Studies of the top quark polarisation in trileptonic tZq final states with ATLAS

To find the top quark's polarisation, the top quark is reconstructed from the trileptonic final state. In the top quark's rest frame the angular distribution of the lepton from the top decay with respect to the top spin can be measured. This distribution is then unfolded to find the distribution on truth-level. Unfolding is a method to find the true distribution of an observable from a distribution as measured in the detector with limited resolution and statistical effects. In the process, the detector effects are simulated to find the truth-distribution with the highest likelihood of being measured as the reconstructed distribution. The top reconstruction yields top candidates with high uncertainties that often do not match the true top quark in the event. Additional uncertainties arise in the unfolding due to these unmatched top candidates. As a result, the found unfolding procedure does not have sufficient accuracy to get meaningful results from measured data.