Ginsparg-Wilson Pions Scattering in a Sea of Staggered Quarks

TL;DR

This paper calculates pion-pion scattering in a mixed action lattice QCD framework, demonstrating that certain scattering parameters can be reliably extracted despite nonunitary effects, and showing that the scattering length has no dependence on some lattice artifacts.

Contribution

It provides a method to determine scattering lengths in mixed action lattice QCD with reduced lattice artifacts and clarifies the volume dependence of two-pion states in such theories.

Findings

Scattering length expressed in physical units is free of mixed action parameters.

Volume dependence of two-pion states matches that of QCD, enabling parameter extraction.

Lattice artifacts cancel out in the scattering length when expressed in physical quantities.

Abstract

We calculate isospin 2 pion-pion scattering in chiral perturbation theory for a partially quenched, mixed action theory with Ginsparg-Wilson valence quarks and staggered sea quarks. We point out that for some scattering channels, the power-law volume dependence of two pion states in nonunitary theories such as partially quenched or mixed action QCD is identical to that of QCD. Thus one can extract infinite volume scattering parameters from mixed action simulations. We then determine the scattering length for both 2 and 2+1 sea quarks in the isospin limit. The scattering length, when expressed in terms of the pion mass and the decay constant measured on the lattice, has no contributions from mixed valence-sea mesons, thus it does not depend upon the parameter, C_Mix, that appears in the chiral Lagrangian of the mixed theory. In addition, the contributions which nominally arise from operators appearing in the mixed action O(a^2 m_q) Lagrangian exactly cancel when the scattering length is written in this form. This is in contrast to the scattering length expressed in terms of the bare parameters of the chiral Lagrangian, which explicitly exhibits all the sicknesses and lattice spacing dependence allowed by a partially quenched mixed action theory. These results hold for both 2 and 2+1 flavors of sea quarks.