Cosmology and Signals of Light Pseudo-Dirac Dark Matter

TL;DR

This paper studies the cosmological evolution, detection prospects, and unique signals of light pseudo-Dirac dark matter with small mass splittings, focusing on sub-GeV mass range and interactions via a dark photon.

Contribution

It provides the first comprehensive analysis of the thermal history, observational constraints, and detection signals for pseudo-Dirac dark matter with splittings from 100 eV to MeV.

Findings

CMB energy injection bounds constrain annihilation and decay of pseudo-Dirac DM.

Distinctive signals include down-scattering on nuclei or electrons, with potential relevance to Xenon1T excess.

Future experiments can probe the parameter space through various detection channels.

Abstract

In this paper, we analyze the cosmological evolution, allowed parameter space, and observational prospects for a dark sector consisting of thermally produced pseudo-Dirac fermions with a small mass splitting, coupled to the Standard Model through a dark photon. This scenario is particularly notable in the context of sub-GeV dark matter, where the mass-off-diagonal leading interaction limits applicability of both CMB energy injection constraints and standard direct detection searches. We present the first general study of the thermal history of pseudo-Dirac DM with splittings from 100 eV to MeV, focusing on the depletion of the heavier "excited" state abundance via scatterings and decays, and on the distinctive signals arising from its small surviving abundance. We analyze CMB energy injection bounds on both DM annihilation and decay, accelerator-based probes, and new line-like direct-detection signals from the excited DM down-scattering on either nuclei or electrons, as well as future search prospects in each channel. We also comment on the relevance of this signal to the few-keV Xenon1T electron excess and on possible diurnal modulation of this signal, and introduce a signal-strength parametrization to facilitate the comparison of future experimental results to theoretical expectations.