Internal down quark's flavour changing contribution to the effective tcZ vertex

The effective vertex may be influenced by tree vertex formed by a mixing with heavy exotic isosinglet down type quarks. To study that the electroweak penguin diagrams involving one insertion of the vertex have been considered and we have calculated the contribution arising out of those diagrams using the Fourth Generation CKM matrix elements; also the applicability of the generalized GIM mechanism is considered. The additional effects of the heavy isosinglets are compared with the effects of exotic heavy isodoublets appearing in multi-generational models. We see that in the effective vertex amplitude, the down flavour changing contribution interferes constructively with the one loop penguin diagrams and experimental data may put constraint on the fourth generation down quark mass. Received: 21 March 1997 / Revised version: 19 August 1997 / Published online: 20 February 1998