Evidence for a Mass Dependent Forward-Backward Asymmetry in Top Quark Pair Production

We present a new measurement of the inclusive forward-backward t-tbar production asymmetry and its rapidity and mass dependence. The measurements are performed with 5.3 fb^{-1} of p-pbar collisions at \sqrt{s} = 1.96 TeV, recorded with CDF II at the Fermilab Tevatron. Significant inclusive asymmetries are observed in the laboratory and t-tbar rest frame, and are consistent with CP conservation under interchange of t and tbar. In the t-tbar rest frame, the asymmetry increases with the t-tbar rapidity difference, \Delta(y), and with the invariant mass M_{t-tbar} of the t-tbar system. 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 M_{t-tbar}. For M_{t-tbar} > 450 GeV/c^2, the parton-level asymmetry in the t-tbar rest frame is A^{t-tbar} = 0.475\pm 0.114 compared to a next-to-leading order QCD prediction of 0.088\pm 0.013.