# The branching fraction and effective lifetime of $B_{(s)}^{0} \rightarrow \mu^+ \mu^-$ at LHCb with Run 1 and Run 2 data

## TL;DR

This paper reports the first observation of the decay $B_{s}^0
ightarrow ^+ ^-$ by LHCb using Run 1 and 2 data, measures its effective lifetime, and finds results consistent with the Standard Model, constraining new physics.

## Contribution

First observation of $B_{s}^0
ightarrow ^+ ^-$ decay by a single experiment and the first measurement of its effective lifetime using combined Run 1 and 2 data.

## Key findings

- Decay observed for the first time by a single experiment.
- Results are consistent with the Standard Model.
- No significant excess of $B^0
ightarrow ^+ ^-$ decays found.

## Abstract

After Run 1 of the LHC, global fits to $b \rightarrow s \ell \ell$ observables show a deviation from the Standard Model (SM) with a significance of $\sim$ 4 standard devations. An example of a $b \rightarrow s \ell \ell$ process is the decay of a $B_{s}^0$ meson into two muons ($B_{s}^0 \rightarrow \mu^+ \mu^-$). The latest analysis of $B_{(s)}^0 \rightarrow \mu^+ \mu^-$ decays by LHCb with Run 1 and Run 2 data is presented. The $B_{s}^0 \rightarrow \mu^+ \mu^-$ decay is observed for the first time by a single experiment. In addition, the first measurement of the $B_{s}^0 \rightarrow \mu^+ \mu^-$ effective lifetime is performed. No significant excess of $B^0 \rightarrow \mu^+ \mu^-$ decays is observed. All results are consistent with the SM and constrain New Physics in $b \rightarrow s \ell \ell$ processes.