Flavor changing neutral currents and rare decays of K-mesons

Processes with $K$-mesons that are due to weak flavor changing neutral currents (FCNC) are considered, primarily, the theoretically clean ‘golden decays’ $K^+\to\pi^+\nu\bar\nu$ and $K_L^0\to\pi^0\nu\bar\nu$. Experimental data for these and other processes violating CP invariance are discussed, and the prospects of future studies are examined. The results of experiments with kaons are compared with those obtained for B-mesons (CP-odd asymmetry in the decays $B_d^0(\bar B_d^0)\to(J/\psi)K_S^0$, $(B^0\leftrightarrows\bar B^0)$ mixing). A possible interpretation of the data on the decays $K\to\pi\nu\bar\nu$, $K_L^0\to\pi^0e^+e^-$, and $K_L^0\to\mu^+\mu^-$ is discussed within the framework of the Standard Model and of New Physics models.