Rare $K$ Decays
  The rare decays of the $K$ meson have had a long tradition as a laboratory for testing the symmetry properties of the weak interactions, and the manner in which these symmetries are broken by higher order effects. Present--day interest is focussed on decays that are suppressed by $CP$--symmetry or GIM symmetry. Such decays, in the standard theory, are sensitive to effects of the virtual top quark, and could also reveal new interactions transcending the standard model. In addition, the radiative decays of the $K$ meson have become a useful testing--ground for effective Lagrangians describing the low energy interactions of pions, kaons and photons. 
L.M. Sehgal