Impact of family-non-universal Z′ boson on pure annihilation Bs→π+π- and Bd→K+K- decays

We study the Bs→π+π- and Bd→K+K- decays in the standard model and the family-non-universal Z′ model. Since none of the quarks in the final states is the same as the initial quark, these decay modes can occur only via power-suppressed annihilation diagrams. Despite the consistency of the standard model prediction with the available data, room remains for a light Z′ boson. Taking into account the Z′ contribution, we find that theoretical results for the branching fractions can better accommodate the data. With the relevant data, we also derive a constraint on the parameter space for the Z′ . Moreover, for Bd→K+K- , both the direct and the mixing-induced CP asymmetry are sensitive to the couplings between Z′ and fermions in the parameter spaces constrained by the data. The measurements at future experimental facilities, including the LHCb, Belle-II, and the proposed high energy e+e- collider, will provide us useful hints for direct searching for the light Z′ boson