TOP PAIR PRODUCTION AT HADRON COLLIDERS IN NONMINIMAL STANDARD MODELS

The cross section of top pair production in hadronic collisions to O(a lpha alpha(s)(2)) is calculated within the General 2-Higgs Doublet Mod el and the Minimal Supersymmetric Standard Model. At the parton level the O(alpha) one-loop corrections to the main production mechanisms, q (q) over bar --> t (t) over bar and gg --> t (t) over bar, significan tly modify the Born cross sections: in the threshold region root (S) o ver cap greater than or similar to 2m(t) they are enhanced up to 50% a nd with increasing cm energy root (S) over cap, they can be reduced by up to the same order of magnitude. In a wide range of the parameter s pace of the models under consideration the observable hadronic cross s ections for top pair production at the Tevatron p (p) over bar --> t(t ) over barX$ and at the LHC pp --> t(t) over barX$ are typically reduc ed by several percent (less than or similar to 10%) compared to the lo west order result. In special regions of the parameter space, that is in the vicinity of the threshold for the top quark decay t --> b H+; < (t chi)over tilde>(0), the radiative corrections are considerably enha nced, comparable in size to QCD effects. (C) 1998 Elsevier Science B.V .