Scheme Dependence of Weak Matrix Elements in the $1/N_c$ Expansion

Abstract

We show how the scheme- and scale-dependence of the short-distance operator product expansion with four-quark operators can be correctly accounted for in the framework of the $1/N_c$-expansion once the hadronization of two-quark currents has been fixed. We show formulas explicitly in the case of the $B_K$-parameter. We then use them with our earlier estimates of the long-distance effects. We compare Chiral Perturbation Theory at Leading- and Next-to-Leading-Order with the ENJL model results in the chiral limit. Good matching between the long- and short-distance regimes is obtained and our final value for the physical scheme independent $\hat B_K$ is $0.77\pm0.05\pm0.05$.Comment: 20 pages, use JHEP.cls and axodraw.sty; small error detected, numerical results changed with about quoted erro