Signatures of a flavor changing Z′ boson in Bq → γZ′

Abstract

Rare B meson decays offer an opportunity to probe a light hidden Z′ boson. In this work we explore a new channel Bq→γZ′ (q=d,s) followed by a cascade decay of Z′ into an invisible (neutrino or dark matter) or charged lepton pair ℓ+ℓ− (ℓ=e,μ). The study is based on a simplified effective model where the down quark sector has tiny flavor-changing neutral current couplings with Z′. For the first time, we calculate BR(Bq→γZ′) at the leading power of 1/mb and 1/Eγ. Confronting with the strong constraints from semi-invisible decays of B meson, we find that the branching ratio for Bd→invisible+γ can be larger than its Standard Model background prediction, leaving a large room for new physics, in particular for light dark matter. Additionally, the branching ratio for Bd→e+e−γ can also be sizable when the corresponding flavor violating Z′ coupling to quarks is of the axial-vector type. On the other hand, the predicted branching ratios of Bd→μ+μ−γ and Bs→ℓ+ℓ−γ are severely constrained by the experimental measurements.