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I give a pedagogical introduction into flavour-changing neutral currentinteractions of kaons and their role to reveal or constrain physics beyond theStandard Model (SM). Then I discuss the measure $\epsilon_K^\prime$ of directCP violation in $K\to \pi\pi$ decays, which deviates from the SM prediction by$2.8\sigma$. A supersymmetric scenario with flavour mixing among left-handedsquarks can accomodate the measured value of $\epsilon_K^\prime$ even for veryheavy sparticles, outside the reach of the LHC. The considered scenario employsmass splittings among the right-handed up and down squarks (to enhance$\epsilon_K^\prime$) and a gluino which is heavier than the left-handedstrange-down mixed squarks by at least a factor of 1.5 (to suppress excessivecontribution to $\epsilon_K$, the measure of indirect CP violation). Thebranching ratios of the rare decays $K^+ \to \pi^+ \nu \bar\nu$ and $K_L \to\pi^0 \nu \bar\nu$, to be measured by the NA62 and KOTO-step2 experiments,respectively, are only moderately affected. These measurements have thecapability to either falsify the model or to constrain the CP phase associatedwith strange-down squark mixing accurately.

$\epsilon_K^\prime/\epsilon_K$: Standard Model and Supersymmetry