RareKdecays

Abstract

This article reviews the current situation in the field of rare K decays: the relevant phenomenology, the present experimental situation, and prospects for the near future. Study of rare K decays can make a significant contribution in a number of different frontier areas of research in high-energy physics. In the area of CP violation, study of such rare decays as Kl-+Tr°e + e~, K®-• 7r°/z + jL4~, K^-^rfivv, and muon polarization in K®->/j,~hfi~ can provide important complementary information to what has been learned from the decay K^-^rnr.Even though experiments with sufficient accuracy to make a meaningful study of CP violation are still a few years away, significant progress has been made in this general area during the last decade. A second major area of interest in the field of rare K decays is the search for processes forbidden in the Standard Model, e.g., K® -+fie and K + ->TT + pi + e~.Various extensions of the Standard Model predict that these processes will occur with branching fractions in the range of 10~1 0 to 10~1 5 . Experiments of the last decade have pushed the limits into the 10 -10 to 10 -11 range, and further improvements in sensitivity of one to two orders of magnitude can be expected in the next few years. K decays allow one also to study higher-order weak-interaction processes such as A^J-•^+/z~~, K®->e + e~, K + ->7r + vv, which are forbidden to first order in the Standard Model. Because of strong suppression, these decay modes offer potential windows on new physics; in addition, they may offer the most reliable measurement of V td , one of the elements of the weak mixing matrix in the quark sector. The studies of the I*I + JJ>~ channel have achieved data samples of close to 1000 events; the other two modes should be observed for the first time in the next few years. Finally, as a byproduct of these studies, one has been able to look simultaneously for new light particles into which the K meson could decay. Limits obtained for various hypothetical particles are summarized. CONTENTS

Weak decays beyond leading logarithms

“…Yet the prospects of getting the required sensitivity of order 10 −11 -10 −12 in five years are encouraging (Littenberg and Valencia, 1993), (Winstein and Wolfenstein, 1993), (Ritchie and Wojcicki, 1993).…”

Direct CP violation in KL → π0e+e− beyond leading logarithms

“…A clear cut observation of direct CP violation, the violation of CP symmetry in the decay amplitudes remains as one of the central targets of high energy physics in the 90's and in the beginning of 21st century [1, 2,3]. The CP violation in the K → ππ decays discovered almost 30 years ago can be accommodated in the standard model as a result of complex phases present in K 0 -K 0 mixing and is usually called the indirect CP violation.…”

Untitled

“…It is well known that the rare kaon decays K + → π + νν and K L → π 0 νν are theoretically very well under control and are therefore expected to play a potentially important role in tests of standard model flavordynamics [29,30,31]. In particular these two decays alone can, at least in principle, fix the unitarity triangle completely [9].…”