Unquenched determination of the kaon parameter B_K from improved staggered fermions

TL;DR

This paper calculates the kaon parameter B_K using improved staggered fermions with dynamical quarks, achieving a more reliable estimate that accounts for quark vacuum polarization effects.

Contribution

It provides the first unquenched calculation of B_K with improved staggered fermions, reducing scaling corrections and including sea quark effects.

Findings

Calculated _K = 0.83 b1 0.18 with dynamical quarks

Demonstrated improved scaling behavior with improved staggered actions

Identified the dominant uncertainty from lattice to continuum matching

Abstract

The use of improved staggered actions (HYP, Asqtad) has been proved to reduce the scaling corrections that affected previous calculations of B_K with unimproved (standard) staggered fermions in the quenched approximation. This improved behaviour allows us to perform a reliable calculation of B_K including quark vacuum polarization effects, using the MILC configurations with n_f=2+1 flavours of sea fermions. We perform such a calculation for a single lattice spacing, a=0.125 fm, and with kaons made up of degenerate quarks with m_s/2. The valence strange quark mass m_s is fixed to its physical value and we use two different values of the light sea quark masses. After a chiral extrapolation of the results to the physical value of the sea quark masses, we find \hat B_K = 0.83+-0.18, where the error is dominated by the uncertainty in the lattice to continuum matching at O(\alpha_s^2). The matching will need to be improved to get the precision needed to make full use of the experimental data on \epsilon_K to constrain the unitarity triangle.