Constraints on the $K_{\mathrm{S}}^0\to {\mu }^{+}{\mu }^{-}$ branching fraction

A search for the decay $K_{\mathrm{S}}^0\to {\mu }^{+}{\mu }^{-}$ 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 $13\mathrm{T}\mathrm{e}\mathrm{V}$. The observed signal yield is consistent with zero, yielding an upper limit of $\mathcal{B}(K_{\mathrm{S}}^0\to {\mu }^{+}{\mu }^{-})<2.2\times {10}^{-10}$ 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.