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T: Fachverband Teilchenphysik
T 98: Topics in flavor physics
T 98.6: Vortrag
Freitag, 3. April 2020, 12:15–12:30, H-HS XII
Measuring lepton flavour universality in W boson decays at the ATLAS experiment — •Nicolas Köhler — CERN, Meyrin, Schweiz
The universality of the lepton couplings to electroweak gauge bosons (lepton flavour universality) is one of the fundamental ingredients of the Standard Model (SM) of particle physics. At LEP, the branching fractions of the W boson into charged leptons have been measured with a high precision, 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 significant deviation from the SM prediction of 2.7σ, thus motivating an independent measurement of this ratio at the LHC. This analysis focusses on leptonic tau decays due to the significantly larger experimental systematic uncertainties for hadronic tau reconstruction. It exploits a Tag&Probe method on tt events where the leptonic W decay of one top quark is used to trigger the event, allowing the lepton of the second W decay, which is unbiased in transverse momentum, to be used for the actual measurement. Leptonic tau decays are distinguished from prompt muons by requirements on the unsigned impact parameter, |d0(µ)|, and the transverse momentum of the muon. This analysis is expected to achieve a 1% precision which will be a significant improvement over the existing LEP results.