Evidence for a mass dependent forward-backward asymmetry in top quark pair production

We present a new measurement of the inclusive forward-backward tt[over-bar] production asymmetry and its rapidity and mass dependence. The measurements are performed with data corresponding to an integrated luminosity of 5.3 fb-1 of pp[over-bar ]collisions at [sqrt]s=1.96 TeV, recorded with the CDF-II Detector at the Fermilab Tevatron. Significant inclusive asymmetries are observed in both the laboratory frame and the tt[over-bar] rest frame, and in both cases are found to be consistent with CP conservation under interchange of t and t[over-bar] . In the tt[over-bar] rest frame, the asymmetry is observed to increase with the tt[over-bar] rapidity difference, delta y, and with the invariant mass Mtt[over-bar] of the tt[over-bar] system. Fully corrected parton-level asymmetries are derived in two regions of each variable, and the asymmetry is found to be most significant at large delta y and Mtt[over-bar] . For Mtt[over-bar] ≥450 GeV/c2, the parton-level asymmetry in the tt[over-bar] rest frame is Att[over-bar] =0.475±0.114 compared to a next-to-leading order QCD prediction of 0.088±0.013.United States. Dept. of EnergyNational Science Foundation (U.S.)Alfred P. Sloan Foundatio