How large can the branching ratio of $B_s\to {\tau }^{+}{\tau }^{-}$ be ?

Abstract

Motivated by the large like-sign dimuon charge asymmetry observed recently, whose explanation would require an enhanced decay rate of $B_s\to {\tau }^{+}{\tau }^{-}$, 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 $b\to s{\tau }^{+}{\tau }^{-}$ modes, as well as measurements of the mass difference, width difference and CP-violating phase in the $B_s$-${\bar{B}}_s$ 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 model with a scalar leptoquark cannot increase the branching ratio to a per cent level. However, an enhancement up to 5% is possible in the model with an extremely light $Z^{\prime }$ with flavor-dependent interactions, even after all the couplings are taken to be perturbative. This however cannot account for the dimuon anomaly completely by itself.

Comments: Typos corrected, some discussions added, accepted for publication in Phys.Rev.D