Properties of H particle-admixed compact star

TL;DR

This work assesses whether the H particle, a flavor-singlet $uuddss$ hexaquark with $J^P=0^+$, can exist as a stable component in neutron-star matter. The mass is estimated in the Chromomagnetic Interaction framework, yielding $m_H=2212.7~\mathrm{MeV}$, below the $\Lambda\Lambda$ threshold, which motivates incorporating the H into a covariant RMF/RMFL description of dense matter. By treating the H–meson couplings $g_{\sigma H}$ and $g_{\omega H}$ as free and allowing density-dependent $g_{\rho}$ in RMFL, the study maps onset densities, stability regions, particle fractions, and the neutron-star equation of state across multiple parameter sets. The results show that the H particle generally softens the EOS and lowers the maximum NS mass, but with sufficiently strong repulsive interactions the presence of the H particle can be compatible with $M_{\rm max}\sim 2~M_\odot$ for some stiff EOSs; overall, the H-dibaryon remains a plausible but tightly constrained degree of freedom in NS interiors.

Abstract

We explore the potential manifestation of a hexaquark, the H particle, as a constituent within neutron stars. The H particle, characterized by a quark composition of $uuddss$, is constructed using the framework of Chromomagnetic Interaction (CMI). Specifically, we contemplate the flavor-singlet state H with $J^P=0^+$. Our computations indicate that the three-flavor hexaquark state, the H particle, possesses a lower mass of $2212.7~\rm{MeV}$ in comparison to the $d^*(2380)$, implying greater stability than the two-flavor $d^*(2380)$. The analysis involving the H particle is carried out using the relativistic mean-field (RMF) model. We investigate the influence of H particle couplings, a key factor in determining the system stability, and focus on the potential existence of H particle within neutron stars. We find that H particle could potentially endure as a stable constituent within neutron stars, and lead to a reduction of the maximum mass.

Figures (6)

• Figure 1: Onset density (in unit of $\rm{fm}^{-3}$) of H particle in $\beta$-stable NS matter as a function of the dimensionless coupling ${\chi}_{\sigma H}$ and ${\chi}_{\omega H}$.
• Figure 2: The solid blue line shows the area boundary of the zone where H particle's appearance leads to decreasing pressure. The red line corresponds to the critical boundary where Yukawa potential equals zero.
• Figure 3: Particle fractions of protons, neutrons, H particles, electrons, and muons ($Y_p, Y_n, Y_H, Y_e, Y_\mu$) as functions of the baryon number density $n_b$ in different models. The three cases of H particle couplings used, (${\chi}_{\sigma H}$, ${\chi}_{\omega H}$)=(0, 0), (0, 0.5), (0.5, 1), correspond to free H particle, purely repulsive interaction and both attractive and repulsive interaction.
• Figure 4: Pressure $P$ as a function of $n_b$ with different couplings. The results using couplings (${\chi}_{\sigma H}$, ${\chi}_{\omega H}$)=(0, 0), (0, 0.5), (0.5, 1), (1, 1.5), (0, 1) are shown. H particle admixed pressures split from those of pure hadronic matter when the H particles appear.
• Figure 5: Mass-radius relations and mass-central density relations for different models and couplings. The results of pure hadronic EOS (dash-dot lines) are compared with those including H particle for different couplings (same as Fig. \ref{['fig:4-nbp']}). The shaded areas correspond to simultaneous measurements of the mass and radius range from NICER for PSR J0030+0451 Riley2019Miller2019 and PSR J0740+6620, respectively Riley2021Miller2021. The hypothesis that the second component of GW190814 is a NS is also depicted Abbott2020. The inferred radius constraint from GW170817 is shown by grey line Annala2018.