Axial-anomaly effects and chiral phase structure in holographic QCD

TL;DR

This paper investigates how axial-anomaly effects influence the chiral phase transition in a holographic QCD model, revealing that different anomaly profiles can lead to diverse phase structures, including crossover, second-order, or first-order transitions.

Contribution

It introduces a holographic model incorporating the axial anomaly with a dynamic $ ext{U}(3)$ extension, analyzing its impact on the QCD phase diagram and the $ ext{η}$-$ ext{η}^ ext{' }$ system.

Findings

Anomaly profile significantly affects the chiral phase transition nature.

Some profiles predict a crossover or second-order transition across all quark masses.

Other profiles produce a first-order transition in the light-quark region.

Abstract

We study the impact of axial-anomaly effects on the chiral phase structure in a $U(3)$-extended soft-wall holographic QCD model. Including the pseudoscalar singlet sector allows for a dynamical description of the $\eta$-$\eta^\prime$ system through a determinant interaction with a holographic-coordinate-dependent strength. Vacuum pseudoscalar observables, particularly the $\eta^\prime$ mass and the $\eta$-$\eta^\prime$ mixing pattern, constrain the overall magnitude of the anomaly contribution but leave its holographic profile largely undetermined. We then examine how different anomaly profiles consistent with vacuum phenomenology affect the finite-temperature chiral transition. Constructing the Columbia plot within this framework, we find that the predicted phase structure depends sensitively on the anomaly implementation: some profiles yield crossover/second-order behavior across the entire quark-mass plane, while others generate a first-order region in the light-quark corner. These results highlight the strong sensitivity of the holographic QCD phase structure to the modeling of axial-anomaly effects.