Effects of a Family Non-universal Z′ Boson in B̄d → μ+μ− and B− → π−μ+μ−, ρ−μ+μ− Decays

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Motivated by the recently updated experimental measurement on B̄d → μ+μ− and B− → π−μ+μ− decays by CDF and Belle collaborations, we revisit these decays, as well as B− → ρ−μ+μ− decay, within the Standard Model, and evaluate the effects of a family non-universal Z′ boson. Under the constraint from ℬ(B− → π−μ+μ−), we find the ranges of the Z′ couplings SLRμμ < −5.2 × 10−2 or DLRμμ < −8.1 × 10−2 are excluded. Within the allowed Z′ parameters spaces, comparing with the SM predictions, we find that ℬ(B̄d → μ+μ−), ℬ(B− → π−μ+μ−), and ℬ(B− → ρ−μ+μ−) could be enhanced by a factor about 226%, 245%, and 254%, respectively, by Z′ contributions. However, they are hardly to be reduced. Furthermore, the zero crossing in the normalized forward-backward asymmetry spectrum of B− → ρ−μ+μ− decay at low dimuon mass always exists.