# Does the present data on B_s - bar B_s mixing rule out a large enhancement in the branching ratio of B_s --> mu+ mu- ?

## TL;DR

This paper analyzes how recent B_s - bar B_s mixing data limits the potential for new physics to significantly enhance the B_s --> mu+ mu- decay rate, showing only modest possible increases over the Standard Model prediction.

## Contribution

It demonstrates that combined constraints from B_s - bar B_s mixing, K0 - bar K0 mixing, and K_L --> mu+ mu- decay severely limit scalar-pseudoscalar new physics contributions to B_s --> mu+ mu-.

## Key findings

- New physics can at most double the B_s --> mu+ mu- branching ratio.
- Data strongly constrains scalar-pseudoscalar contributions.
- Standard Model predictions remain robust against these new physics scenarios.

## Abstract

In this letter, we consider the constraints imposed by the recent measurement of B_s - bar B_s mixing on the new physics contribution to the rare decay B_s --> mu+ mu-. New physics in the form vector and axial-vector couplings is already severely constrained by the data on B --> (K,K*) mu+ mu-. Here, we show that B_s - bar B_s mixing data, together with the data on K0 - bar K0 mixing and K_L --> mu+ mu- decay rate, strongly constrain the scalar-pseudoscalar contribution to B_s --> mu+ mu-. We conclude that new physics can at best lead to a factor of 2 increase in the branching ratio of B_s --> mu+ mu- compared to its Standard Model expectation.