Searches for hidden sectors using K+ → π+X decays

A bstract Results from the study of the rare decays K⁺→ π⁺ν ν̄ K + → π + ν ν ¯ , K + → π + μ + μ − and K + → π + γγ at the NA62 experiment at CERN are interpreted in terms of improved limits for B(K⁺→ π⁺X) B K + → π + 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.