Measurement of the forward–backward asymmetry of top-quark and antiquark pairs using the full CDF Run II data set

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Abstract

We measure the forward--backward asymmetry of the production of top-quark and antiquark pairs in proton-antiproton collisions at center-of-mass energy $\sqrt{s}=1.96\text{ }\text{ }\mathrm{TeV}$ using the full data set collected by the Collider Detector at Fermilab (CDF) in Tevatron Run II corresponding to an integrated luminosity of $9.1\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$. The asymmetry is characterized by the rapidity difference between top quarks and antiquarks ($\mathrm{\ensuremath{\Delta}}y$) and measured in the final state with two charged leptons (electrons and muons). The inclusive asymmetry, corrected to the entire phase space at parton level, is measured to be ${A}_{\mathrm{FB}}^{t\overline{t}}=0.12\ifmmode\pm\else\textpm\fi{}0.13$, consistent with the expectations from the standard model (SM) and previous CDF results in the final state with a single charged lepton. The combination of the CDF measurements of the inclusive ${A}_{\mathrm{FB}}^{t\overline{t}}$ in both final states yields ${A}_{\mathrm{FB}}^{t\overline{t}}=0.160\ifmmode\pm\else\textpm\fi{}0.045$, which is consistent with the SM predictions. We also measure the differential asymmetry as a function of $\mathrm{\ensuremath{\Delta}}y$. A linear fit to ${A}_{\mathrm{FB}}^{t\overline{t}}(|\mathrm{\ensuremath{\Delta}}y|)$, assuming zero asymmetry at $\mathrm{\ensuremath{\Delta}}y=0$, yields a slope of $\ensuremath{\alpha}=0.14\ifmmode\pm\else\textpm\fi{}0.15$, consistent with the SM prediction and the previous CDF determination in the final state with a single charged lepton. The combined slope of ${A}_{\mathrm{FB}}^{t\overline{t}}(|\mathrm{\ensuremath{\Delta}}y|)$ in the two final states is $\ensuremath{\alpha}=0.227\ifmmode\pm\else\textpm\fi{}0.057$, which is $2.0\ensuremath{\sigma}$ larger than the SM prediction.

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