Neutron star with dark matter using vector portal

Abstract

Compact astrophysical objects, such as neutron stars, can provide a unique environment where the interplay between strongly interacting nuclear matter and dark matter (DM) can yield possible observable signatures. We investigate here the impact of fermionic DM interacting with nucleons via a vector mediator ($Z^\prime$) portal inside neutron stars using the relativistic mean field (RMF) framework. Unlike scalar portal DM models, which primarily modify the effective nucleon mass through scalar interactions, vector mediators introduce additional repulsive interactions that directly affect the baryonic chemical potential and the pressure of dense matter. We show that the precise measurements of neutron star properties, including the mass radius relation and tidal deformability from gravitational wave observations, X-ray and radio observations of pulsars, can shed light on properties of DM. We study the gross structural properties of a neutron star using the Tolman Oppenheimer Volkoff (TOV) equations, employing an equation of state (EOS) for neutron star matter in the presence of vector portal-assisted DM. The resulting stellar configurations consistent with observational bounds from gravitational wave observations in LIGO/Virgo, and X-ray observations of pulsars in NICER, are shown to constrain the vector portal DM parameters. It is observed that, while large portal mass can soften the EOS of the DM admixed neutron star matter, the light portal mass can make the EOS stiffer at large densities resulting in distinct mass-radius relation and the tidal deformability between the two scenarios. The vector portal DM scenario, with DM interaction with quarks via $Z^\prime$ vector boson, can establish a direct connection to terrestrial searches, including direct and indirect detection and collider searches for the $Z^\prime$ boson.

Figures (11)

• Figure 1: Variation of the mean meson fields $\sigma_0$, $\omega_0$, $\rho_0$ as a function of baryon density $n_{B}$ for NS matter in the presence of vector portal DM for parameter set 1. The left most panel corresponds to a scalar field $\sigma_0$, middle panel for a vector meson field $\omega_0$ , and right most panel for a isovector field $\rho_0^3$. The numerical results are shown for pure nuclear matter (NM) and different DM Fermi momenta $k_{\rm F\chi} = 10,\, 20,\ 30\, {\rm MeV}$.
• Figure 2: Mean-field value of the vector portal mediator $Z'_0$ as a function of baryon density $n_B$ for different DM Fermi momenta $k_{F\chi} = 10,\, 20,\, 30$ MeV, shown for two representative parameter sets: (a) Set 1 corresponding to a heavy mediator (left-panel), and (b) Set 3 corresponding to a light mediator (right-panel).
• Figure 3: Equation of state (pressure $\mathcal{P}$ as a function of energy density $\mathcal{E}$) for NS matter admixed with fermionic DM via a vector portal interaction, shown for set 1 (left-panel). The corresponding mass radius relation ($M-R$ curve) is shown in the right-panel with various astrophysical observations (see text). The red solid curve corresponds to pure nuclear matter (NM), while dashed, dot-dashed, and dotted curves represent DM admixture with Fermi momenta $k_{F\chi} = 10,\, 20,\, 30$ MeV, respectively.
• Figure 4: EoS for NS matter admixed with fermionic DM via a vector portal interaction, shown for set 2 (left-panel). The corresponding $M-R$ curve is shown in the right-panel. The red solid curve corresponds to pure nuclear matter (NM), while dashed, dot-dashed, and dotted curves represent DM admixture with Fermi momenta $k_{F\chi} = 10,\, 20,\, 30$ MeV, respectively.
• Figure 5: Equation of state (left panel) and corresponding mass–radius relations (right panel) for NS matter admixed with fermionic DM via a vector portal interaction for Set 3, corresponding to a light mediator ($m_{Z^\prime} = \hbox{100,MeV}$). The red solid curves represent pure nuclear matter (NM), while dashed, dot-dashed, and dotted curves correspond to DM admixture with Fermi momenta $k_{F\chi} = 10,\, 20,\, 30$ MeV respectively.