Finite density nuclear matter and neutron stars in hard-wall AdS/QCD model

TL;DR

The paper develops a bottom-up hard-wall AdS/QCD framework with baryons treated as solitons and employs a homogeneous, z-dependent ansatz that includes the pure gauge sector to study dense nuclear matter. By minimizing the grand potential Ω with respect to density and chiral order parameters, it achieves decoupling of chiral restoration from baryon onset in some parameter regimes and reveals a pseudoconformal-like approach of the speed of sound toward the conformal limit near $6\rho_0$ while $T^\mu_\mu$ remains nonzero. The resulting equation of state is tightly constrained and yields neutron-star mass–radius relations in agreement with current observations, with the chiral sector playing a key role in high-density behavior. The work also identifies avenues for refinement, including breaking the U(2) symmetry, incorporating isospin and spin, and exploring explicit chiral-symmetry breaking effects to improve quantitative fits.

Abstract

We investigate properties of nuclear matter, equation of state (EOS) of neutron stars and its mass-radius relation in a hard-wall AdS/QCD model by regarding baryons as solitonic configurations in gauge fields. Compared with previous approaches, we employ a different homogeneous approximation that takes into account the equations of motion for the pure gauge fields. By choosing appropriate parameters, we realize a chiral phase transition within the baryonic phase, where the chiral condensate decreases with the baryon chemical potential, until it reaches zero -- chiral symmetry is restored. In addition, independent of the existence of chiral phase transition, we also find that the speed of sound converges to the conformal limit at the density relevant to cores of massive stars but the trace of energy-momentum tensor does not vanish which indicates the pseudoconformal structure and intrinsic manifestation of scale symmetry in compact star matter. Through calculations, we obtain an equation of state that is more tightly constrained than previous works, and the resulting mass-radius relation of neutron stars is consistent with current observations.

Figures (10)

• Figure 1: The image obtained under different $\mu_c$, with all other parameters fixed. The solution when $\mu_c \ne 0$ is equivalent to the solution when $\mu_c = 0$, but shifted by constant $\mu_c$.
• Figure 2: Contour plot of the grand potential $\Omega$ without including scalar field $\Phi$.
• Figure 3: The log-log plot of baryon chemical potential $\mu$ versus $\rho/\rho_0$.
• Figure 4: The sound speed plots under three different sets of parameters.
• Figure 5: The value of $\mathcal{E}-3P$ from three sets of parameters.