# Measurement of the $B^0_s\to\mu^+\mu^-$ branching fraction and effective lifetime and search for $B^0\to\mu^+\mu^-$ decays

## TL;DR

This paper reports the first observation of the rare decay $B^0_s o\mu^+\mu^-$ with a measured branching fraction and lifetime, and sets an upper limit on $B^0 o\mu^+\mu^-$, all consistent with the Standard Model.

## Contribution

The paper presents the first observation of $B^0_s o\mu^+\mu^-$ decay in a single experiment, including its branching fraction and effective lifetime, and provides a new upper limit for $B^0 o\mu^+\mu^-$.

## Key findings

- Observation of $B^0_s o\mu^+\mu^-$ with 7.8 sigma significance
- Measured branching fraction ${
m B}(B^0_s o\mu^+\mu^-) = (3.0 imes 10^{-9})$
- First measurement of the $B^0_s o\mu^+\mu^-$ effective lifetime

## Abstract

A search for the rare decays $B^0_s\to\mu^+\mu^-$ and $B^0\to\mu^+\mu^-$ is performed at the LHCb experiment using data collected in $pp$ collisions corresponding to a total integrated luminosity of 4.4 fb$^{-1}$. An excess of $B^0_s\to\mu^+\mu^-$ decays is observed with a significance of 7.8 standard deviations, representing the first observation of this decay in a single experiment. The branching fraction is measured to be ${\cal B}(B^0_s\to\mu^+\mu^-)=\left(3.0\pm 0.6^{+0.3}_{-0.2}\right)\times 10^{-9}$, where the first uncertainty is statistical and the second systematic. The first measurement of the $B^0_s\to\mu^+\mu^-$ effective lifetime, $\tau(B^0_s\to\mu^+\mu^-)=2.04\pm 0.44\pm 0.05$ ps, is reported. No significant excess of $B^0\to\mu^+\mu^-$ decays is found and a 95 % confidence level upper limit, ${\cal B}(B^0\to\mu^+\mu^-)<3.4\times 10^{-10}$, is determined. All results are in agreement with the Standard Model expectations.