The t->WZb decay in the Standard Model: A Critical Reanalysis
  We compute the t->WZb decay rate, in the Standard Model, at the leading order in perturbation theory, with special attention to the effects of the finite widths of the W and Z bosons. These effects are extremely important, since the t->WZb decay occurs near its kinematical threshold. They increase the value of the decay rate by orders of magnitude near threshold or allow it below the nominal threshold. We discuss a procedure to take into account the finite-width effects and compare the results with previous studies of this decay. Within the Standard Model, for a top quark mass in the range between 170 and 180 GeV, we find BR(t->WZb) ~ 2 x 10^{-6}, which makes the observation at the LHC very difficult if at all possible. 
G. Altarelli, L. Conti and V. Lubicz