Spin measurements in top-quark events at the LHC

Abstract

Measurements of polarisation and spin correlations are presented in events with top quarks produced in pp collisions at the LHC. The data correspond to integrated luminosities of $$5 fb^{-1}$$ at $$\sqrt{s}$$ = 7 TeV and 20 $$fb^{-1}$$ at $$\sqrt{s}$$ = 8 TeV collected with the ATLAS and CMS detectors. The top quark polarization is measured in both single top quark production in the t-channel and $$t\bar{t}$$ pair-production, from the angular distributions of charged leptons in the rest frame of their parent top quark. The spin correlations are measured in $$t\bar{t}$$ events using various angular distributions of the decay products. The measurements are made using both template fitting methods and by unfolding the distributions to the parton-level, where differential measurements with respect to the invariant mass, rapidity, and transverse momentum of the $$t\bar{t}$$ system are also made. The spin correlation measurements are used to search for new physics in the form of a light top squark or an anomalous top quark chromo-magnetic dipole moment. All measurements are found to be in agreement with predictions of the standard model.