Dirac vs. Majorana dark matter imprints on neutron star observables

The fundamental character of a fermionic dark matter (DM), 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 motivating 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 the structure of neutron stars. Within a relativistic mean-field framework extended by a scalar (or Higgs-like) portal coupling between DM and nucleons, we construct self-consistent equations of state 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 generally softens the equation of state more strongly than Majorana DM, 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.