Measurement of spin correlation between top and antitop quarks produced in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV

We present a measurement of the correlation between the spins of t and tbar quarks produced in proton-antiproton collisions at the Tevatron Collider at a center-of-mass energy of 1.96 TeV. We apply a matrix element technique to dilepton and single-lepton+jets final states in data accumulated with the D0 detector that correspond to an integrated luminosity of 9.7 fb${}^{-1}$. The measured value of the correlation coefficient in the off-diagonal basis, $O_{off}=0.89\pm 0.22$ (stat + syst), is in agreement with the standard model prediction, and represents evidence for a top-antitop quark spin correlation difference from zero at a level of 4.2 standard deviations.