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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.

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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.

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

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$\textrm{CL}_{\textrm{s}}$ 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 $95\%$ ($2\sigma$).

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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.

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