ε′/ε and Chiral Dynamics

The long-distance contributions to K → 2π amplitudes can be pinned down, using well established Chiral Perturbation Theory techniques. The strong S–wave rescattering of the two final pions generates sizeable chiral loop corrections, which have an important impact on the direct CP violation ratio ε/ε [1,2]. Including all large logarithmic corrections, both at short and long distances, the Standard Model Prediction for this observable is found to be [2] Re(ε/ε) = (1.7± 0.9) · 10, in good agreement with the most recent experimental measurements. A better estimate of the strange quark mass could reduce the theoretical uncertainty to 30%.