Zt, γt and t production at hadron colliders via strong flavour-changing neutral couplings

We consider gq -> Zt, gq -> gamma t and gq -> t production (q = u, c) mediated by strong flavour-changing neutral interactions within an effective operator framework. We provide total cross sections for Tevatron and LHC, showing explicitly that the six processes can be described in full generality in terms of only two parameters (anomalous couplings) for q =u plus two for q = c. In our work we take into account and study in detail the effects of top quark decay. For gamma t, the inclusion of the top quark decay in the matrix element reveals an striking result: the largest contribution to the final state, e.g. gamma l nu b with l = e, mu, tau, does not result from gq -> gamma t -> gamma l nu b but from on-shell gq -> t production with t -> gamma l nu b, being the photon radiated off the top decay products. This contribution, missed in previous literature, increases the signal cross sections by factors ranging between 3 and 6.5. (C) 2010 Elsevier B.V. All rights reserved.