Search for a feebly interacting particle X in the decay K ⁺ → π ⁺ X

A search for the K⁺ → π⁺X decay, where X is a long-lived feebly interacting particle, is performed through an interpretation of the K⁺ → π⁺νν¯ analysis of data collected in 2017 by the NA62 experiment at CERN. Two ranges of X masses, 0–110 MeV/c² and 154–260 MeV/c², and lifetimes above 100 ps are considered. The limits set on the branching ratio, BR(K⁺ → π⁺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.