New Probes of New Physics with Leptonic Rare B Decays

Abstract: Decays of the kind $B^0_{s,d}\to\ell^+\ell^-$ belong to the most favourable processes for probing the flavour structure of the Standard Model, with outstanding sensitivity to new (pseudo)-scalar contributions. While the branching ratio of $B^0_s\to\mu^+\mu^-$ has already been measured at the LHC in the ballpark of the Standard Model expectation, there is still significant room for New-Physics effects. We discuss how these may be revealed in the future super-high precision era of $B$-decay studies by utilising new theoretically clean observables, including CP-violating asymmetries. Another promising decay is $B^0_{s}\to e^+e^-$, which has received little attention in view of its enormously helicity suppressed Standard Model branching ratio, with the most recent experimental upper bound dating back to 2009. Using the current constraints on New Physics from $B^0_s\to\mu^+\mu^-$ as a guideline, we show that the $B^0_s\to e^+e^-$ branching ratio may be hugely enhanced through new (pseudo)-scalar contributions up to the regime of $B^0_{s}\to\mu^+\mu^-$.