Estimating epsilon'/epsilon. A Review
Abstract
The real part of eps'/eps measures direct CP violation in the decays of a neutral kaon in two pions. Its determination may answer the question of whether CP violation is present only in the mass matrix of neutral kaons (the superweak scenario) or also at work directly in the decays. After a brief historical summary, we discuss the present and expected experimental sensitivities. In the light of these, we come to the problem of estimating eps'/eps in the standard model. We review the present (beginning of 1998) status of the theoretical prediction of eps'/eps. The short-distance part of the computation is now known to the next-to-leading order in QCD and QED and therefore well under control. On the other hand, the evaluation of the hadronic matrix element of the relevant operators is where most of the theoretical uncertainty still resides. We analyze the results for the vacuum saturation approximation at O(p^2) in the momenta, the M\"unchen phenomenological approach, the lattice approach, and the chiral quark model. The values of the B_i parameters in the various approaches are discussed, together with the allowed range of the relevant combination of the Cabibbo-Kobayashi-Maskawa entries Im V_{td}V^*_{ts}. We conclude by summarizing and comparing all up-to-date estimates of eps'/eps. Because of the intrinsic uncertainties of the long-distance computations, values ranging from 10^{-4} to a few times 10^{-3} may be accounted for in the standard model. Since this range covers most of the present experimental uncertainty, it is unlikely that new physics effects can be disentangled from the standard model prediction. For updates on the review and additional material see http://www.he.sissa.it/review/.Keywords
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