ar X iv : h ep - p h / 00 05 28 3 v 1 2 9 M ay 2 00 0 1 Calculational techniques ( not only ) for single top production

The top quark, discovered 1995 at Fermilab, is special among the quarks due to its large mass: Its lifetime is shorter than the characteristic hadronization time scale and therefore top bound states do not have time to form. From a calculational point of view this allows an immediate application of perturbation theory to top physics. Up to now top quarks have only been produced in pairs through the strong interaction. With the upcoming Run II at the Tevatron and later with LHC one expects to produce a top quark also through an electroweak Wtb-vertex. The principal production mechanisms for single top production are flavour excitation, W-gluon fusion, s-channel production and associated Wproduction. Some representative Feynman diagrams are shown in fig. 1. Single top production will be an essential input for a direct measurement of the CKM matrix element Vtb. In addition, a discovery of non-standard charged-current top couplings might give a hint on new physics. Since the electroweak decay of the top quark proceeds so rapidly before any hadronization effects can take place, the decay products of the top quark are correlated with the top quark spin. Of particular importance will be the semileptonic decay t → bl̄ν, since a detector signal corresponding to hadronic top decays will suffer from large QCD backgrounds. Single top production, in which a top quark is produced through a left-handed interaction, offers therefore an opportunity for polarisation studies. Due to the short lifetime of the top quark, production and decay of a top quark should be considered together and the releq q