Observation of $W \to \tau \nu_{\tau}$ Decays with the ATLAS Experiment

Physics studies of processes with tau leptons in the final state, while challenging at hadron colliders, are of great importance at the LHC. The tau 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 tau leptons, in particular Z-> tau tau and W-> tau nu, are important background processes in those searches and their cross sections need to be measured first. This thesis reports the first observation of W-> tau nu decays and of hadronically decaying tau leptons with the ATLAS experiment at the LHC. The analysis is based on a data sample corresponding to an integrated luminosity of 546nb-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 W and Z decays. The observed excess of data events over the total background is compatible with the SM expectation for W-> tau nu decays, both in the number of events and in the shapes of distributions of characteristic variables.