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CMB limits on the absorption of light vector and axial-vector dark matter

Gabriele Montefalcone1,*, Nicola Bellomo1,2,3,4, and Kimberly K. Boddy1

  • *Contact author: montefalcone@utexas.edu
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Phys. Rev. D 114, 023548 – Published 22 July, 2026

DOI: https://doi.org/10.1103/nkk2-k7f9

Abstract

Leptophilic sub-MeV spin-1 dark matter (DM) can be converted into a photon via inelastic scattering with a free electron or absorption by a neutral hydrogen atom in the primordial plasma. We study for the first time the impact of the energy injection resulting from such processes on cosmic microwave background (CMB) anisotropies. We obtain upper limits on the vector and axial-vector DM-electron couplings using Planck 2018 temperature, polarization, and lensing data for DM masses between 100 eV and 100 keV. We find that, due to the suppression of the hydrogen atomic form factor at high energies, inelastic scattering provides the dominant constraint for DM masses above the keV scale. At lower masses, hydrogen ionization through DM absorption is the leading channel, driven by the higher efficiency of postrecombination energy injection in modifying the free-electron fraction. Although the bounds we derive are considerably weaker than existing laboratory and astrophysical limits, they provide a robust and independent cosmological probe of leptophilic DM interactions.

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Phys. Rev. D 114, 023548– Published 22 July, 2026
  • Received 27 May 2026
  • Accepted 9 July 2026
Article part of CHORUS
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