Kaon Portal to Freeze-in Dark Matter

Abstract

We investigate freeze-in production of light dark matter through the quark flavor-changing operator $(\bar{s}{\gamma }_{\mu }d)(\bar{\chi }{\gamma }^{\mu }\chi )$ in a low-reheating cosmology. For reheating temperatures below the QCD crossover, kaon decays and scatterings generate the dark matter abundance through $K\to \pi \chi \bar{\chi }$ and $K\pi \to \chi \bar{\chi }$. The same interaction induces the rare kaon decays $K^{+}\to {\pi }^{+}\chi \bar{\chi }$ and $K_L\to {\pi }^0\chi \bar{\chi }$. This links the freeze-in relic abundance to searches at NA62, KOTO, and KOTO II. We find that lower reheating temperatures require larger couplings to compensate for the Boltzmann-suppressed kaon abundance, making kaon-driven freeze-in dark matter testable at rare kaon decay experiments.

Comments: 15 pages, 3 figures