Quarkyonic or baryquark matter? On the dynamical generation of momentum space shell structure

TL;DR

This study compares two models of dense QCD matter, quarkyonic and baryquark, finding baryquark matter energetically favored and highlighting the need for modifications to favor quarkyonic matter.

Contribution

It introduces a detailed analysis of quarkyonic versus baryquark matter, demonstrating the energetic preference for baryquark matter in a simplified model.

Findings

Baryquark matter is energetically favored over quarkyonic matter.

Both models show similar hadron-quark transition behavior.

Baryquark matter exhibits physically acceptable sound speed without regulators.

Abstract

We study the equation of state of a mixture of (quasi-)free constituent quarks and nucleons with hard-core repulsion at zero temperature. Two opposite scenarios for the realization of the Pauli exclusion principle are considered: (i) a Fermi sea of quarks surrounded by a shell of baryons -- the quarkyonic matter, and (ii) a Fermi sea of nucleons surrounded by a shell of quarks which we call \emph{baryquark matter}. In both scenarios, the sizes of the Fermi sea and shell are fixed through energy minimization at fixed baryon number density. While both cases yield a qualitatively similar transition from hadronic to quark matter, we find that baryquark matter is energetically favored in this setup and yields a physically acceptable behavior of the speed of sound without the need to introduce an infrared regulator. In order to retain the theoretically more appealing quarkyonic matter as the preferred form of dense QCD matter will thus require modifications to the existing dynamical generation mechanisms, such as, for example, the introduction of momentum-dependent nuclear interactions.