Perturbative corrections to four-fermion operators on the lattice

We calculate the relation between the four-fermion operators of the effective weak Hamiltonian in the continuum and on the lattice. We include, at this stage, only those lattice operators which have the same dimension as the continuum operators. We consider the case where the operators contain fields which can contract with each other. We present, in a rather pedagogical fashion, various methods for performing such calculations and describe several short cuts that we have found useful.