Connecting the quenched and unquenched worlds via the large Nc world

In the large Nc (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 (Nc=3) quenched QCD and QCD are the same up to higher-order corrections in the 1/Nc expansion. Thus the calculation of the non-leptonic kaon decays relevant for the ΔI=1/2 rule in the quenched approximation is expected to differ from the unquenched one by an O(1/Nc) correction. However, the calculation relevant to the CP-violation parameter ϵ′/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.