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

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 √ s = 1.96 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 fb−1. The asymmetry is characterized by the rapidity difference between top quarks and antiquarks (∆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 tt¯ FB = 0.12 ± 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 tt¯ FB in both final states yields A tt¯ FB = 0.160 ± 0.045, which is consistent with the SM predictions. We also measure the differential asymmetry as a function of ∆y. A linear fit to A tt¯FB(|∆y|), assuming zero asymmetry at ∆y = 0, yields a slope of α = 0.14 ± 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 tt¯ FB(|∆y|) in the two final states is α = 0.227 ± 0.057, which is 2.0σ larger than the SM prediction.