Enhancing $B_s \to e^+ e^-$ to an Observable Level in the Two-Higgs-Doublet Model

As a result of the helicity suppression effect, within the Standard Model the rare decay channel $B_s \to e^+ e^-$ has a decay probability which is five orders of magnitude below current experimental limits. Thus, any observation of this channel within the current or forthcoming experiments will give unambiguous evidence of Physics Beyond the Standard Model. In this work, we present for the first time a New Physics scenario in which the branching fraction $\bar{\mathcal{B}}r(B_s \to e^+ e^-)$ is enhanced up to values which saturate the current experimental bounds. More concretely, we study the general Two-Higgs-Doublet Model (2HDM) with a pseudoscalar coupling to electrons unsuppressed by the electron mass. Furthermore, we demonstrate how this scenario can arise from a UV-complete theory of quark-lepton unification that can live at a low scale. This latter step allows us to establish correlations between $B_s \to e^+ e^-$ and the lepton-flavour-violating decays $\tau^{-}\to \mu^{-}e^+ e^-$ and $\tau\to \mu \gamma$.