CHIRAL APPROACH TO THE KL-KS MASS DIFFERENCE

A complete analysis of the long range part of the K(L) - K(S) mass difference in next-to-leading-order of chiral perturbation theory is presented. We find a large positive contribution from K0 - KBAR0 transitions via DELTA-S = 2 vertices and irreducible loop diagrams involving two separate weak interactions. Single-particle pole contributions are small if an approximate nonet symmetry for the couplings of the singlet eta-1 holds. They tend to cancel the short distance contribution from box diagrams. In total, good agreement with experiment is obtained. In a second part we study the K(L) - K(S) mass difference in a phenomenological model of nonleptonic weak transitions which explains the DELTA-I = 1/2 rule in terms of non-perturbative quark-quark correlations. The numerical results agree well with those presented before.