Possible s -wave annihilation for MeV dark matter with the 21-cm absorption

Lian-Bao Jia,Xu Liao

doi:10.1103/physrevd.100.035012

Abstract

The CMB observation sets stringent constraints on MeV dark matter (DM) annihilating into charged states/photons in s-wave, and the recent observation of the 21-cm absorption at the cosmic dawn reported by EDGES is also very strict for s-wave annihilations of MeV DM. The millicharged DM with p-wave dominant annihilations during the freeze-out period are considered in literatures to give an explanation about the 21-cm absorption, with photon mediated scattering cooling the hydrogen. In this paper, we focus on the annihilation of millicharged DM being s-wave dominant. To explain the 21-cm absorption and meanwhile be compatible with the CMB and 21-cm absorption bounds on DM annihilations, we consider the annihilation close to the resonance, with the new mediator (here is dark photon) mass being slightly above twice of the millicharged DM mass. In this case, the annihilation cross section at the temperature $T \to 0$ could be much smaller than that at $T_f$, which would be tolerated by the bounds on DM annihilations, avoiding the excess heating from DM s-wave annihilations to the hydrogen gas. The beam dump and lepton collider experiments can be employed to hunt for millicharged DM via the production of the invisible dark photon.

Highlights

For dark matter (DM) particles with masses in the range of ten MeV to hundreds of TeV, the relic abundance of DM can be obtained via the thermal freeze-out of DM
For s-wave-dominant DM annihilations, to be compatible with stringent constraints from cosmic microwave background (CMB) and the anomalous 21-cm absorption, the annihilation is considered to be close to the resonance, with ξ (ξ 1⁄4 mA0=2mχ) being slightly above 1
For ξ in a range of 0.004 ≲ ξ − 1 ≲ 0.085, the weighted annihilation cross section f2DMhσ2vri0 could be ≲10−30 cm3=s, which is tolerated by constraints from the anomalous 21-cm absorption with Tm < 4 K (z 1⁄4 17.2), avoiding excess heating to the hydrogen

Summary

INTRODUCTION

For dark matter (DM) particles with masses in the range of ten MeV to hundreds of TeV, the relic abundance of DM can be obtained via the thermal freeze-out of DM. For the mediator mass being slightly below twice the DM mass, the annihilation cross section of DM at T → 0 could be larger than that at the freeze-out temperature Tf; for the mediator mass being slightly above twice the DM mass, the annihilation cross section of DM at T → 0 could be smaller than that at T 1⁄4 Tf. In the case of the new mediator mass being slightly above twice the millicharged DM mass, the millicharged DM with s-wave-dominant annihilations may cause the 21-cm anomaly and evade constraints from the CMB and 21-cm absorption. In the case of the new mediator mass being slightly above twice the millicharged DM mass, the millicharged DM with s-wave-dominant annihilations may cause the 21-cm anomaly and evade constraints from the CMB and 21-cm absorption This will be investigated in this paper

ANNIHILATIONS OF MILLICHARGED DM

NUMERICAL ANALYSIS

CONCLUSION AND DISCUSSION