1/N_c Corrections to the Hadronic Matrix Elements of Q_6 and Q_8 in K   --> pi pi Decays
  We calculate long-distance contributions to the amplitudes A(K^0 --> pi pi, I) induced by the gluon and the electroweak penguin operators Q_6 and Q_8, respectively. We use the 1/N_c expansion within the effective chiral lagrangian for pseudoscalar mesons. In addition, we adopt a modified prescription for the identification of meson momenta in the chiral loop corrections in order to achieve a consistent matching to the short-distance part. Our approach leads to an explicit classification of the loop diagrams into non-factorizable and factorizable, the scale dependence of the latter being absorbed in the low-energy coefficients of the effective theory. Along these lines we calculate the one-loop corrections to the O(p^0) term in the chiral expansion of both operators. In the numerical results, we obtain moderate corrections to B_6^(1/2) and a substantial reduction of B_8^(3/2). 
T. Hambye, G.O. Koehler, E.A. Paschos, P.H. Soldan (Dortmund U.), and   W.A. Bardeen (Fermilab)