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Abstract

The ratio of branching ratios of the W boson to muons and electrons, R μ/e W = 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 crosssection in the ee, eμ and μμ dilepton final states. To reduce systematic uncertainties, it is normalised by the square root of the corresponding ratio R μμ/ee Z for the Z boson measured in inclusive Z → ee and Z → μμ events. By using the precise value of R μμ/ee Z determined from e+e− colliders, the ratio R μ/e W is determined to be R μ/e W = 0.9995 ± 0.0022 (stat) ± 0.0036 (syst) ± 0.0014 (ext). The three uncertainties correspond to data statistics, experimental systematics and the external measurement of R μμ/ee Z , giving a total uncertainty of 0.0045, and confirming the Standard Model assumption of lepton flavour universality in Wboson decays at the 0.5% level.