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

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 $\mathcal{B}(B^0_s \to \mu^+\mu^-) = (3.0 \pm 0.6^{+0.3}_{−0.2}) \times 10^{−9}$, 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, $\mathcal{B}(B^0 \to \mu^+\mu^-) < 3.4 \times 10^{−10}$, is determined. All results are in agreement with the standard model expectations.