Observation of Z->tau_h tau_l Decays with the ATLAS detector

A study of Z->tau tau decays has been performed with the ATLAS experiment at the LHC. The channel with one tau lepton decaying into electron or muon and the second one into hadrons has been considered. The analysis is based on a data sample corresponding to an integrated luminosity of of 8.32 pb-1 for the electron channel and 8.53 pb-1 for the muon channel, at a proton-proton centre-of-mass energy of 7 TeV. In the muon channel a total of 51 data events are selected, with a background of 5.2+-0.7(stat.)+-0.8(syst.) events from QCD processes, and of 4.67+-0.45(stat.)+-1.48(syst.) events from other W, Z and ttbar decays. In the electron channel a total of 29 data events are selected, with a background of 6.8+-0.6(stat.)+-0.7(syst.) events from QCD processes, and of 4.98+-0.55(stat.)+-1.40(syst.) events from other W, Z and ttbar decays. The observed excess of data events over the total background is compatible with the Standard Model signal expectation, 39.85+-1.83(stat.)+-6.83(syst.) events in the muon channel and 24.5+-1.42(stat.)+-5.93(syst.) events in the electron channel. This is the first observation of Z->tau tau decays in ATLAS.