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

The ratio of branching ratios of the W boson to muons and electrons, R_W^μ/e=B(W→μν)/B(W→eν), has been measured using 140fb^-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 R_Z^μμ/ee for the Z boson measured in inclusive Z→ee and Z→μμ events. By using the precise value of R_Z^μμ/ee determined from e⁺e^- colliders, the ratio R_W^μ/e is determined to be (Formula presented.) The three uncertainties correspond to data statistics, experimental systematics and the external measurement of R_Z^μμ/ee, 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.