An Estimate of the K0-K0bar Mixing Amplitude

We computed the B_K parameter on the lattice by using a non-perturbatively improved Wilson action. From our quenched simulation, in the MSbar scheme and at mu=2 GeV, we obtain B_K=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.