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

A search for the rare decays $B_s^0\to {\mu }^{+}{\mu }^{-}$ 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 $B_s^0\to {\mu }^{+}{\mu }^{-}$ 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 $B_s^0\to {\mu }^{+}{\mu }^{-}$ effective lifetime, $\tau (B_s^0\to {\mu }^{+}{\mu }^{-})=2.04\pm 0.44\pm 0.05$ 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.