Equation of state for hyperonic neutron-star matter in SU(3) flavor symmetry

TL;DR

This work extends relativistic mean-field theory to SU(3) flavor symmetry to model hyperonic neutron-star matter, incorporating strange mesons and SU(3)-invariant vector-meson couplings. By introducing a quartic φ self-interaction and φ–ρ mixing, the model preserves SU(6) tendencies while enabling SU(3) flexibility, with the vector couplings tuned via α_v, θ_v, and z_v. The key finding is that the maximum neutron-star mass is governed mainly by z_v, allowing $M_{ ext{max}} o 2M_{ ext{⊙}}$ for $z_v o ext{low}$ values (e.g., $z_v \,\le\; 0.15$), while α_v also aids in sustaining large masses and θ_v has a smaller impact. The SU(3) framework thus reconciles nuclear and astrophysical constraints and offers a plausible resolution to the hyperon puzzle, predicting delayed hyperon onset and a feasible EoS compatible with NICER and GW170817 data.

Abstract

Using a relativistic mean-field model calibrated to finite-nucleus observables and bulk properties of dense nuclear matter, we investigate hyperonic neutron-star matter within an SU(3) flavor-symmetry scheme. To retain SU(6)-based couplings within SU(3) flavor symmetry, we add a quartic $φ$ self-interaction and $φ$-$ρ$ mixing. We demonstrate the roles of $α_{v}$ ($F/(F+D)$ ratio), $θ_{v}$ (mixing angle), and $z_{v}$ (singlet-to-octet coupling ratio) in SU(3)-invariant vector-meson couplings. It is found that $z_{v}$ predominantly controls the maximum mass of a neutron star, and $2M_{\odot}$ neutron stars can be supported for $z_{v}\le0.15$. The $α_{v}$ also helps sustain large masses, whereas $θ_{v}$ has a smaller effect on neutron-star properties. This SU(3) framework reconciles nuclear and astrophysical constraints, and offers a plausible resolution to the hyperon puzzle.

Figures (7)

• Figure 1: Equations of state for neutron stars. The SU(6)-symmetry cases are shown with and without hyperons, together with the SU(3)-symmetry cases for $z_{v}=0.00$, $0.25$, and $0.50$ with $\alpha_{v}=\alpha_{v}^{id}$ and $\theta_{v}=\theta_{v}^{id}$ (case A). The filled circles and triangles denote the maximum-mass points for SU(3) and SU(6) symmetry, respectively.
• Figure 2: Mass-radius relations of neutron stars. The observational data are supplemented by the NICER constraints Vinciguerra:2023qxqChoudhury:2024xbkSalmi:2024aumSalmi:2024bssMauviard:2025dmd. We show the cases for $z_{v}=0.00$, $0.25$, and $0.50$ with $\alpha_{v}=\alpha_{v}^{id}$ and $\theta_{v}=\theta_{v}^{id}$ in SU(3) symmetry (case A).
• Figure 3: Correlation between $\theta_{v}$ and $z_{v}$ in the maximum mass of neutron stars, $M_{\textrm{max}}$, for $\alpha_{v}=\alpha_{v}^{id}$. The red thick line is the $2M_{\odot}$ limit, while the white lines denote contours of $M_{\textrm{max}}/M_{\odot}$ at intervals of 0.1. The magenta dot represents the case of SU(6) symmetry.
• Figure 4: Correlation between $\alpha_{v}$ and $z_{v}$ in the maximum mass of neutron stars, $M_{\textrm{max}}$, for $\theta_{v}=36.5^{\circ}$ParticleDataGroup:2024cfk. The magenta dot represents the case of the ideal mixing. The colored lines are the same as in Fig. \ref{['fig:NSmax-alpha']}.
• Figure 5: Chemical potential, $\mu_{B}$, and partial fractions, $Y_{i}$, in neutron-star matter as a function of $n_{B}/n_{0}$. We show the cases for $z_{v}=0.00$, $0.25$, and $0.50$ with $\alpha_{v}=\alpha_{v}^{id}$ and $\theta_{v}=\theta_{v}^{id}$ in SU(3) symmetry (case A). The filled circle denotes the onset of hyperons in the upper panels.The thick vertical lines in the lower panels shows the density at which a neutron star reaches the maximum-mass point by solving the TOV equation.