BK-parameter from Nf = 2 twisted mass lattice QCD

Abstract

We present an unquenched N f = 2 lattice computation of the B K parameter which controls K 0 − ¯¯¯¯ 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 nonperturbative RI-MOM scheme, in the continuum limit and at the physical value of the pion mass we get B RGI K = 0.729 ± 0.030 , a number well in line with the existing quenched and unquenched determinations.