Superfluid helium II as the QCD vacuum

TL;DR

This paper draws parallels between superfluid helium II and QCD by comparing their topological susceptibilities, suggesting that similar long-distance physics and contact terms underpin both systems, which could deepen understanding of their topological properties.

Contribution

It demonstrates that the contact terms in superfluid helium II and QCD share a common origin related to long-distance physics beyond microscopic degrees of freedom.

Findings

Contact terms in superfluid and QCD have the same origin.

Long-distance physics plays a crucial role in topological susceptibilities.

Conceptual similarities may enhance understanding of QCD vacuum.

Abstract

We study the winding number susceptibility < I^2> in a superfluid system and the topological susceptibility < Q^2> in QCD. We argue that both correlation functions exhibit similar structures, including the generation of the contact terms. We discuss the nature of the contact term in superfluid system and argue that it has exactly the same origin as in QCD, and it is related to the long distance physics which cannot be associated with conventional microscopical degrees of freedom such as phonons and rotons. We emphasize that the conceptual similarities between superfluid system and QCD may lead, hopefully, to a deeper understanding of the topological features of a superfluid system as well as the QCD vacuum.