  We calculate the corrections to the amputated Green's functions of 4-fermion
operators, in 1-loop Lattice Perturbation theory. The novel aspect of our
calculations is that they are carried out to second order in the lattice
spacing, O(a^2). We employ the Wilson/clover action for massless fermions (also
applicable for the twisted mass action in the chiral limit) and the Symanzik
improved action for gluons. Our calculations have been carried out in a general
covariant gauge. Results have been obtained for several popular choices of
values for the Symanzik coefficients (Plaquette, Tree-level Symanzik, Iwasaki,
TILW and DBW2 action). We pay particular attention to $\Delta F=2$ operators,
both Parity Conserving and Parity Violating ($F$ stands for flavour: S, C, B).
We study the mixing pattern of these operators, to O(a^2), using the
appropriate projectors. Our results for the corresponding renormalization
matrices are given as a function of a large number of parameters: coupling
constant, clover parameter, number of colors, lattice spacing, external
momentum and gauge parameter. The O(a^2) correction terms (along with our
previous O(a^2) calculation of $Z_\Psi$) are essential ingredients for
minimizing the lattice artifacts which are present in non-perturbative
evaluations of renormalization constants with the RI'-MOM method. A longer
write-up of this work, including non-perturbative results, is in preparation
together with members of the ETM Collaboration.
