Measurement of the Tau Polarization in $Z\rightarrow\tau\tau$ Decays with the ATLAS Detector

This thesis presents a measurement of the tau polarization in Z decays in proton-proton collision at √s = 7 TeV. The measurement is based on 4.6 fb−1 data that was collected in 2011 with the ATLAS experiment situated at the Large Hadron Collider. The polarization was measured in one prong hadronic tau decays, by use of a variable that measures the energy asymmetry between charged and neutral pions. Templates from right- and left-handed taus were fitted to the data. A measurement of the tau polarization was performed and the systematic uncertainties on background and signal were investigated. The result of the measurement was Pτ =−0.258±0.048(stat.)±0.028(MC)±0.073(sys.).