Measurement of the top quark polarization and $\mathrm{t \bar t}$ spin correlations in dilepton final states at $\sqrt{s}=13~\mathrm{TeV}$

Measurements of the top quark polarization and top quark pair ($\mathrm{t \bar t}$) spin correlations are presented using events containing two leptons produced in proton-proton collisions at a center-of-mass energy of $\sqrt{s}=13~\mathrm{TeV}$. The data were recorded by the CMS experiment at the CERN LHC in 2016 and correspond to an integrated luminosity of $35.9~\mathrm{fb}^{-1}$. A set of parton-level normalized differential cross sections, sensitive to each of the independent coefficients of the spin-dependent parts of the $\mathrm{t \bar t}$ production density matrix, is measured for the first time at $\sqrt{s}=13~\mathrm{TeV}$. The measured distributions and extracted coefficients are compared with standard model predictions from Monte Carlo simulations with next-to-leading-order (NLO) accuracy in quantum chromodynamics (QCD) and from NLO QCD calculations including weak and mixed QCD-weak corrections. All measurements are found to be consistent with the expectations of the standard model, and are used in a simultaneous fit to constrain the anomalous chromomagnetic dipole moment of the top quark to $-0.07 < C_\text{tG}/\Lambda^{2} < 0.16~\mathrm{TeV}^{-2}$ at $95\%$ confidence level.