Search for K⁺ decays into the π⁺e⁺e⁻e⁺e⁻ final state

The first search for ultra-rare K⁺ decays into the π⁺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 1.4×10⁻⁸ at 90% CL is obtained for the branching ratio of the K⁺→π⁺e⁺e⁻e⁺e⁻ decay, predicted in the Standard Model to be (7.2±0.7)×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⁺→π⁺aa, a→e⁺e⁻ and K⁺→π⁺S, S→A^′A^′, A^′→e⁺e⁻.