Non-perturbative renormalisation of four fermion operators and B 0 − B̄ 0 mixing with Wilson fermions ∗ † SPQcdR Collaboration

Matrix elements of the four fermion (4f) operators play an important rôle in the studies of CP-violation in the Standard Model and beyond. Of particular interest are the matrix elements describing the B − B̄ and K − K̄ mixing amplitudes. Lattice QCD is at present the best suited method to compute such quantities non-perturbatively (NP). To keep the theoretical uncertainties under control, it is of vital interest to renormalise the corresponding 4f-operators non-perturbatively. An additional difficulty arises when working with Wilson fermions on the lattice, namely all the parity even (∆F = 2) operators mix among themselves. Therefore, besides the overall renormalisation, the spurious mixing should be subtracted.