Phase Transitions, theta Behavior and Instantons in QCD and its Holographic Model

TL;DR

This paper explores the phase transition in QCD and its holographic model, focusing on theta dependence, instanton behavior, and the dissociation into fractional instanton quarks at the confinement-deconfinement transition.

Contribution

It introduces a new picture of the confinement-deconfinement transition involving instanton dissociation into fractional instanton quarks within a holographic QCD framework.

Findings

Theta dependence sharply changes at the phase transition.

Instanton expansion breaks down at critical temperature T_c.

High temperature phase is a dilute instanton gas; low temperature phase has fractional instanton quarks.

Abstract

In the holographic model of QCD, theta dependence sharply changes at the point of confinement deconfinement phase transition. In large N QCD such a change in theta behavior can be related to the breakdown of the instanton expansion at some critical temperature T_c. Associating this temperature with confinement-deconfinement phase transition leads to the description of the latter in terms of dissociation of instantons into the fractionally charged instanton quarks. To elucidate this picture, we introduce the nonvanishing chiral condensate in the deconfining phase and assume a specific lagrangian for the eta' field in the confining phase. In the resulting picture the high temperature phase of the theory consists of the dilute gas of instantons, while the low temperature phase is described in terms of freely moving fractional instanton quarks.