Estimating ε′/ε in the Standard Model

I discuss the comparison of the current theoretical calculations of ε/ε with the experimental data. Lacking reliable “first principle” calculations, phenomenological approaches may help in understanding correlations among different contributions and available experimental data. In particular, in the chiral quark model approach the same dynamics which underlies the ∆I = 1/2 selection rule in kaon decays appears to enhance the K → ππ matrix element of the Q6 gluonic penguin, thus driving ε/ε in the range of the recent experimental measurements. The results announced by the KTeV Collaboration last February and by the NA48 Collaboration at this conference [1] (albeit preliminary) have marked a great experimental achievement, establishing 35 years after the discovery of CP violation in the neutral kaon system the existence of a much smaller violation acting directly in the decays. While the Standard Model (SM) of strong and electroweak interactions provides an economical and elegant understanding of the presence of indirect (ε) and direct (ε) CP violation in term of a single phase, the detailed calculation of the size of these effects implies mastering strong interactions at a scale where perturbative methods break down. In addition, CP violation in K → ππ decays is the result of a destructive interference between two sets of contributions (for a suggestive picture of the gluonic and electroweak penguin diagrams see the talk by Buras at this conference [2]), thus potentially inflating up to an order of magnitude the uncertainties on the individual hadronic matrix elements of the effective four-quark operators.