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

The ratio of branching ratios of the W boson to muons and electrons, $R^{\mu / e}_{W} = \mathcal{B}(W \to \mu \nu)/(\mathcal{B}(W \to e \nu)$, has been measured using 140 fb$^{-1}$ of pp collision data at $\sqrt{s}$ = 13 TeV collected with the ATLAS detector at the LHC, probing the universality of lepton couplings. The ratio is obtained from measurements of the $t\bar{t}$ production cross-section in the $ee, e\mu$ and $\mu \mu$ dilepton final states. To reduce systematic uncertainties, it is normalised by the square root of the corresponding ratio $R^{\mu \mu / ee}_{Z}$ for the Z boson measured in inclusive Z $\to ee$ and Z $\to \mu \mu$ events. By using the precise value of $R^{\mu \mu / ee}_{Z}$ determined from $e^{+}e^{-}$ colliders, the ratio $R^{\mu / e}_{W}$ is determined to be $R^{\mu / e}_{W} = 0.9995 \pm 0.0022 (stat) \pm 0.0036 (syst) \pm 0.0014 (ext)$. The three uncertainties correspond to data statistics, experimental systematics and the external measurement of $R^{\mu \mu / ee}_{Z}$, giving a total uncertainty of 0.0045, and confirming the Standard Model assumption of lepton flavour universality in W-boson decays at the 0.5% level.