Measurements of ttbar Spin Correlations and Top-Quark Polarization Using Dilepton Final States in pp Collisions at sqrt(s) = 8 TeV

Spin correlations and polarization are measured in top quark-antiquark pairs produced in $\mathrm{pp}$ collisions at the LHC at $\sqrt{s}=8\:\mathrm{TeV}$. The data correspond to an integrated luminosity of $19.5\:\mathrm{fb^{-1}}$ collected with the CMS detector. The measurements are performed using events with two oppositely charged leptons (electrons or muons) and two or more jets, where at least one of the jets is identified as originating from a b quark. The spin correlations and polarization are measured from the angular distributions of the two selected leptons, unfolded to the parton level, both inclusively and differentially with respect to the invariant mass, rapidity, and transverse momentum of the $\mathrm{t\bar{t}}$ system. All measurements are found to be in agreement with predictions of the standard model of particle physics. A search for new physics in the form of a top quark chromo-magnetic anomalous coupling is performed, and the measured value of the real part of the chomo-magnetic coupling $Re(\hat{\mu}_t)$ is found to be $-0.013 \pm 0.032$ with a corresponding $95\%$ confidence interval of $-0.050< Re(\hat{\mu}_t) < 0.076$.