Measurement of spin correlation in $t \bar{t}$ production from $pp$ collisions at $\sqrt{s}=7$ TeV using the ATLAS detector

The ATLAS collaboration has performed a study of spin correlation in $t\bar{t}$ production from proton-proton collisions at a centre-of-mass energy of 7 TeV using 0.70 fb$^{-1}$ of data. The lifetime of the top quark is approximately $5 \times 10^{-25}$ s, implying that the top quark decays before there is time for its spin to be flipped by the strong interaction. This allows the spin information to be accessed via the angular distributions of its decay products. 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 $A_{helicity} = 0.34_{-0.11}^{+0.15}$, which is in agreement with the NLO Standard Model prediction.