Search for $K_{\mathrm{S}(\mathrm{L})}^0\to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ decays at LHCb

A search for $K_{\text{S(L)}}^0\to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-}$ decays is performed using proton-proton collision data collected by the LHCb experiment at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.1 $f{b}^{-1}$. No evidence for signal is found. The 90% confidence level upper limits are the first set for both decays and are $B(K_{\text{S}}^0\to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-})<5.1\times {10}^{-12}$ and $B(K_{\text{L}}^0\to {\mu }^{+}{\mu }^{-}{\mu }^{+}{\mu }^{-})<2.3\times {10}^{-9}$.