Mahboubeh Shahrbaf

We investigate the impact of a hypothetical bosonic dark matter (DM) candidate, the sexaquark, on the fundamental ( f -mode) oscillations of neutron stars (NSs). By varying the DM particle mass and considering different core compositions including hypernuclear matter, sexaquark DM, and deconfined quark matter (QM), we construct hybrid equations of state (EOS) with a smooth hadron-quark crossover that remain consistent with current astrophysical constraints on mass ( M ), radius ( R ), and tidal deformability (Λ). Our analysis shows that the presence of these exotic components systematically alters quasi-universal f -mode relations considering f -mode frequency ( f ), damping time ( τ ), compactness ( C ), and angular velocity ( ω ). In particular, relations involving f -√( M / R ³ ), ( R ⁴ / M ³ τ )( C ), ω M ( C ), require higher-order polynomial fits compared to standard studies. Quadratic forms remain sufficient for f -√( M / R ³ ) and ω M ( C ), while damping-time relations such as ( R ⁴ / M ³ τ)( C ) demand higher-order corrections to capture their curvature. For f (Λ), a cubic fit provides a satisfactory description. Within this extended framework the relations remain tight and effectively composition independent. These results suggest that precise f -mode measurements with future gravitational-wave detectors could provide clear signatures of DM and other exotic matter in NS interiors.

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 _⊙ 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.

We investigate the equation of state (EOS) and macroscopic properties of neutron stars (NSs) and hyperonic stars within the framework of the lowest-order constrained variational (LOCV) method, extended to include interacting Λ hyperons. The nucleon–nucleon interaction is modeled using the AV18 potential supplemented by Urbana three-body forces, while Λ N and ΛΛ interactions are described by phenomenological spin- and parity-dependent potentials calibrated to available hypernuclear data. Cold, charge-neutral, and β -equilibrated matter composed of neutrons, protons, electrons, muons, and Λ hyperons is considered. We compute particle fractions, chemical potentials, the EOS, speed of sound, tidal deformability, and stellar structure by solving the Tolman–Oppenheimer–Volkoff equations, and compare our results with recent NICER and gravitational-wave observations. The inclusion of Λ hyperons leads to EOS softening, reducing the maximum NS mass from 2.34 M _⊙ to 2.07 M _⊙ , while keeping it consistent with the 2 M _⊙ mass constraint. At 1.4 M _⊙ , the model satisfies observational limits on radius and tidal deformability, with the Λ onset occurring below this mass. Comparison with other microscopic and relativistic mean-field models shows that our EOS remains consistent with the allowed pressure–energy density range, while also permitting even canonical-mass NSs of about 1.4 M _⊙ to accommodate hyperons. These results suggest that hyperons can appear in NSs across the observed mass range without violating current astrophysical constraints, and that the extended LOCV method provides a consistent, microscopic approach to modeling dense hypernuclear matter.

We study the appearance of sexaquark (uuddss) as a candidate of dark matter in the core of neutron stars while there has been no experimental evidence of this particle in laboratories so far. Regarding the observational constraints of supermassive pulsars as well as gravitational waves, we encounter a sexaquark dilemma for which we introduce a solution by quark deconfinement fulfilling all present constraints from multi-messenger astronomy, i.e., tidal deformability in accordance with GW170817 and maximum mass above the lower limit from PSR J0740+6620.

We present a systematic investigation of the possible locations for the special point (SP), a unique feature of hybrid neutron stars in the massradius diagram. The study is performed within the two-phase approach where the high-density (quark matter) phase is described by the covariant nonlocal Nambu–Jona-Lasinio (nlNJL) model equation of state (EOS) which is shown to be equivalent to a constant-sound-speed (CSS) EOS. For the nuclear matter phase around saturation density different relativistic density functional EOSs are used: DD2p00, its excluded-volume modification DD2p40 and the hypernuclear EOS DD2Y-T. In the present contribution we apply the Maxwell construction scheme for the deconfinement transition and demonstrate that a simultaneous variation of the vector and diquark coupling constants results in the occurrence of SP "trains" which are invariant against changing the nuclear matter EOS. We propose that the SP train corresponding to a variation of the diquark coupling at constant vector coupling is special since it serves as a lower bound for the line of maximum masses and accessible radii of massive hybrid stars.

We present a systematic investigation of the possible locations for the special point (SP), a unique feature of hybrid neutron stars in the mass-radius diagram. The study is performed within the two-phase approach where the high-density (quark matter) phase is described by the covariant nonlocal Nambu--Jona-Lasinio (nlNJL) model equation of state (EOS) which is shown to be equivalent to a constant-sound-speed (CSS) EOS. For the nuclear matter phase around saturation density different relativistic density functional EOSs are used: DD2p00, its excluded-volume modification DD2p40 and the hypernuclear EOS DD2Y-T. In the present contribution we apply the Maxwell construction scheme for the deconfinement transition and demonstrate that a simultaneous variation of the vector and diquark coupling constants results in the occurrence of SP "trains" which are invariant against changing the nuclear matter EOS. We propose that the SP train corresponding to a variation of the diquark coupling at constant vector coupling is special since it serves as a lower bound for the line of maximum masses and accessible radii of massive hybrid stars.