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

TL;DR

This paper reports the first search for ultra-rare $K^+$ decays into a $ ext{pi}^+ e^+ e^- e^+ e^-$ final state, setting upper limits on the branching ratios and exploring potential hidden-sector mediators.

Contribution

It provides the first experimental upper limits on the branching ratios of these rare decays and investigates possible new physics involving hidden-sector particles.

Findings

Upper limit of $1.4 imes 10^{-8}$ for $K^+ o ext{pi}^+ e^+ e^- e^+ e^-$ decay.

Upper limits around $10^{-9}$ for decays involving hidden-sector mediators.

No observation of the rare decays within the sensitivity of the dataset.

Abstract

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 $(7.2\pm0.7)\times 10^{-11}$. 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 $K^+\to\pi^+S$, $S\to A^\prime A^\prime$, $A^\prime\to e^+e^-$.