BK-parameter from Nf = 2 twisted mass lattice QCD

We present an unquenched $N_f=2$ lattice computation of the $B_K$ parameter which controls $K^0-{\bar{K}}^0$ oscillations. A partially quenched setup is employed with two maximally twisted dynamical (sea) light Wilson quarks, and valence quarks of both the maximally twisted and the Osterwalder–Seiler variety. Suitable combinations of these two kinds of valence quarks lead to a lattice definition of the $B_K$ parameter which is both multiplicatively renormalizable and O($a$) improved. Employing the non-perturbative RI-MOM scheme, in the continuum limit and at the physical value of the pion mass we get $B_K^{\mathrm{R}\mathrm{G}\mathrm{I}}=0.729\pm 0.030$, a number well in line with the existing quenched and unquenched determinations.