NLO QCD corrections to t(t)over-bar-b(b)over-bar production at the LHC: 1. quark-antiquark annihilation

The process pp. t (t) over barb (b) over bar + X represents a very important background reaction to searches at the LHC, in particular to t (t) over barH production where the Higgs boson decays into a b (b) over bar pair. A successful analysis of t (t) over barH at the LHC requires the knowledge of direct t (t) over barb (b) over bar production at next-to-leading order in QCD. We take the first step in this direction upon calculating the next-to-leading-order QCD corrections to the subprocess initiated by q (q) over bar annihilation. We devote an appendix to the general issue of rational terms resulting from ultraviolet or infrared ( soft or collinear) singularities within dimensional regularization. There we show that, for arbitrary processes, in the Feynman gauge, rational terms of infrared origin cancel in truncated one-loop diagrams and result only from trivial self-energy corrections.