Abstract
A strong absorption profile was reported by the EDGES Collaboration, which indicates the hydrogen gas being colder than expected. It could be signatures of non-gravitational interactions between normal matter and dark matter (DM), and a possible explanation is that a small fraction of millicharged DM scatters with normal matter, with the DM mass in tens of MeV. To obtain the small fraction of millicharged DM and meanwhile being tolerant as regards the constraints, the dark photon portal scalar and vector millicharged DM are explored in this paper. We consider the mass of dark photon to be slightly above twice the millicharged DM mass, and thus the millicharged DM predominantly annihilates in p-wave during the freeze-out period, with the annihilation being enhanced near the resonance. The dark photon mainly decays into millicharged DM, and couplings of the dark photon with SM particles could be allowed by the lepton collision experiments. The corresponding parameter spaces are derived. Future lepton collision experiments can be employed to search for millicharged DM via the production of the invisible dark photon.
Highlights
In the universe, about 84% of the cosmological matter density is contributed by dark matter (DM) [1], while little has become known about the particle properties of DM
The annihilation mode φφ∗ (V V ∗) → γ → e+e− is a p-wave process, which is suppressed by the millicharge parameter η2, and the corresponding thermally averaged annihilation cross section is in the range of about 10−32–10−26 cm3/s
As mentioned in the Introduction, to cool the hydrogen gas indicated by the EDGES observation, the fraction of millicharged DM fDM is about 0.003–0.02, with DM mass ∼ 10–80 MeV
Summary
About 84% of the cosmological matter density is contributed by dark matter (DM) [1], while little has become known about the particle properties of DM . An absorption profile around 78 MHz in the skyaveraged spectrum was reported by the EDGES Collaboration [2], and the magnitude of the absorption was enhanced with 3.8σ discrepancy. This enhancement would indicate the hydrogen gas to be colder than expected, and it may be signatures of non-gravitational interactions between normal matter and DM from the cosmic dawn [2–6]. [12–15], with a DM mass about 10–80 MeV and the photon as the mediator in the scattering of cooling the hydrogen. Due to a small fraction of DM being millicharged, the couplings of the dark photon with SM particles may be not very small, and this will be restricted by the lepton collision
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