How large can the branching ratio of B_s→τ⁺τ⁻ be?

Abstract

Motivated by the large like-sign dimuon charge asymmetry observed recently, whose explanation would require an enhanced decay rate of B_s→τ⁺τ⁻, we explore how large a branching ratio of this decay mode is allowed by the present constraints. We use bounds from the lifetimes of B_d and B_s, constraints from the branching ratios of related b→sτ⁺τ⁻ modes, as well as measurements of the mass difference, width difference and CP-violating phase in the B_s−B̅_s system. Using an effective field theory approach, we show that a branching ratio as high as 15% may be allowed while being consistent with the above constraints. The measurement of this decay, therefore, may be within the reach of current experiments and can point toward a specific class of new physics models. We also explore the possible enhancement of this decay in models with leptoquarks and Z′ and find that, in the latter case, the branching ratio may be as much as 5%, which can alleviate the dimuon anomaly to some extent.