Lepton-flavor violating B decays in generic Z′ models

Abstract

LHCb has reported deviations from the Standard Model in b→sμ+μ- 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 RK, it is interesting to examine the possibility that also lepton flavor is violated, especially in the light of the CMS excess in h→τ±μ. In this article, we investigate the perspectives to discover the lepton-flavor violating modes B→K(∗)τ±μ, Bs→τ±μ and B→K(∗)μ±e, Bs→μ±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 τ→3μ, τ→μνν¯, μ→eγ, gμ-2, semileptonic b→sμ+μ- decays, B→K(∗)νν¯ and Bs-B¯s mixing are examined. From these decays, we determine upper bounds on the decay rates of lepton-flavor violating B decays. Br(B→Kνν¯) limits the branching ratios of lepton-flavor violating B decays to be smaller than 8×10-5(2×10-5) for vectorial (left-handed) lepton couplings. However, much stronger bounds can be obtained by a combined analysis of Bs-B¯s, τ→3μ, τ→μνν¯ and other rare decays. The bounds depend on the amount of fine-tuning among the contributions to Bs-B¯s mixing. Allowing for a fine-tuning at the percent level we find upper bounds of the order of 10-6 for branching ratios into τμ final states, while Bs→μ±e is strongly suppressed and only B→K(∗)μ±e can be experimentally accessible (with a branching ratio of order 10-7).