Discretization effects and the scalar meson correlator in mixed-action lattice simulations

TL;DR

This paper investigates discretization effects in mixed-action lattice QCD with domain-wall valence and staggered sea quarks, demonstrating how these effects influence the scalar meson correlator and can be modeled with mixed-action chiral perturbation theory.

Contribution

The study identifies new parameters arising from discretization effects in mixed-action lattice theories and validates MAChPT as an effective tool to predict and correct unitarity-violating effects.

Findings

Residual quark mass is smaller than the lightest valence quark mass.

Mixed valence-sea meson mass-splitting is comparable to staggered taste-splittings.

Scalar correlator behavior matches MAChPT predictions.

Abstract

We study discretization effects in a mixed-action lattice theory with domain-wall valence quarks and Asqtad-improved staggered sea quarks. At the level of the chiral effective Lagrangian, discretization effects in the mixed-action theory give rise to two new parameters as compared to the lowest order Lagrangian for staggered fermions -- the residual quark mass, m_res, and the mixed valence-sea meson mass-splitting, Delta_mix. We find that the size of m_res is approximately four times smaller than our lightest valence quark mass on our coarser lattice spacing, and comparable to that of simulations by RBC and UKQCD. We also find that the size of Delta_mix is comparable to the smallest of the staggered meson taste-splittings measured by MILC. Because lattice artifacts are different in the valence and sea sectors of the mixed-action theory, they give rise to unitarity-violating effects that disappear in the continuum limit. Such effects are expected to be mild for many quantities of interest, but are significant in the case of the isovector scalar (a_0) correlator. Specifically, once m_res, Delta_mix, and two other parameters that can be determined from the light pseudoscalar spectrum are known, the two-particle intermediate state "bubble" contribution to the scalar correlator is completely predicted within mixed-action chiral perturbation theory (MAChPT). We find that the behavior of the scalar meson correlator is quantitatively consistent with the MAChPT prediction; this supports the claim that MAChPT describes the dominant unitarity-violating effects in the mixed-action theory and can be used to remove lattice artifacts and recover physical quantities.