Weak K-amplitudes in the chiral and 1/Nc-expansions

Abstract

It is shown that there exist symmetry constraints for non-leptonic weak amplitudes which emerge when the 1/Nc-expansion restricted to the leading and next-to-leading approximations only is systematically combined with χPT limited to the lowest non-trivial order. We discuss these constraints for the couplings g8 and g27 of ΔS = 1 transitions and the BK-parameter of $K^0-{\bar K}^0$ mixing.