CP violation in leptonic rare B⁰ₛBs0 decays as a probe of new physics

The decay B⁰ₛ→ μ ⁺μ ⁻ is a key probe for the search of physics beyond the Standard Model. While the current measurements of the corresponding branching ratio agree with the Standard Model within the uncertainties, significant New-Physics effects may still be hiding in B⁰ₛ→ μ ⁺μ ⁻ . In order to reveal them, the observable Aμ μΔ Γ ₛ , which is provided by the decay width difference Δ Γ ₛ of the B⁰ₛ -meson system, plays a central role. We point out that a measurement of a CP-violating observable S_μ μ , which is induced through interference between B⁰ₛ – B̄⁰ₛ mixing and Bₛ→ μ ⁺μ ⁻ decay processes, is essential to obtain the full picture, in particular to establish new scalar contributions and CP-violating phases. We illustrate these findings with future scenarios for the upgrade(s) of the LHC, exploiting also relations which emerge within an effective field theory description of the Standard Model, complemented with New Physics entering significantly beyond the electroweak scale.