  The transformation properties of strong penguin operators under the action of
the flavor group change when they are considered as operators in (partially)
quenched QCD instead of the unquenched theory. An ambiguity arises, which is
parameterized by new low-energy constants in the effective theory describing
non-leptonic kaon decays in the (partially) quenched setting. Here we summarize
results of the analysis for the complete set of three-flavor strong penguin
operators, consisting of products of two left-handed flavor currents, or a
left- and a right-handed current. Our results imply that (partially) quenched
lattice computations of the Delta I=1/2 rule and epsilon'/epsilon are both
affected by ambiguities intrinsic to the use of the quenched approximation at
leading order in the chiral expansion. The only exception is the partially
quenched case with three light sea quarks, consistent with general
expectations. We also address the issue of quenched ambiguities in the case of
an active charm, correcting and extending that in Phys. Rev. D 74, 014509
(2006).
