Z-Z' Mixing and Z-Mediated FCNCs in SU(3)_C x SU(3)_L x U(1)_X Models

Abstract

Most of the existing analyses of FCNC processes in the 331 models, based on the gauge group SU(3)_C x SU(3)_L x U(1)_X, take only into account tree-level exchanges of a new heavy neutral gauge boson Z'. However due to the Z-Z' mixing also corresponding contributions from Z boson are present that are usually neglected. We calculate the impact of these contributions on Delta F=2 processes and rare K, B_s and B_d decays for different values of a parameter beta, which distinguishes between various 331 models and for different fermion representations under the SU(3)_L group. We find a general expression for the Z-Z' mixing in terms beta, M_Z, M_Z' and tan(bar{beta}), familiar from 2 Higgs Doublet models, that differs from the one quoted in the literature. We study in particular the models with beta=+-n/sqrt{3} with n=1,2 which have recently been investigated by us in the context of new data on B_{s,d}->mu^+ mu^- and B_d->K^*(K)mu^+ mu^-. We find that these new contributions can indeed be neglected in the case of Delta F=2 transitions and decays, like B_d->K^*mu^+mu^-, where they are suppressed by the small vectorial Z coupling to charged leptons. However the contributions of tree-level Z exchanges to decays sensitive to axial-vector couplings, like B_{s,d}->mu^+ mu^- and B_d->K mu^+ mu^-, and those with neutrinos in the final state, like b->s nu bar{nu} transitions, K^+->pi^+ nu bar{nu} and K_L->pi^0 nu bar{nu} cannot be generally neglected with size of Z contributions depending on beta, tan(bar{beta}) and M_Z'. We analyze for the first time the ratio epsilon'/epsilon in these models including both Z' and Z contributions. Our analysis of electroweak precision observables within 331 models demonstrates transparently that the interplay of NP effects in electroweak precision observables and those in flavour observables could allow in the future to identify the favourite 331 model.