Non-perturbative scale evolution of four-fermion operators in two-flavour QCD ∗

We apply finite-size recursion techniques based on the Schro dinger functional formalism to determine the renormalization group running of four-fermion operators which appear in the ΔS = 2 effective weak Hamiltonian of the Standard Model. Our calculations are done using O(a) improved Wilson fermions with Nf = 2 dynamical flavours. Preliminary results are presented for the four-fermion operator which determines the BK-parameter in tmQCD.