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

We present a new measurement of the inclusive forward-backward $t\overline{t}$ production asymmetry and its rapidity and mass dependence. The measurements are performed with data corresponding to an integrated luminosity of $5.3{\mathrm{fb}}^{-1}$ of $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$, recorded with the CDF-II Detector at the Fermilab Tevatron. Significant inclusive asymmetries are observed in both the laboratory frame and the $t\overline{t}$ rest frame, and in both cases are found to be consistent with $CP$ conservation under interchange of $t$ and $\overline{t}$. In the $t\overline{t}$ rest frame, the asymmetry is observed to increase with the $t\overline{t}$ rapidity difference, $\Delta y$, and with the invariant mass $M_{t\overline{t}}$ of the $t\overline{t}$ 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 $M_{t\overline{t}}$. For $M_{t\overline{t}}\ge 450\mathrm{GeV}/c^2$, the parton-level asymmetry in the $t\overline{t}$ rest frame is $A^{t\overline{t}}=0.475\pm 0.114$ compared to a next-to-leading order QCD prediction of $0.088\pm 0.013$.