CMB Limits on the Absorption of Light Vector and Axial-Vector Dark Matter

TL;DR

This paper investigates how light vector and axial-vector leptophilic dark matter interactions affect cosmic microwave background anisotropies, setting new cosmological constraints on their couplings for masses between 100 eV and 100 keV.

Contribution

It provides the first analysis of CMB constraints on leptophilic dark matter absorption and inelastic scattering processes, extending the mass range and including both vector and axial-vector interactions.

Findings

Upper limits on DM-electron couplings derived from Planck 2018 data.

Inelastic scattering dominates constraints above the keV mass range.

Hydrogen ionization absorption is the main process below the keV mass range.

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 post-recombination 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.