Dynamical study of BK with improved staggered quarks

We report here on the preliminary results for the calculation of the Kaon parameter BK using improved staggered fermions. The scaling corrections that affected previous staggered determinations of BK have been proved to be reduced by using improved actions (HYP, Asqtad) in the quenched approximation. This improved behaviour allows us to perform a reliable dynamical calculation of BK including quark vacuum polarization effects using the MILC (2+1) flavour dynamical configurations. We also discuss the renormalization effects with the Asqtad action and the possible ways to improve the calculation.