Search for $K^{+}$ decays into the ${\pi }^{+}{e}^{+}{e}^{-}{e}^{+}{e}^{-}$ final state

The first search for ultra-rare $K^{+}$ decays into the ${\pi }^{+}{e}^{+}{e}^{-}{e}^{+}{e}^{-}$ final state is reported, using a dataset collected by the NA62 experiment at CERN in 2017-2018. An upper limit of $1.4\times {10}^{-8}$ at 90% CL is obtained for the branching ratio of the $K^{+}\to {\pi }^{+}{e}^{+}{e}^{-}{e}^{+}{e}^{-}$ decay, predicted in the Standard Model to be $(7.2\pm 0.7)\times {10}^{-11}$. Upper limits at 90% CL are obtained at the level of ${10}^{-9}$ for the branching ratios of two prompt decay chains involving pair-production of hidden-sector mediators: $K^{+}\to {\pi }^{+}aa$, $a\to e^{+}{e}^{-}$ and $K^{+}\to {\pi }^{+}S$, $S\to A^{\mathrm{\prime }}{A}^{\mathrm{\prime }}$, $A^{\mathrm{\prime }}\to e^{+}{e}^{-}$.