Strong isospin-breaking effects in K -> 2 pi at next-to-leading order in the chiral expansion

Strong isospin-breaking (LB) contributions to both the octet and 27-plet we ak K --> pi pi transitions are evaluated at next-to-lending order (NLO) in the chiral expansion. NLO contributions are shown to significantly reduce t he leading order result for the potentially large contribution to the Delta I = 3/2 amplitude resulting from strong isospin-breaking modifications to the weak Delta I = 1/2 amplitude. The ratio of strong IB 27-plet to strong IB octet contributions is found to be small for all decay amplitudes. Combi ned with recent results on the corresponding electromagnetic contributions, we find that the ratio of the intrinsic strengths of octet and 27-plet eff ective weak operators can be taken to be that obtained from experimental da ta, analyzed ignoring isospin breaking, to an accuracy better than of order similar to 10%. (C) 2000 Elsevier Science B.V. All rights reserved.