Millicharged Fermions as Dark Matter: Theoretical Modeling and Experimental Detection

Fermions with a small effective charge, qeff≪1, arise from kinetic mixing between an unbroken (or nearly unbroken) dark U(1)′ and electromagnetism, or from a genuinely fractional charge. Millicharged particles (mCPs) inherit the Pauli-exclusion constraints on fermionic dark matter (DM) in galaxies, while adding a long-range plasma phenomenology absent for neutral candidates. This review connects three strands: (i) the symmetry structure, cosmology and astrophysical bounds of millicharged fermions, comparing kinetic-mixing and fractional-charge realizations; (ii) the role of mCPs in galactic halo formation and dynamics, and when they fit the Ruffini–Argüelles–Rueda (RAR) self-gravitating core–halo model; and (iii) experimental techniques for detecting mCPs trapped on Earth, arriving from the galactic halo, or emitted by the Sun. We show that a collisionless halo of keV–MeV millicharged fermions requires α′≲7×10−13–2×10−10, depending on mass and velocity, orders of magnitude below the coupling needed for thermal freeze-out via χχ¯→A′A′ in the Milky Way and dwarf galaxies. A RAR halo made of the mCP itself thus needs non-thermal production, a charge-symmetric plasma and very small dark coupling, whereas a sub-dominant mCP component can be added without changing the halo. We close with open theoretical and experimental challenges and a summary of the allowed mCP parameter space.