Next-to-leading order QCD corrections to top quark spin correlations at hadron colliders:: the reactions gg → t(t)over-bar(g) and gq((q)over-bar) → t(t)over-barq→((q)over-bar)

We have computed the cross section for t (t) over bar production by gluon-gluon fusion at next-to-leading order (NLO) in the QCD coupling. keeping the full dependence on the t (t) over bar spins. Furthermore, we have determined to the same order the spin dependent cross sections for the processes g + q((q) over bar) --> t (t) over bar + q((q) over bar) Together with our previous results [Phys. Lett. B 483 (2000) 99] for q + (q) over bar --> t (t) over bar (g) these results allow for predictions, at NLO QCD, of the hadronic production of t (t) over bar pairs in a general spin configuration. As an application we have determined the degree of correlation of the t and (t) over bar spins at NLO. using various spin quantisation axes. (C) 2001 Elsevier Science B.V. All rights reserved.