# Improved limit on the branching fraction of the rare decay $K^0_{\scriptscriptstyle S}\to\mu^+\mu^-$

## TL;DR

This paper reports a new, more stringent upper limit on the rare decay $K^0_S o \mu^+\mu^-$, using LHCb data, significantly improving previous constraints and supporting the background-only hypothesis.

## Contribution

The study provides the most restrictive upper limit to date on the branching fraction of $K^0_S o \mu^+\mu^-$ decay, based on 3 fb$^{-1}$ of LHCb data at 7 and 8 TeV.

## Key findings

- No significant signal observed, consistent with background.
- Upper limit on branching fraction set at $< 0.8 imes 10^{-9}$ (90\\%).
- Improved the previous limit by an order of magnitude.

## Abstract

A search for the decay $K^0_{\scriptscriptstyle S}\to\mu^+\mu^-$ is performed, based on a data sample of proton-proton collisions corresponding to an integrated luminosity of 3 fb$^{-1}$, collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The observed yield is consistent with the background-only hypothesis, yielding a limit on the branching fraction of ${\cal B}(K^0_{\scriptscriptstyle S}\to\mu^+\mu^-) < 0.8~(1.0) \times 10^{-9}$ at $90\%~(95\%)$ confidence level. This result improves the previous upper limit on the branching fraction by an order of magnitude.