Abstract
The ratio of branching ratios of the W boson to muons and electrons, Rμ/eW=B(W→μν)/B(W→eν), has been measured using 140 fb−1 of pp collision data at 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 tt¯ production cross-section in the ee, eμ and μμ dilepton final states. To reduce systematic uncertainties, it is normalised by the square root of the corresponding ratio Rμμ/eeZ for the Z boson measured in inclusive Z→ee and Z→μμ events. By using the precise value of Rμμ/eeZ determined from e+e− colliders, the ratio Rμ/eW is determined to be Rμ/eW=0.9995±0.0022±0.0036±0.0014. The three uncertainties correspond to data statistics, experimental systematics and the external measurement of Rμμ/eeZ, 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.