Searches for hidden sectors using $K^{+}\to \ddot{I}{€}^{+}X$ decays

Abstract

Results from the study of the rare decays $K^{+}\to \ddot{I}{€}^{+}\hat{I}½\bar{\hat{I}}½$, $K^{+}\to \ddot{I}{€}^{+}\hat{I}{¼}^{+}\hat{I}{¼}^{-}$ and $K^{+}\to \ddot{I}{€}^{+}{\hat{I}}^3{\hat{I}}^3$ at the NA62 experiment at CERN are interpreted in terms of improved limits for $\mathrm{B}({\mathrm{K}}^{+}\to \ddot{\mathrm{I}}{€}^{+}\mathrm{X})$ and coupling parameters of hidden-sector models, where $X$ is a mediator. World-leading limits are achieved for dark photon, dark scalar and axion-like particle models.