Search for a feebly interacting particle $X$ in the decay $K^{+}\to {\pi }^{+}X$

Abstract

A search for the $K^{+}\to {\pi }^{+}X$ decay, where $X$ is a long-lived feebly interacting particle, is performed through an interpretation of the $K^{+}\to {\pi }^{+}\nu \bar{\nu }$ analysis of data collected in 2017 by the NA62 experiment at CERN. Two ranges of $X$ masses, $0$-$110\text{MeV}/c^2$ and $154$-$260\text{MeV}/c^2$, and lifetimes above $100\text{ps}$ are considered. The limits set on the branching ratio, $\text{BR}(K^{+}\to {\pi }^{+}X)$, are competitive with previously reported searches in the first mass range, and improve on current limits in the second mass range by more than an order of magnitude.

arXiv comment14 pages, 6 figures, accepted for publication by JHEP

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