Measurement of the $B^0_s \to \mu^+ \mu^-$ branching fraction and search for $B^0 \to \mu^+ \mu^-$ decays at the LHCb experiment

TL;DR

This paper reports the observation of the rare decay $B^0_s o \mu^+\mu^-$ with a significance of 4.0 sigma and measures its branching fraction, while setting an upper limit for $B^0 o \mu^+\mu^-$, consistent with Standard Model predictions.

Contribution

First measurement of the $B^0_s o \\mu^+\\mu^-$ branching fraction at LHCb with a 4 sigma significance, and the most stringent limit on $B^0 o \\mu^+\\mu^-$ to date.

Findings

Branching fraction of $B^0_s o \\mu^+\\mu^-$ is $(2.9^{+1.1}_{-1.0}) imes 10^{-9}$.

Significance of $B^0_s o \\mu^+\\mu^-$ signal is 4.0 sigma.

Upper limit for $B^0 o \\mu^+\\mu^-$ is $7.4 imes 10^{-10}$ at 95% CL.

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. The data analysed correspond to an integrated luminosity of 1 fb$^{-1}$ of $pp$ collisions at a centre-of-mass energy of 7 TeV and 2 fb$^{-1}$ at 8 TeV. An excess of $B^0_s \to\mu^+\mu^-$ signal candidates with respect to the background expectation is seen with a significance of 4.0 standard deviations. A time-integrated branching fraction of ${\cal B}(B^0_s \to\mu^+\mu^-) = (2.9^{+1.1}_{-1.0})\times 10^{-9}$ is obtained and an upper limit of ${\cal B}(B^0 \to\mu^+\mu^-) < 7.4\times 10^{-10}$ at 95% confidence level is set. These results are consistent with the Standard Model expectations.