Impact of Dynamical Fermions on QCD Vacuum Structure

TL;DR

This study investigates how dynamical fermions influence the topological structure of the QCD vacuum, revealing increased topological charge fluctuations and larger instanton-like objects compared to quenched configurations.

Contribution

It introduces a new over-improved stout-link smearing algorithm that preserves topological objects and compares the topological features of quenched and dynamical QCD vacua.

Findings

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

Dynamical gauge fields contain more and larger instanton-like objects.

Topological charge density visualization shows enhanced topological fluctuations with dynamical fermions.

Abstract

We examine how dynamical fermions affect both the UV and infrared structure of the QCD vacuum. We consider large $28^3 \times 96$ lattices from the MILC collaboration, using a 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 nontrivial topological objects including instantons. At short distances we focus on 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 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. At large distances we examine the extent to which instanton-like objects are found on the lattice, and how their distributions vary between quenched and dynamical gauge fields. We show that dynamical gauge fields contain more instanton-like objects with an average size greater than in the quenched vacuum. Finally, we directly visualize the topological charge density in order to investigate the effects of dynamical sea-quark degrees of freedom on topology.