Dirac vs. Majorana Dark Matter Imprints on Neutron Star Observables

TL;DR

This paper explores how the fundamental fermionic nature of dark matter, whether Dirac or Majorana, influences neutron star properties and how astrophysical observations can distinguish between these two possibilities.

Contribution

It develops self-consistent equations of state for Dirac and Majorana dark matter coupled to nuclear matter and analyzes their impact on neutron star structure within a relativistic mean-field framework.

Findings

Dirac dark matter softens the equation of state more than Majorana dark matter.

Dirac dark matter leads to smaller neutron star radii and lower maximum masses.

Current observations can potentially discriminate between Dirac and Majorana dark matter models.

Abstract

The fundamental character of a fermionic dark matter, whether it is a Dirac or Majorana particle remains a key unresolved issue whose answer would profoundly affect dark-sector phenomenology and detection strategies thereby motivates complementary probes across particle and astrophysical experiments. Compact stars, particularly neutron stars, offer unique astrophysical laboratories for probing such fundamental properties under extreme densities. The presence of a fermionic DM admixed with nuclear matter can modify the equation of state, thereby affecting observable quantities such as the mass-radius (M-R) relation and tidal deformability. In this work, we investigate how the intrinsic particle nature of fermionic DM influences neutron star structure. Within a relativistic mean-field framework extended by a scalar (or Higgs like) portal coupling between DM and nucleons, we construct self-consistent equation of states for both Dirac and Majorana cases and solve the Tolman-Oppenheimer-Volkoff equations to obtain stellar configurations. Owing to the difference in internal degrees of freedom, Dirac DM (four degrees of freedom) generally softens the equation of state more strongly than Majorana DM (two degrees of freedom), leading to smaller radii and lower maximum masses. We identify the parameter space consistent with current NICER and gravitational-wave constraints, highlighting the potential of compact-star observations to discriminate between Dirac and Majorana dark matter.