Estimating epsilon'/epsilon. A Review

The real part of epsilon'/epsilon measures direct CP violation in the decays of a neutral kaon in two pions. It is a fundamental quantity which has justly attracted a great deal of theoretical as well as experimental work. 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 epsilon'/epsilon in the standard model. We review the present (circa 1998) status of the theoretical predictions of epsilon'/epsilon. 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 of the currently most developed calculations. 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 predictions of epsilon'/epsilon. 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/.