Could the 21-cm absorption be explained by the dark matter suggested by $^8$Be transitions?

TL;DR

This paper investigates whether dark matter, suggested by anomalies in $^8$Be transitions, could explain the unexpectedly strong 21-cm absorption observed by EDGES, through photon-mediated scattering involving millicharged fermionic dark matter.

Contribution

It proposes a model linking $^8$Be anomalies to 21-cm absorption via pseudoscalar-mediated millicharged dark matter, analyzing annihilation channels and experimental constraints.

Findings

P-wave annihilation dominates during freeze-out.

s-wave annihilation is consistent with CMB and 21-cm constraints.

Future experiments like NA62 could detect millicharged dark matter.

Abstract

The stronger than expected 21-cm absorption was observed by EDGES recently, and another anomaly of $^8$Be transitions would be signatures of new interactions. These two issues may be related to each other, e.g., pseudoscalar $A$ mediated fermionic millicharged dark matter (DM), and the 21-cm absorption could be induced by photon mediated scattering between MeV millicharged DM and hydrogen. This will be explored in this paper. For fermionic millicharged DM $\bar{\chi} \chi$ with masses in a range of $2 m_A < 2 m_{\chi} < 3 m_A$, the p-wave annihilation $\bar{\chi} \chi \to A A$ would be dominant during DM freeze-out. The s-wave annihilation $\bar{\chi} \chi$ $\to A, \gamma $ $\to e^+ e^-$ is tolerant by constraints from CMB and the 21-cm absorption. The millicharged DM can evade constraints from direct detection experiments. The process of $K^+ \to \pi^+ \pi^0$ with the invisible decay $\pi^0 \to \bar{\chi} \chi$ could be employed to search for the millicharged DM, and future high intensity $K^+$ sources, such as NA62, will do the job.