Holographic Soliton Crystals for Dense Nuclear Matter and Neutron Stars

Abstract

We construct a nuclear-matter equation of state (EOS) from holographic QCD in the Witten-Sakai-Sugimoto (WSS) model, going beyond the homogeneous ansatz by building dense baryonic matter more directly from the solitonic description of holographic baryons. Employing the two-baryon interaction potential obtained from the linearized soliton tails sourced in the curved background by point-like (core) instantons, we assemble an infinite face-centered cubic (FCC) crystal with nearest neighbors in the most attractive channel as an approximation to the quantum liquid of baryons and compute the energy density $\mathcal{E}(n_B)$, the chemical potential $\mu_B$, and the pressure $P$. We calibrate the symmetric-matter EOS by fixing the 't Hooft coupling $\lambda$ and the WSS scale $M_{\rm KK}$ to saturation-density and onset-chemical-potential properties, and by including a quark-mass term to reproduce the physical pion mass $m_\pi=135\,\mathrm{MeV}$. This fit turns out to remain reasonably close to the parameters required by the vacuum meson sector, while their modification is consistent with Brown-Rho scaling in a dense medium, and the incompressibility at saturation is of the correct order of magnitude, both in clear contrast to the homogeneous approximation. We then extend to beta-equilibrated matter, using phenomenological input for the symmetry energy, and obtain hybrid EOS and neutron-star observables that are compatible with the NICER constraints.