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Strong constraints on the $K^0_{\mathrm{S}}\rightarrow\mu^{+}\mu^{-}$ branching fraction

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Abstract

A search for the decay $K^0_{\mathrm{S}}\rightarrow\mu^{+}\mu^{-}$ is performed using proton-proton collision data, corresponding to an integrated luminosity of $5.6 \text{fb}^{-1}$ and collected with the LHCb experiment during 2016, 2017 and 2018 at a center-of-mass energy of $13 \mathrm{TeV}$. The observed signal yield is consistent with zero, yielding an upper limit of ${\cal B}(K^0_{\mathrm{S}}\rightarrow\mu^{+}\mu^{-}) < 2.2 \times 10^{-10}$ at 90 CL. The limit reduces to ${\cal B}(K^0_{\mathrm{S}}\rightarrow\mu^{+}\mu^{-}) < 2.1 \times 10^{-10}$ at 90 CL once combined with the result from data taken in 2011 and 2012.

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Diagrams representing SM contributions to the $ K ^0_{\mathrm{S}} \rightarrow \mu^+\mu^-$ decay amplitude: (top) long-distance contribution, generated by two intermediate photons, and (bottom) short-distance contributions.

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Invariant-mass distribution of $ K ^0_{\mathrm{S}} \rightarrow \pi^+\pi^-$ candidates in 2016 trigger-unbiased data (points with error bars) and corrected simulation (solid histogram). The histogram of simulated candidates is normalized to data.

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Projection of the fit to the dimuon invariant mass distribution for the (top left) TIS and (bottom left) XTOS trigger categories. The plots on the right correspond to the projection of the fit in the BDT bins with the highest signal-to-background ratio for the (top right) TIS and (bottom right) XTOS trigger categories. The dashed orange line shows the signal contribution, the dotted green line the $ K ^0_{\mathrm{L}} \rightarrow \mu^+\mu^-$ contribution, the dash dotted red line the $ K ^0_{\mathrm{S}} \rightarrow \pi^+\pi^-$ contribution, the loosely dotted brown line the background from random combination of tracks and material interactions, and the solid blue line the total probability density function. For clarity, empty bins are not shown.

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Evaluation of $-2\Delta \log {\cal L}$, where ${\cal L}$ is the likelihood of the fit model, as a function of $ {\cal B}( K ^0_{\mathrm{S}} \rightarrow \mu^+\mu^-)$ . The dotted orange line corresponds to the Run 1 result, the dashed blue line to the Run 2 result, and the solid green line shows the combination. The two vertical lines show the location of the upper limit of the combined result at 90% and 95% confidence level.

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Position of the secondary vertices for $ K ^0_{\mathrm{S}} \rightarrow \mu^+\mu^-$ candidates satisfying the requirement \(m_{ $\mu ^+\mu ^-$ } > 520 $\text{ Me V /}c^2$ \). The pattern of the innermost subdetector of the LHCb , the VELO , can be seen, together with that of the surrounding cavity.

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Animated gif made out of all figures.

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Supplementary Material [file]

Supplementary material full pdf

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This ZIP file contains supplemetary material for the publication LHCb-PAPER-2019-038. The files are: supplementary.pdf : An overview of the extra material *.pdf, *.png, *.eps : The figures in variuous formats

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Created on 27 February 2021.