Heavy Neutron Star Phenomenology with an H-dibaryon

The equation of state for dense nuclear matter in $β$-equilibrium is explored including the possibility of a doubly-strange H-particle. Consistent with experimental constraints, the mass of the H in free space is taken to be near the $Λ, Λ$ threshold. Within the quark-meson coupling model, which we use, no new parameters are required to describe the interaction between the H-dibaryon and the other baryons. The maximum mass is only slightly reduced, and the tidal deformability is essentially unchanged with this addition. In heavy neutron stars the H is abundant and extends as far as 6 km from the center of the core.

💡 Research Summary

This paper investigates the impact of a doubly‑strange H‑dibaryon on the equation of state (EoS) of dense β‑equilibrated nuclear matter and on the observable properties of neutron stars (NS). The authors work within the quark‑meson coupling (QMC) framework, a relativistic mean‑field model in which the σ, ω and ρ mesons couple directly to the confined quarks inside baryons. Because the meson‑quark couplings are fixed by the underlying bag‑model description, the interaction of the H‑dibaryon with the other baryons is determined without introducing any new free parameters; only the free‑space mass of the H, (M_H), must be specified.

Experimental limits from double‑Λ hypernuclei place the H mass just above the ΛΛ threshold, while recent lattice QCD calculations suggest a bound state very close to that threshold. The authors therefore adopt a mass range of 2247–2269 MeV, corresponding to a binding energy of roughly –26 ± 11 MeV relative to two Λ’s. Within QMC the effective in‑medium mass (M_H^*) is expressed as a linear‑plus‑quadratic function of the scalar field σ, with weight factors roughly twice those of the Λ because the H contains twice as many light quarks. The scalar polarizability of the bag reduces the σ‑H coupling at high density, while the vector ω coupling provides a repulsive contribution that dominates the net H‑H interaction.

Solving the mean‑field equations shows that the H appears at a baryon density of about 0.6 fm⁻³ (≈3 n₀), i.e. after the Λ hyperon but before the appearance of Ξ hyperons. When the H condensates, the ω field weakens and the σ field strengthens, reflecting the balance between repulsive ω exchange and attractive σ exchange. The H does not couple to the isovector ρ meson, so its presence slightly reduces the isovector mean field through the altered neutron fraction.

The authors incorporate a short‑range “overlap” repulsion term, previously used in QMC to mimic excluded‑volume effects at high density. This term mitigates the softening of the EoS that normally accompanies the onset of new degrees of freedom. Consequently, the pressure–energy density relation is only modestly softened when the H appears, and the maximum NS mass is reduced by at most ~0.04 M⊙ compared with the H‑free QMC model. All three mass choices still yield (M_{\rm max}>2.0,M_{\odot}), comfortably satisfying the most massive pulsar observations.

Particle‑fraction plots reveal that the H can dominate the central composition of the most massive stars, extending outward to a radius of roughly 6 km. Its presence pushes the threshold densities for the Ξ⁰ and Ξ⁻ hyperons to higher values; in some cases Ξ⁰ never appears at all. Because the H only appears at densities well above three times nuclear saturation, the low‑mass (≈1.4 M⊙) stars remain unchanged, and the tidal deformability Λ₁.₄ stays at the previously reported value of ~430. Thus the inclusion of the H does not affect the agreement with GW170817 constraints.

In summary, the study demonstrates that a near‑threshold H‑dibaryon can be accommodated within a self‑consistent QMC description of dense matter without violating current astrophysical constraints. The H reduces the central density and pressure of the heaviest stars but leaves the observable mass–radius relation and tidal deformability essentially intact. This work therefore supports the possibility that the H‑dibaryon, if it exists, could be a genuine component of neutron‑star cores, and it motivates further experimental searches and lattice QCD refinements to pin down its mass and interaction strengths.