Observational Probes of the Neutron Star Equation of State with Hyperons, Bosonic Dark Matter, and Quark Matter

TL;DR

The paper investigates whether a hybrid neutron-star EOS that includes hyperons, bosonic sexaquark dark matter, and deconfined quark matter can satisfy current observational constraints. It employs a DD2Y-T hadronic EOS with hyperons and S, a covariant nlNJL quark-matter EOS, and a smooth replacement interpolation crossover to connect phases, analyzing a range of sexaquark masses. Bayesian inference incorporating NICER mass–radius measurements and GW170817 tidal deformability bounds places the allowed S mass in the narrow window $m_S  [1885,1935]$ MeV, with a preferred value near $m_S  1900$ MeV and a DM fraction of about $12$–$15 ext{ }$ in the hadronic phase. The results show that including S DM softens the EOS sufficiently to reconcile stiff hadronic models with tidal and radius constraints, highlighting sexaquarks as a viable DM candidate in NS interiors within a model-dependent framework. Future observations and more detailed microphysical treatments will further test the role of such exotic particles in dense matter.

Abstract

Context. The presence of dark matter in neutron stars is of growing interest due to its potential impact on the structure and observable properties of these objects. Among the various candidates, the hypothetical sexaquark has emerged as a promising bosonic dark matter particle, potentially forming under extreme conditions in neutron star cores. Aims. We investigate whether a hybrid neutron star model that includes hyperons, bosonic dark matter (in the form of sexaquarks), and deconfined quark matter can satisfy all current observational constraints. We particularly focus on identifying the range of sexaquark masses consistent with mass-radius measurements and the tidal deformability limit. Methods. We used the DD2Y-T model for the hadronic phase, which includes hyperons, and a nonlocal Nambu-Jona-Lasinio model for the deconfined quark phase. The phase transition was modeled as a smooth crossover using the replacement interpolation construction method. Sexaquark-baryon interactions were introduced via an effective mass shift representing repulsion. We incorporated the full set of current observational data, including NICER measurements of PSRs J0437-4715 and newly published J0614-3329 data, and performed a Bayesian analysis to constrain the sexaquark mass. Results. Our results show that the presence of the sexaquark softens the equation of state, enabling the hybrid model to satisfy both the radius and tidal deformability constraints around the canonical 1.4 M_\odot neutron stars. We find that hybrid EOSs with a sexaquark mass around 1900 MeV are in agreement with all available constraints, including those from HESS J1731-347 and PSR J0952-0607, which represent the lowest and highest mass neutron stars observed to date. The Bayesian analysis favors a sexaquark mass range of 1885-1935 MeV, supporting the potential relevance of this exotic particle in neutron star interiors.

Figures (10)

• Figure 1: Pressure as a function of baryonic chemical potential for both hadronic phase and quark phase. The solid lines correspond to the hadronic phase with S for which the coupling strength of S remains constant ($x_\text{S}=0.03$), while different values of the mass of S are supposed. The QM EOSs shown with dotted lines are characterized by two key parameters: the diquark coupling ($\eta_D$) and the vector meson coupling ($\eta_V$), respectively. The hadronic EOS without S is also shown with the dashed blue line as DD2Y-T for comparison. The colored dots show the critical chemical potential in Eq. (\ref{['eq:mechequi']}) at which the RIC has been applied.
• Figure 2: Pressure as a function of baryonic chemical potential for the hadronic phase, QM phase, and the hybrid EOS constructed within the framework of RIC for $m_\text{S}=1900$ MeV.
• Figure 3: Pressure as a function of energy density for the baseline hadronic EOSs (DD2 and DD2Y-T) and the constructed hybrid EOSs. The yellow shaded region and dashed green line indicate the constraints from Hebeler:2013nza and Miller:2021qha, respectively.
• Figure 4: Fraction of S particles as a DM candidate in the hadronic phase calculated using the DD2Y-T model for various S particle masses. The fractions are shown from the respective onset densities up to densities beyond the central density of NSs, illustrating how the S fraction evolves within the purely hadronic phase. The black and red dash-dotted lines indicate the onset densities for the transition to deconfined QM, corresponding to $\mu_H$ for $m_{S}=1890$ MeV and $m_{S}=2054$ MeV, respectively.
• Figure 5: Mass-radius relations for NSs with a purely hadronic core (dashed blue line), hybrid stars without DM (dash-dotted red line), and hybrid stars including DM for different S particle masses (solid colored lines). The colored regions represent observational constraints for comparison. The 1-$\sigma$ M-R constraint from the NICER analysis of PSR J0740+6620 is shown in turquoise dittmann:2024:10215108. The updated $95\%$ and $68\%$ credible NICER results for PSR J0030+0451 are displayed as nested light and dark cyan regions vinciguerra_2023_8239000. The hatched magenta and green regions show the high-mass BW pulsar PSR J0952-0607 Romani:2022jhd and PSR J0348+0432 Antoniadis:2013pzd, respectively. The credible intervals for HESS J1731-347 are shown as nested green regions, representing the $68\%$ (inner) and $90\%$ (outer) levels based on X-ray spectral modeling doroshenko:2023:8232233. The M-R constraints for PSR J0437-4715 are represented by overlapping dark and light blue regions, corresponding to the $68\%$ (inner) and $90\%$ (outer) credible intervals choudhury:2024:13766753. The nested light and dark maroon regions show the most recent NICER analysis for the mass and radius of PSR J0614-3329, $95\%$ and $68\%$ credible results, respectively mauviard_2025_15603406. The zoomed-in region (a) highlights that all RIC EOSs including S DM pass through the $95\%$ credible region of PSR J0614–3329, whereas the RIC_DD2Y-T and DD2Y-T curves do not satisfy this constraint. The zoomed-in region (b) illustrates that for S masses larger than $1930$ MeV, the NS radius lies outside the $68\%$ credible region for PSR J0437-4715.