Measurement of Tau Polarization in $\boldmath{\Wtaunu}$ Decays with the ATLAS Detector in pp Collisions at $\sqrt{s}~=~7~\TeV$

In this paper, the measurement of tau polarization in hadronic tau decays is presented. It is measured from the momenta and energies of a single final state charged particle in W->taunu decays. The data, corresponding to an integrated luminosity of 24 pb^-1, were collected by the ATLAS experiment in the year 2010. The measured value of the tau polarization is P_tau=-1.06+/-0.04(stat)+0.05(syst)-0.07(syst), in agreement with the Standard Model predictions, and is consistent with a physcially allowed 95% credibility interval [-1,-0.91]. The measurement of tau polarization in hadronic tau decays described in this paper marks the first measurement of tau polarization at a hadron collider and the first such measurement in W->taunu decays. Tau polarization is a powerful probe of potential new phenomena at the Large Hadron Collider. Applications include sensitivity to the nature of the spin of a theorized Higgs boson that decays to tau pairs.