Measurement of the Correlation Between the Polar Angles of Leptons from Top Quark Decays in the Helicity Basis at √s̅ = 7 TeV using the ATLAS Detector

A measurement of the correlations between the polar angles of leptons from the decay of pair-produced t and t̅ quarks in the helicity basis is reported, using proton-proton collision data collected by the ATLAS detector at the LHC. The dataset corresponds to an integrated luminosity of 4.6  fb⁻¹ at a center-of-mass energy of √s̅ = 7  TeV collected during 2011. Candidate events are selected in the dilepton topology with large missing transverse momentum and at least two jets. The angles θ₁ and θ₂ between the charged leptons and the direction of motion of the parent quarks in the tt̅ rest frame are sensitive to the spin information, and the distribution of cosθ₁ ⋅ cosθ₂ is sensitive to the spin correlation between the t and t̅ quarks. The distribution is unfolded to parton level and compared to the next-to-leading order prediction. A good agreement is observed.