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Abstract

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−.

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