$ε_K^\prime/ε_K$: 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 $ε_K^\prime$ of direct CP violation in $K\to ππ$ 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 $ε_K^\prime$ 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 $ε_K^\prime$) 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 $ε_K$, the measure of indirect CP violation). The branching ratios of the rare decays $K^+ \to π^+ ν\barν$ and $K_L \to π^0 ν\barν$, 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.

Talk at conference C17-03-18, 52nd Rencontres de Moriond on EW Interactions and Unified Theories (Moriond EW 2017)