K0-K0 bar mixing in the Standard Model from Nf=2+1+1 Twisted Mass Lattice QCD

We present preliminary results at β = 1.95 (a = 0.077 fm) on the first unquenched Nf = 2 + 1+1 lattice computation of the BK parameter which controls the neutral kaon oscillations in the Standard Model. Using Nf = 2 + 1 + 1 maximally twisted sea quarks and Osterwalder-Seiler valence quarks we achieve O(a) improvement and a continuum-like renormalization pattern for the four-fermion operator. Our results are extrapolated/interpolated to the physical light/strange quark mass but not yet to the continuum limit. The computation of the relevant renormalization constants is performed non perturbatively in the RI’-MOM scheme using dedicated simulations with Nf = 4 degenerate sea quark flavours produced by the ETM collaboration. We get B^RGI_K(a = 0.077) = 0.747(18), which when compared to our previous unquenched Nf = 2 determination and most of the existing results, suggests a rather weak B^RGI_K dependence on the number of dynamical flavours. We are at the moment analysing lattice data at two additional β values which will allow us to perform an extrapolation to the continuum limit.