Measurements of Top Quark Pair Spin Correlations in pp Collisions at a centre-of-mass energy of 13 TeV with CMS

Measurements of the top quark polarization and top quark pair spin correlations are presented using events containing two leptons produced in proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the CMS experiment at the CERN LHC during the full Run II and correspond to an integrated luminosity of 137/fb. A set of multi-differential cross section distributions are measured for each of the independent coefficients of the top-spin dependent parts of the production density matrix for the \mathrm{t \bar t}$ system, unfolded to particle and parton-level, and compared to standard model predictions from Monte Carlo simulations with next-to-leading-order (NLO) accuracy in quantum chromodynamics (QCD) at matrix-element level interfaced to parton-shower simulations. The spin coefficients are extracted from the measured distributions and compared to values from fixed order calculations at NLO in QCD with electroweak corrections. The measurement provides a precision test of the Standard Model of particle physics and probes for New Physics.