Measurement of top quark polarisation in $t\bar{t}$ events with the ATLAS detector in proton-proton collisions at $\sqrt{s}=7\, \mathrm{TeV}$

This note presents a measurement of the top quark polarisation in $t\bar{t}$ events using the lepton plus jets final state, where one $W$ boson decays leptonically and the other hadronically. The decay of the $t\bar{t}$ pair is fully reconstructed using a likelihood method in order to calculate the rest frame of the leptonically decaying top quark. A template fit to the distribution of lepton polar angles in the parent top quark's rest frame is used to extract the fraction of positively polarised top quarks. The full 2011 ATLAS 7 TeV centre of mass energy $pp$ collisions dataset from the LHC ($4.66\, \mathrm{fb}^{-1}$) is used to perform the measurement. The fraction of positive ly polarised top quarks is found to be $f= 0.470 \pm 0.009 {\rm (stat)} ^{+0.023}_{-0.032} {\rm (syst)}$, compatible with the Standard Model prediction of $f = 0.5$.