Next-to-leading order QCD corrections to top quark spin correlations at hadron colliders: the reactions $g g \to t {\bar t} (g)$ and $g q ({\bar q}) \to t {\bar t} q ({\bar q})$

We have computed the cross section for $t\bar t$ production by gluon-gluon fusion at next-to-leading order (NLO) in the QCD coupling, keeping the full dependence on the $t\bar t$ spins. Furthermore we have determined to the same order the spin dependent cross sections for the processes $g + q ({\bar q})\to t {\bar t} + q ({\bar q})$. Together with our previous results for $q + {\bar q} \to t {\bar t} (g)$ these results allow for predictions, at NLO QCD, of the hadronic production of $t\bar t$ pairs in a general spin configuration. As an application we have determined the degree of correlation of the $t$ and $\bar t$ spins at NLO, using various spin quantisation axes.