# Search for the rare decay $K_{\rm\scriptscriptstyle S}^0\rightarrow\mu^{+}\mu^{-}$

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## Abstract

A search for the decay KS -> mu+ mu- is performed, based on a data sample of 1.0 fb^-1 of pp collisions at \sqrt{s}=7 TeV collected by the LHCb experiment at the Large Hadron Collider. The observed number of candidates is consistent with the background-only hypothesis, yielding an upper limit of BR(KS -> mu+ mu-) < 11 (9) x 10^-9 at 95 (90)% confidence level. This limit is a factor of thirty below the previous measurement.

## Figures and captions

 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. Fig1.pdf [17 KiB] HiDef png [143 KiB] Thumbnail [57 KiB] *.C file 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. Fig2left.pdf [27 KiB] HiDef png [550 KiB] Thumbnail [167 KiB] *.C file Fig2right.pdf [39 KiB] HiDef png [801 KiB] Thumbnail [192 KiB] *.C file Background model fitted to the data separated along (left) TIS and (right) TOS trigger categories. The vertical lines delimit the search window. Fig3left.pdf [20 KiB] HiDef png [175 KiB] Thumbnail [70 KiB] *.C file Fig3right.pdf [20 KiB] HiDef png [172 KiB] Thumbnail [69 KiB] *.C file $\textrm{CL}_{\textrm{s}}$ 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 $95\%$ ($2\sigma$). Fig4a.pdf [15 KiB] HiDef png [150 KiB] Thumbnail [47 KiB] *.C file Fig4b.pdf [18 KiB] HiDef png [148 KiB] Thumbnail [51 KiB] *.C file [Failure to get the plot] Animated gif made out of all figures. PAPER-2012-023.gif Thumbnail

## Tables and captions

 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. Table_1.pdf [39 KiB] HiDef png [38 KiB] Thumbnail [5 KiB] tex code

Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.