ϵ'K/ϵK: Standard model and supersymmetry

Abstract

I give a pedagogical introduction into flavour-changing neutral current interactions of kaons and their role to reveal or constrain physics beyond the Standard Model (SM). Then I discuss the measure ϵ'K of direct CP violation in K → ππ decays, which deviates from the SM prediction by 2.8σ. A supersymmetric scenario with flavour mixing among left-handed squarks can accomodate the measured value of ϵ'K even for very heavy sparticles, outside the reach of the LHC. The considered scenario employs mass splittings among the right-handed up and down squarks (to enhance eK) and a gluino which is heavier than the left-handed strange-down mixed squarks by at least a factor of 1.5 (to suppress excessive contribution to ϵK, the measure of indirect CP violation). The branching ratios of the rare decays K+ → π+νν and KL → π0νν, to be measured by the NA62 and KOTO-step2 experiments, respectively, are only moderately affected. These measurements have the capability to either falsify the model or to constrain the CP phase associated with strange-down squark mixing accurately.