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.
Categories: hep-ex