Measurement of the W-boson polarisation in top-quark decays with the ATLAS detector

A measurement of the W-boson polarisation in top-quark decays is presented. The top-antitop events were produced in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}$ = 7 TeV at the LHC. The data set corresponds to an integrated luminosity of 4.7 fb$^{-1}$ and was collected by the ATLAS experiment. The measurement was performed in the lepton+jets channel which is characterised by an isolated electron or muon, missing transverse energy and at least four jets. One of the jets has to be identified as a b-jet. The W-boson polarisation was studied using the angular distribution of the charged lepton in the rest frame of the W-boson. The event is fully reconstructed using a kinematic likelihood fit. The fractions of left-handed, right-handed and longitudinally polarised W-bosons were estimated from the data distribution using a binned likelihood fit.