Light hadron spectroscopy using domain wall valence quarks on an Asqtad sea

TL;DR

This study computes the light hadron spectrum in full QCD using mixed-action lattice techniques, revealing unexpected linear behavior of nucleon mass with pion mass and highlighting challenges in chiral extrapolations.

Contribution

It introduces a mixed-action approach with domain wall valence quarks on Asqtad sea quarks and analyzes baryon masses with advanced chiral perturbation theory.

Findings

Nucleon mass is nearly linear in pion mass across the studied range.

Next-to-next-to-leading order SU(2) fits match experimental nucleon mass.

Unexpected linearity of nucleon mass with pion mass observed in dynamical calculations.

Abstract

We calculate the light hadron spectrum in full QCD using two plus one flavor Asqtad sea quarks and domain wall valence quarks. Meson and baryon masses are calculated on a lattice of spatial size $L \approx 2.5$\texttt{fm}, and a lattice spacing of $a \approx 0.124$\texttt{fm}, for pion masses as light as $m_\pi \approx 300$\texttt{MeV}, and compared with the results by the MILC collaboration with Asqtad valence quarks at the same lattice spacing. Two- and three-flavor chiral extrapolations of the baryon masses are performed using both continuum and mixed-action heavy baryon chiral perturbation theory. Both the three-flavor and two-flavor functional forms describe our lattice results, although the low-energy constants from the next-to-leading order SU(3) fits are inconsistent with their phenomenological values. Next-to-next-to-leading order SU(2) continuum formulae provide a good fit to the data and yield and extrapolated nucleon mass consistent with experiment, but the convergence pattern indicates that even our lightest pion mass may be at the upper end of the chiral regime. Surprisingly, our nucleon masses are essentially lineaer in $m_\pi$ over our full range of pion masses, and we show this feature is common to all recent dynamical calculations of the nucleon mass. The origin of this linearity is not presently understood, and lighter pion masses and increased control of systematic errors will be needed to resolve this puzzling behavior.