Abstract
The ratio of branching ratios of the boson to muons and electrons, _∧/ = ( → )/( → ) , has been measured using 140 fb⁻¹ of 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 ̄ production cross-section in the , and dilepton final states. To reduce systematic uncertainties, it is normalised by the square root of the corresponding ratio _∧/ for the boson measured in inclusive → and → events.
By using the precise value of _∧/ determined from ⁺ ⁻ colliders, the ratio _∧ / is determined to be
_∧/ = 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 _∧/, giving a total uncertainty of 0.0045, and confirming the Standard Model assumption of lepton flavour universality in -boson decays at the 0.5% level
