Abstract
In the large N c(number of colors) limit, quenched QCD and QCD are identical. This implies that, in the effective field theory framework, some of the low energy constants in ( N c = 3) quenched QCD and QCD are the same up to higher-order corrections in the 1/ N c expansion. Thus the calculation of the nonleptonic kaon decays relevant for theδ I =½ rule in the quenched approximation is expected to differ from the unquenched one by anO(1/ N c ) correction. However, the calculation relevant to the CP-violation parameterε′/ε would have a relatively big higher-order correction due to the large cancellation in the leading order. Some important weak matrix elements are poorly known that even constraints with 100% errors are interesting. In those cases, quenched calculations will be very useful.
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