The impact of flavour changing neutral gauge bosons on (B)over-bar → Xsγ

The branching ratio of the rare decay (B) over bar -> X-s gamma provides potentially strong constraints on models beyond the Standard Model. Considering a general scenario with new heavy neutral gauge bosons, present in particular in Z' and gauge flavour models, we point out two new contributions to the (B) over bar -> X-s gamma decay. The fi rst one originates from one- loop diagrams mediated by gauge bosons and heavy exotic quarks with electric charge -1/3. The second contribution stems from the QCD mixing of neutral current- current operators generated by heavy neutral gauge bosons and the dipole operators responsible for the (B) over bar -> X-s gamma decay. The latter mixing is calculated here for the fi rst time. We discuss general sum rules which have to be satisfied in any model of this type. We emphasise that the neutral gauge bosons in question could also significantly affect other fermion radiative decays as well as non-leptonic two-body B decays, epsilon'/epsilon, anomalous (g - 2)(mu) and electric dipole moments.