Two-Loop Fermionic Corrections to Heavy-Quark Pair Production: the   Quark-Antiquark Channel
  We evaluate the fermionic two-loop QCD corrections to the heavy-quark pair production process in the quark-antiquark channel. We obtain analytic results which are valid for any value of the Mandelstam invariants s and t, and of the heavy quark mass m. Our findings confirm previous results for the analytic evaluation in the small-mass limit and numerical results for the exact amplitude. We furthermore provide the expansion of the two-loop amplitude at the production threshold s ~ 4m^2. 
R. Bonciani, A. Ferroglia, T. Gehrmann, D. Maitre, and C. Studerus