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

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 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^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 W-boson decays at the 0.5% level.