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 $4.4{\mathrm{fb}}^{-1}$. 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 $\mathcal{B}(B_s^0\to {\mu }^{+}{\mu }^{-})=(3.0\pm 0.6_{-0.2}^{+0.3})\times {10}^{-9}$, where the first uncertainty is statistical and the second systematic. The first measurement of the $B_s^0\to {\mu }^{+}{\mu }^{-}$ effective lifetime, $\tau (B_s^0\to {\mu }^{+}{\mu }^{-})=2.04\pm 0.44\pm 0.05\mathrm{ps}$, 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.

Figure 1

Mass distribution of the selected $B_{(s)}^0\to {\mu }^{+}{\mu }^{-}$ candidates (black dots) with $\mathrm{BDT}>0.5$. The result of the fit is overlaid, and the different components are detailed.

Figure 2

(Top) Mass distribution of the selected $B_{(s)}^0\to {\mu }^{+}{\mu }^{-}$ candidates (black dots) with $\mathrm{BDT}>0.55$. The result of the fit is overlaid together with the $B_s^0\to {\mu }^{+}{\mu }^{-}$ (red dashed line) and the combinatorial background (blue dashed line) components. (Bottom) Background-subtracted $B_s^0\to {\mu }^{+}{\mu }^{-}$ decay-time distribution with the fit result superimposed.