Chiral symmetry restoration and hyperon suppression in neutron stars

TL;DR

The paper tackles the hyperon puzzle in neutron stars by formulating an SU(3) linear parity doublet model in the representation $(3,\bar{3})+(\bar{3},3)$, consistently incorporating chiral symmetry restoration and the chiral invariant mass $m_0$. Hyperon onset densities rise with increasing $m_0$, and for $m_0 \gtrsim 750$ MeV hyperons can be delayed beyond the typical quark-hadron transition density ($2$--$5 n_0$), enabling deconfinement before hyperons appear and preventing excessive EoS softening. The approach provides a natural, symmetry-driven mechanism to address the hyperon puzzle without ad hoc repulsive hyperon interactions, while highlighting the dependence on model choices and the potential need for higher chiral representations to reproduce hyperon spectra and potentials. The results have implications for dense-matter composition, NS cooling, and gravitational-wave signals, and point to future work incorporating additional chiral channels and refined vector interactions.

Abstract

The ``hyperon puzzle'' remains a fundamental challenge in nuclear astrophysics. We investigate hyperon emergence in neutron star matter using the $SU(3)$ parity doublet model with chiral representation $(3,\bar{3}) + (\bar{3},3)$. This framework naturally incorporates chiral symmetry restoration and provides a systematic description of baryon masses in dense matter through the interplay between the chiral condensate and the chiral invariant mass $m_0$. We find that the hyperon onset density exhibits strong sensitivity to $m_0$: for $m_0 = 500$ MeV, hyperons first appear at $1.9n_0$ while for $m_0 \gtrsim 750$ MeV, hyperons emerge only above $5n_0$. This delayed onset arises from the weakened density dependence of baryon masses at larger $m_0$ values. When the hyperon onset density exceeds the expected quark-hadron transition range ($2$--$5n_0$), matter undergoes deconfinement before hyperons populate, avoiding the EoS softening while maintaining consistency with massive neutron star observations. Our results demonstrate that chiral dynamics provides a natural resolution to the hyperon puzzle without requiring ad hoc repulsive hyperon interactions.

Figures (8)

• Figure 1: Masses of $N$ and $\Xi$ as functions of the chiral condensate $\sigma$, with $m_0$ fixed to be 800 MeV.
• Figure 2: Vacuum potential for $\sigma$ and $\sigma_s$ with $m_0$ fixed to be 800 MeV
• Figure 3: EoS in the low-density regime. Upper panel: $L = 50, 60, 70, 75$ MeV with fixed $m_0=800$ MeV. Lower panel: $m_0 = 500, 600, 700, 800, 900$ MeV with fixed $L=70$ MeV. The shaded blue band shows the 1$\sigma$ uncertainty range from $\chi$EFT Drischler:2017wttDrischler:2020fvzKeller:2022crb.
• Figure 4: Masses of $N, N^* \Xi, \Xi^*$ and $\Lambda, \Lambda^*$ as functions of the normalized baryon number density, with $m_0=600, 800$ MeV and $g_{\phi NN}=0$. The solid curves denote for the ground state mass while the dashed curves denote for its excited state.
• Figure 5: Particle fractions $n_H/n_B$ as functions of normalized baryon density $n_B/n_0$ for different chiral invariant masses $m_0 = 600, 700, 800, 900$ MeV with $g_{\phi NN}=0$. The figure shows the evolution of neutrons (n), protons (p), hyperons ($\Xi^-, \Xi^0$), and leptons (e, $\mu$) in neutron star matter under $\beta$-equilibrium and charge neutrality conditions. The onset densities of hyperons increase significantly with larger $m_0$ values.