Observation of W → τν_τ Decays with the ATLAS Experiment

Abstract

Physics studies of processes with t leptons in the final state, while challenging at hadron colliders, are of great importance at the LHC. The τ leptons provide important signatures in searches for the Higgs boson as well as for new physics in a wide range of theoretical models. Decays of Standard Model particles to τ leptons, in particular <em>Z</em>→ττ and <em>W</em> → τν_τ, are important background processes in those searches and their cross sections need to be measured first. This thesis reports the first observation of <em>W</em> → τν_τ decays and of hadronically decaying τ leptons with the ATLAS experiment at the LHC. The analysis is based on a data sample corresponding to an integrated luminosity of 546 nb^-1, which was recorded at a proton-proton centre-of-mass energy of 7TeV. A total of 78 data events are selected, with an estimated background of 11.1±2.3_(stat.) ±3.2_(syst.) events from QCD processes, and of 11.8±0.4_(stat.) ±3.7_(syst.) events from other <em>W</em> and <em>Z</em> decays. The observed excess of data events over the total background is compatible with the SM expectation for <em>W</em> → τν_τ decays, both in the number of events and in the shapes of distributions of characteristic variables.