Kaon portal to freeze-in dark matter

A

bstract

We investigate freeze-in production of light dark matter through the quark flavor-changing operator

in a low-reheating cosmology. For reheating temperatures below the QCD crossover, kaon decays and scatterings generate the dark matter abundance through

$$ K\to \pi \chi \overline{\chi} $$ K → πχ χ ¯

and

$$ K\pi \to \chi \overline{\chi} $$ Kπ → χ χ ¯

. The same interaction induces the rare kaon decays

$$ {K}^{+}\to {\pi}^{+}\chi \overline{\chi} $$ K + → π + χ χ ¯

and

$$ {K}_L\to {\pi}^0\chi \overline{\chi} $$ K L → π 0 χ χ ¯

. 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.