Aspects of QCD Vacuum Structure

TL;DR

This paper investigates how dynamical fermions influence the topological structure of the QCD vacuum using lattice simulations, revealing enhanced topological features and a layered sign-alternating structure.

Contribution

It introduces a new gluonic definition of topological charge density with an over-improved stout-link smearing algorithm, and studies the effects of dynamical fermions on QCD topology.

Findings

Dynamical fermions increase the negative dip in the topological charge correlator.

The positive contact term in the correlator is also enhanced with dynamical fermions.

The layered sign-alternating structure of the QCD vacuum is confirmed.

Abstract

The impact of dynamical fermions on the vacuum structure of QCD is explored. Of particular interest is the topological charge correlator, $<q(x) q(0) >$, where negative values at small $x$ reveal a sign-alternating layered structure to the topological-charge density of the QCD vacuum. We consider large $28^3 \times 96$ lattices from the MILC collaboration, and develop a new gluonic definition of the topological charge density, founded on a new over-improved stout-link smearing algorithm. The algorithm reproduces established results from the overlap formalism and is designed to preserve instantons. We examine the extent to which instanton-like objects are found on the lattice. Finally, we investigate the effects of dynamical sea-quark degrees of freedom on topology and find that the magnitudes of the negative dip in the $<q(x)q(0)>$ correlator and the positive $<q(0)^2>$ contact term are both increased with the introduction of dynamical fermion degrees of freedom. This is in accord with expectations based on charge renormalization and the vanishing of the topological susceptibility in the chiral limit.