Observation of W->tau nu Decays with the ATLAS Experiment

A search for W -> tau nu decays, with the tau lepton decaying into hadrons, has been performed 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 7 TeV. A total of 78 data events are selected, with a 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 Standard Model signal expectation, both in number of events and in shapes of distributions of kinematical variables and variables used in the tau identification. This is the first observation of W -> tau nu decays and of hadronically decaying tau leptons in ATLAS.