Layers of deformed instantons in holographic baryonic matter

TL;DR

This paper improves the modeling of baryonic matter in holographic QCD by allowing anisotropic instanton deformations and multiple layers, revealing phase transitions and conditions for more realistic dense matter descriptions.

Contribution

It introduces an enhanced approximation for holographic baryonic matter that includes anisotropic instantons and multiple layers, enabling more realistic phase transition predictions.

Findings

Baryon onset is a second-order phase transition under initial assumptions.

Imposing shape constraints leads to a first-order baryon onset and high-density chiral restoration.

Two instanton layers can form dynamically at high densities.

Abstract

We discuss homogeneous baryonic matter in the decompactified limit of the Sakai-Sugimoto model, improving existing approximations based on flat-space instantons. We allow for an anisotropic deformation of the instantons in the holographic and spatial directions and for a density-dependent distribution of arbitrarily many instanton layers in the bulk. Within our approximation, the baryon onset turns out to be a second-order phase transition, at odds with nature, and there is no transition to quark matter at high densities, at odds with expectations from QCD. This changes when we impose certain constraints on the shape of single instantons, motivated by known features of holographic baryons in the vacuum. Then, a first-order baryon onset and chiral restoration at high density are possible, and at sufficiently large densities two instanton layers are formed dynamically. Our results are a further step towards describing realistic, strongly interacting matter over a large density regime within a single model, desirable for studies of compact stars.