Measurement of the W-boson polarisation in top quark decays in pp collision data at sqrt(s) = 7 TeV using the ATLAS detector

The polarisation of W bosons in top-quark decays is sensitive to the Wtb vertex structure. In this analysis the W-boson helicity fractions and angular asymmetries are measured in top-quark pair production. Final states with a single charged lepton, missing transverse energy and at least four jets, where at least one of them is b-tagged, are considered. Data collected by the ATLAS experiment at a centre-of-mass energy of 7 TeV are used, corresponding to an integrated luminosity of 35 pb^-1. The results are in agreement with the Standard Model and are used to set limits on anomalous contributions to the Wtb vertex.