncertainties.

Contribution

It provides a precise test of lepton flavor universality in W decays using a large dataset and novel analysis techniques to distinguish decay modes.

Findings

Measured ratio R_{τ/e} = 0.975 ± 0.012 (stat.) ± 0.020 (syst.)

Result consistent with Standard Model predictions

Utilized impact parameter and transverse momentum differences for decay mode separation

Abstract

A measurement of the ratio of the branching fractions, $R_{\tau/e} = B(W \to \tau \nu)/ B(W \to e \nu)$, is performed using a sample of $W$ bosons originating from top-quark decays to final states containing $\tau$-leptons or electrons. This measurement uses $pp$ collisions at $\sqrt{s}=13$ TeV, collected by the ATLAS experiment at the Large Hadron Collider during Run 2, corresponding to an integrated luminosity of 140 fb$^{-1}$. The $W \to \tau \nu_\tau$ (with $\tau \to e \nu_e \nu_\tau$) and $W \to e \nu_e$ decays are distinguished using the differences in the impact parameter distributions and transverse momentum spectra of the electrons. The measured ratio of branching fractions $R_{\tau/e} = 0.975 \pm 0.012 \textrm{(stat.)} \pm 0.020 \textrm{(syst.)}$, is consistent with the Standard Model assumption of lepton flavour universality in $W$-boson decays.