Measurement of leptonic asymmetries and top-quark polarization in tt-bar production

Abstract

We present measurements of lepton (ℓ) angular distributions in top-quark (t) pair production and tt-bar→W+bW−b-bar→ℓ+νbℓ−ν-b-ar b-bar decays produced in pp-bar collisions at a center-of-mass energy of s√=1.96 TeV, where ℓ is an electron or muon. Using data corresponding to an integrated luminosity of 5.4 fb(−1), collected with the D0 detector at the Fermilab Tevatron Collider, we measure for the first time the leptonic forward-backward asymmetry in dilepton final states and obtain A(ℓ)(FB)=(5.8±5.1(stat)±1.3(syst))%, corrected for detector acceptance. This is compared to the standard model prediction of A(ℓ)(FB)(predicted)=(4.7±0.1)%. A deviation from the standard model prediction as previously seen in a D0 measurement based on the analysis of the ℓ+jets final state is not observed in these dilepton final states. The two results differ from each other by 1.4 standard deviations and are combined to obtain A(ℓ)(FB)=(11.8±3.2)%. Furthermore, we present a first study of the top-quark polarization.