Testing improved actions for dynamical Kogut-Susskind quarks
Kostas Orginos, Doug Toussaint
TL;DR
The paper addresses whether Naik and fat-link improvements to the Kogut-Susskind quark action extend from quenched to dynamical QCD and explores flavor-symmetry restoration across the full pion set to guide lattice-spacing choices. Using a Symanzik improved gauge action and four valence KS actions on dynamical two- and four-flavor lattices, the authors measure the pion spectrum and dispersion and interpolate to $m_\pi/m_\rho=0.55$ for comparison, focusing on $c^2 = \frac{E^2(\vec{k})-E^2(\vec{0})}{\vec{k}^2}$ and $\delta_2=\frac{M_\pi^2-M_G^2}{M_\rho^2-M_G^2}$. They find that the Naik term improves dispersion (bringing $c^2$ closer to 1) and fat-link averaging reduces flavor-symmetry breaking, though nonlocal pions remain heavier than the Goldstone and receive less benefit from valence-fatting; gauge-action improvements also help. The results validate using quenched-improvement insights in full QCD and provide practical guidance on lattice spacing and temporal extent for finite-temperature QCD studies, including rough estimates of required time slices for target temperatures.
Abstract
We extend tests of "Naik" and "fat link" improvements of the Kogut-Susskind quark action to full QCD simulations, and verify that the improvements previously demonstrated in the quenched approximation apply also to dynamical quark simulations. We extend the study of flavor symmetry improvement to the complete set of pions, and find that the nonlocal pions are significantly heavier than the local non-Goldstone pion. These results can be used to estimate the lattice spacing necessary for realistic simulations with this action.