Two-parton contribution to the heavy-quark forward–backward asymmetry in NNLO QCD

Forward–backward asymmetries, AFBQ, are important observables for the determination of the neutral-current couplings of heavy quarks in inclusive heavy quark production, e+e−→γ∗,Z∗→Q+Xe+e−→γ∗,Z∗→Q+X. In view of the measurement perspectives on AFBQ at a future linear collider, precise predictions of AFBQ are required for massive quarks. We compute the contribution of the QQ¯ final state to AFBQ to order αs2 in the QCD coupling. We provide general formulae, and we show that this contribution to AFBQ is infrared-finite. We evaluate these two-parton contributions for b and c quarks on and near the Z resonance, and for t   quarks above threshold. Moreover, near the tt¯ threshold we obtain, by expanding in the heavy-quark velocity β  , an expression for AFBtt¯ to order αs2 and NNLL in β. This quantity is equal, to this order in β  , to the complete forward–backward asymmetry AFBt.