Distribution of selected candidates (black points) in the (left) $ B^0_s \to\mu^+\mu^-$ and (right) $ B^0\to\mu^+\mu^-$ mass window for BDT$>$0.5, and expectations for, from the top, $ B^{0}_{(s)}\to\mu^+\mu^-$ SM signal (gray), combinatorial background (light gray), $ B^0_{(s)}\to h^+{h'}^{-}$ background (black), and cross-feed of the two modes (dark gray). The hatched area depicts the uncertainty on the sum of the expected contributions.

$\textrm{CL}_{\textrm{s}}$ as a function of the assumed $\cal B$ for (left) $ B^0_s \to\mu^+\mu^-$ and (right) $ B^0\to\mu^+\mu^-$ decays. The long dashed black curves are the medians of the expected $\textrm{CL}_{\textrm{s}}$ distributions for $ B^0_s \to\mu^+\mu^-$ , if background and SM signal were observed, and for $ B^0\to\mu^+\mu^-$ , if background only was observed. The yellow areas cover, for each $\cal B$ , 34% of the expected $\textrm{CL}_{\textrm{s}}$ distribution on each side of its median. The solid blue curves are the observed $\textrm{CL}_{\textrm{s}}$ . The upper limits at 90 % (95 %) CL are indicated by the dotted (solid) horizontal lines in red (dark gray) for the observation and in gray for the expectation.

Animated gif made out of all figures.

Expected combinatorial background, $ B^0_{(s)}\to h^+{h'}^{-}$ background, cross-feed, and signal events assuming SM predictions, together with the number of observed events in the $ B^0_s \to\mu^+\mu^-$ and $ B^0\to\mu^+\mu^-$ mass signal regions, in bins of BDT.

Expected and observed limits on the $ B^{0}_{(s)}\to\mu^+\mu^-$ branching fractions.