An estimate of the K0-K0bar mixing amplitude

We computed the BK parameter on the lattice by using a non-perturbatively improved Wilson action. From our quenched simulation, in the MSbar scheme and at μ = 2 GeV, we obtain BK = 0.73(7)+0.05−0.01. To gain some insight in the systematic errors due to the use of the quenched approximation, we also made the unquenched computation with Wilson fermions. We do not observe any significant deviation with respect to the quenched result.