CMB Spectral Distortions from Resonant Conversions in Atomic Dark Sectors

TL;DR

This paper investigates how resonant conversions between dark and Standard Model photons in atomic dark sectors can produce detectable CMB spectral distortions, constraining dark sector parameters with current and future observations.

Contribution

It introduces a detailed modeling of dark photon conversions and thermal mass evolution, providing new constraints and forecasts for atomic dark sector parameters using CMB spectral data.

Findings

Current COBE/FIRAS data rules out certain dark sector parameter regions.

Resonant conversions can produce observable spectral distortions in the CMB.

Future FOSSIL satellite observations could probe ultra-low dark electric charges.

Abstract

Dark sectors consisting of atomic constituents (electrons, protons, and photons) offer a well-motivated extension to the Standard Model while providing multiple avenues for phenomenological study. In this work, we explore the impact of conversions between the dark and Standard Model photons in the primordial CMB spectral distortion epoch ($10^3 \lesssim z \lesssim 10^6$). These conversions are resonantly enhanced when the induced thermal masses of both photonic species are equal, thus leading to the possibility that sizeable distortions can be produced. To this end, we solve the Boltzmann equation at early times to determine the (irreducible) freeze-in or freeze-out abundance of dark photons. This procedure also allows us to update the limits on generic milli-charged dark sectors using the ACT DR6 bound on the number of effective radiative degrees of freedom ($N_{\rm eff}$). By then modeling the evolution of the thermal masses in both sectors, we compute the primordial CMB distortion using the Landau-Zener formalism. We find that when the dark electron and proton are roughly similar in mass (the positronium limit), current spectral distortion data from the COBE/FIRAS instrument is able to rule out novel regions of parameter space. We also forecast bounds from the proposed FOSSIL satellite, finding that spectral distortions can also be used to probe the ultra-low dark electric charge regions of parameter space, which are difficult to investigate by other means.