Asymmetries in the angular distributions in the tt̄ system

Asymmetries in the pair production of top–antitop quarks in hadronic collisions have gained a lot of attention because the size of the effect exceeded the predictions based on the Standard Model (SM). To understand the basis for these predictions as well as the importance of this measurement and its experimental subtleties, it is instructive to review its historic predecessor — the asymmetry of fermion pair production in electron–positron collisions. One of the first indications of the existence of the Z-boson was an observation of the forward–backward asymmetry in the production of muon pairs in ee collisions observed at PETRA at the center-of-mass energy of 30 GeV/c, significantly below the Z-boson mass. The observed negative asymmetry of−8% indicated that the mediator of the electroweak interaction has a nonzero axial coupling to fermions, or in other words, its coupling to left-handed fermions is not equal to that to right-handed fermions. By analogy, should a strong interaction be left–right asymmetric, as was hypothesized in some models, it would be manifested in the forward–backward asymmetry of quark– antiquark production in pp̄ collisions. Since the top quark is the heaviest known quark, measuring asymmetry in top-pair production probes the nature of strong interactions at the highest energy scale. The raw negative measured asymmetry in muon pair production was corrected for a small positive asymmetry expected purely due to the next-to-leading (NLO) order quantum electrodynamics (QED) effect, which is an electromagnetic