Measurement of the $B^0_s\toμ^+μ^-$ branching fraction and effective lifetime and search for $B^0\toμ^+μ^-$ decays

Abstract

A search for the rare decays $B^0_s\toμ^+μ^-$ and $B^0\toμ^+μ^-$ 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 $B^0_s\toμ^+μ^-$ 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 ${\cal B}(B^0_s\toμ^+μ^-)=\left(3.0\pm 0.6^{+0.3}_{-0.2}\right)\times 10^{-9}$, where the first uncertainty is statistical and the second systematic. The first measurement of the $B^0_s\toμ^+μ^-$ effective lifetime, $τ(B^0_s\toμ^+μ^-)=2.04\pm 0.44\pm 0.05$ ps, is reported. No significant excess of $B^0\toμ^+μ^-$ decays is found and a 95 % confidence level upper limit, ${\cal B}(B^0\toμ^+μ^-)<3.4\times 10^{-10}$, is determined. All results are in agreement with the Standard Model expectations.

All figures and tables, along with any supplementary material and additional information, are available at https://lhcbproject.web.cern.ch/lhcbproject/Publications/LHCbProjectPublic/LHCb-PAPER-2017-001.html