– 1 – Rare Kaon Decays

A. Introduction: There are several useful reviews on rare kaon decays and related topics [1–14]. Activity in rare kaon decays can be divided roughly into four categories: 1. Searches for explicit violations of the Standard Model 2. Measurements of Standard Model parameters 3. Searches for CP violation 4. Studies of strong interactions at low energy. The paradigm of Category 1 is the lepton flavor violating decay K L → μe. Category 2 includes processes such as K + → π + νν, which is sensitive to |V td |. Much of the interest in Category 3 is focused on the decays K L → π 0 , where ≡ e, μ, ν. Category 4 includes reactions like K + → π + + − which constitute a testing ground for the ideas of chiral perturbation theory. Category 4 also includes K L → π 0 γγ and K L → + − γ. The former is important in understanding a CP-conserving contribution to K L → π 0 + − , whereas the latter could shed light on long distance contributions to K L → μ + μ −. The interplay between Categories 2-4 can be illustrated in Fig. 1. The modes K → πνν are the cleanest ones theoretically. They can provide accurate determinations of certain CKM parameters (shown in the figure). In combination with alternate determinations of these parameters, they also constrain new interactions. The modes K L → π 0 e + e − and K L → μ + μ − are also sensitive to CKM parameters. However, they suffer from a series of hadronic uncertainties that can be addressed, at least in part, through a systematic study of the additional modes indicated in the figure.