An anomalous Wtb coupling at a linear collider

Differential cross sections of secondary particles in a process of top quark pair production and decay into six fermions at a linear collider with an unpolarized and a longitudinally polarized electron beam are computed to the lowest order in the standard model and in the presence of an anomalous Wtb coupling. It is illustrated that the latter has a little impact on the differential cross sections. In particular, it is shown that the angular distribution of a secondary lepton receives practically no contribution from the anomalous Wtb coupling, even if the top quark is produced off shell and the non-double resonance background contributions are taken into account. This finding is in accordance with the decoupling theorem that has been proven in literature [Phys. Lett. B 476 (2000) 87; Phys. Lett. B 529 (2002) 82; Phys. Lett. B 557 (2003) 55; Pramana 54 (2000) 791, hep-ph/0002006] in the narrow top quark width approximation