Lepton-flavor violating B decays in generic Z′ models

LHCb has reported deviations from the Standard Model in b -> s mu(+)mu(-) transitions for which a new neutral gauge boson is a prime candidate for an explanation. As this gauge boson has to couple in a flavor nonuniversal way to muons and electrons in order to explain R-K, it is interesting to examine the possibility that also lepton flavor is violated, especially in the light of the CMS excess in h -> tau(+/-)mu(+/-). In this article, we investigate the perspectives to discover the lepton-flavor violating modes B -> K-(*())tau(+/-)mu(+/-), B-s -> tau(+/-)mu(+/-) and B -> K-(*())mu(+/-)e(+/-), B-s -> mu(+/-)e(+/-). For this purpose we consider a simplified model in which new-physics effects originate from an additional neutral gauge boson (Z') with generic couplings to quarks and leptons. The constraints from tau -> 3 mu, tau -> mu nu(nu) over bar, mu -> e gamma, g(mu) - 2, semileptonic b -> s mu(+)mu(-) decays, B -> K-(*())nu(nu) over bar and B-s-(B) over bar (s) mixing are examined. From these decays, we determine upper bounds on the decay rates of lepton-flavor violating B decays. Br(B -> K nu(nu) over bar) limits the branching ratios of lepton-flavor violating B decays to be smaller than 8 x 10(-5) (2 x 10(-5)) for vectorial (left-handed) lepton couplings. However, much stronger bounds can be obtained by a combined analysis of B-s-(B) over bar (s), t -> 3 mu, tau -> mu nu(nu) over bar and other rare decays. The bounds depend on the amount of fine-tuning among the contributions to B-s-(B) over bars mixing. Allowing for a fine-tuning at the percent level we find upper bounds of the order of 10(-6) for branching ratios into tau mu final states, while B-s -> mu(+/-)e(+/-) is strongly suppressed and only B -> K-(*())mu(+/-)e(+/-) can be experimentally accessible (with a branching ratio of order 10(-7)).