Rare K and μ decays: Theoretical and experimental status and prospects

Abstract

I. THEORETICAL CONSIDERATIONS At present there is a remarkably successful standard model of electroweak and strong interactions. There are no contradictions between experiment and theory~ Nevertheless, there are many questions which the standard model does not answer, questions of the why kind. The origin of quark and lepton masses is not understood, nor is the reason for generations , or how many of these there are. In the most fundamental sense, it is hoped that rare decays will occur that are not required by the standard model (that is, their rates will be nonzero if predicted to be zero in the standard model, or will differ significantly from a nonzero standard model prediction). If indeed this happens, one may expect that it will provide important clues to help answer the open questions. There are two classes of decays. Some are expected to occur at some level in the standard model (henceforth denoted as SM), and provide tests of this model. Some examples of these are given in Table i. New clues could arise from finding rates or other observables which differ from those predicted by the SM. For the second type of decays, the SM prediction is zero--they do not occur at any level. Finding them at all would demonstrate the existence of new interactions. For the various processes to be discussed below, these SM predictions will be mentioned; they are summarized in the first line of Table i. Some of the decays might give results different from the SM because of the existence of new interactions, or equivalently, the exchange of new particles. Others might be of interest as a way to discover neutrino masses because of modified kinematical distributions, or a decay into a new kind of particle. In this section we shall provide a brief description of some ideas that might lead to observable new effects. While many of the ideas suggest that new effects should occur, they do not require that they occur at any specified level. Sometimes particular models may predict an effect, but at the present level of understanding, one can usually find another model which predicts a smaller or larger effect. Rather than trying to provide complete and detailed references in the proceedings of a workshop such as this, we shall only mention a few reviews from which the literature can be traced. I Partly this is because of the perspective that the ideas discussed do provide good motivation that some effects should occur somewher~ but no partic ufar idea should be interpreted to imply that a specific prediction (beyond the SM) is more than suggestive.