In pursuit of new physics with Bs,d0\u2009\u2192\u2009\u2113+\u2113\u2212

Abstract

Leptonic rare decays of Bs,d0 mesons offer a powerful tool to search for physics beyond the Standard Model. The Bs0 → μ+μ− decay has been observed at the Large Hadron Collider and the first measurement of the effective lifetime of this channel was presented, in accordance with the Standard Model. On the other hand, Bs0 → τ+τ− and Bs0 → e+e− have received considerably less attention: while LHCb has recently reported a first upper limit of 6.8 × 10−3 (95% C.L.) for the Bs0 → τ+τ− branching ratio, the upper bound 2.8 × 10−7 (90% C.L.) for the branching ratio of Bs0 → e+e− was reported by CDF back in 2009. We discuss the current status of the interpretation of the measurement of Bs0 → μ+μ−, and explore the space for New-Physics effects in the other Bs,d0 → ℓ+ℓ− decays in a scenario assuming flavour-universal Wilson coefficients of the relevant four-fermion operators. While the New-Physics effects are then strongly suppressed by the ratio mμ/mτ of the lepton masses in Bs0 → τ+τ−, they are hugely enhanced by mμ/me in Bs0 → e+e− and may result in a Bs0 → e+e− branching ratio as large as about 5 times the one of Bs0 → μ+μ−, which is about a factor of 20 below the CDF bound; a similar feature arises in Bd0 → e+e−. Consequently, it would be most interesting to search for the Bs,d0 → e+e− channels at the LHC and Belle II, which may result in an unambiguous signal for physics beyond the Standard Model.