$t\bar{t}$ spin correlation at the LHC

The ATLAS collaboration has performed a study of spin correlation in ttbar production from proton-proton collisions at the Large Hadron Collier (LHC) at a centre-of-mass energy of 7 TeV using 0.70 fb-1 of data. Candidate events are selected in the dilepton topology with large missing transverse energy and at least two jets. The difference in azimuthal angle between the two charged leptons is compared to the expected distributions in the Standard Model, and to the case where the top quarks are produced with uncorrelated spin. Using the helicity basis as the quantisation axis, the strength of the spin correlation between the top and antitop quark is measured to be A_helicity = 0.34 +0.15-0.11, which is in agreement with the NLO Standard Model prediction.