The ratio of branching ratios of the W boson tomuons and electrons, R μ/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 ratiois 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 rootof the corresponding ratio R μμ/eeZ for the Z boson measured in inclusive Z → ee and Z → μμ events. By using the precise value of R μμ/eeZ determined from e+e− colliders, the ratio R μ/eW is determined to be
R μ/eW = 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/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