Dortmund 2021 – scientific programme

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T: Fachverband Teilchenphysik

T 78: Electroweak Interactions II

T 78.2: Talk

Thursday, March 18, 2021, 16:15–16:30, Tc

Test of the universality of tau and muon lepton couplings in W boson decays with the ATLAS detector — •Nicolas Köhler — CERN, Meyrin, Schweiz

The universality of the lepton couplings to the electroweak gauge bosons is one of the fundamental axioms of the Standard Model (SM) of particle physics. At LEP, the branching fractions of the W boson into charged leptons have been precisely measured, however, the uncertainty on the ratio of BR(W→τν)/BR(W→µν) still remains at approximately 2.4%. The PDG combined measurement of BR(W→τν)/BR(W→µν) shows a 2.7σ deviation from the SM prediction, motivating an independent measurement of this ratio at the LHC. ATLAS recently performed a measurement of this quantity using 139 fb⁻¹ of pp collision data recorded during LHC Run 2. To obtain a large unbiased sample of W bosons, di-leptonic tt events are selected. Muons originating from W bosons and those originating from an intermediate tau lepton are distinguished through the muon transverse impact parameter and the muon transverse momentum spectra. The result is in agreement with a universal coupling as postulated in the SM and the measurement is the most precise one to date.