Measurement of spin correlation in tt̄ production using a matrix element approach

We determine the fraction of tt̄ events with spin correlation, assuming that the spin of the top quark is either correlated with the spin of the top antiquark as predicted by the standard model or is uncorrelated. For the first time we use a matrix-element-based approach to study tt̄ spin correlation. We use tt̄→W+bW -b̄→+νb-ν̄b̄ final states produced in pp̄ collisions at a center-of-mass energy √s=1.96TeV, where denotes an electron or a muon. The data correspond to an integrated luminosity of 5.4fb-1 and were collected with the D0 detector at the Fermilab Tevatron collider. The result agrees with the standard model prediction. We exclude the hypothesis that the spins of the tt̄ are uncorrelated at the 97.7% C.L.