Mass spectrum for selected $ K ^0_{\rm\scriptscriptstyle S} \rightarrow \pi^+\pi^-$ candidates in the MB sample. The black points correspond to the mass reconstructed under the $\pi\pi$ mass hypothesis for the daughters, while the red triangles correspond to the mass reconstructed under the $\mu\mu$ mass hypothesis.

Position in the $x-z$ plane of the secondary vertices of the background candidates found in the high mass sideband for (left) TIS candidates and (right) TOS candidates. The lighter coloured areas correspond to higher density of points.

Background model fitted to the data separated along (left) TIS and (right) TOS trigger categories. The vertical lines delimit the search window.

$\textrm{CL}_{\textrm{s}}$ curves for (a) TIS, (b) TOS categories and for (c) the combined sample. The solid line corresponds to the observed $\textrm{CL}_{\textrm{s}}$ . The dashed line corresponds to the median of the $\textrm{CL}_{\textrm{s}}$ for an ensemble of background-alone experiments. In each plot, two bands are shown. The green (dark) band covers $68\%$ ($1\sigma$) of the $\textrm{CL}_{\textrm{s}}$ curves obtained in the background only pseudo-experiments, while the yellow (light) band covers $95\%$ ($2\sigma$).

Animated gif made out of all figures.

Upper limits on $ {\cal B}( K ^0_{\rm\scriptscriptstyle S} \rightarrow \mu^+\mu^-)$ for the TIS and the TOS categories separately, and for the combined analysis. The last entry in the table is the $p$-value of the background-only hypothesis.