Sensitivity of Hongmeng 21cm experiment on scattering dark matter

TL;DR

This work assesses Hongmeng's ability to probe DM–baryon scattering through the global 21 cm signal by jointly forward‑modeling the cosmological signal, astrophysical backgrounds, and foregrounds within a lunar‑orbit mission framework. Using a Fisher forecast, the authors show that the full 5‑year Hongmeng mission can tighten existing CMB limits on the DM–baryon cross‑section by a factor of about 21, reaching $\sigma_0 \lesssim 4 \times 10^{-43}\,\mathrm{cm^2}$ for $0.1\mathrm{ MeV} \lesssim m_\chi \lesssim 0.3\mathrm{ GeV}$, while even a month of data provides a ~3× improvement. The analysis highlights the importance of jointly marginalizing Pop II and Pop III astrophysical parameters, since ignoring Pop III contributions can spuriously tighten constraints. It also uncovers potential non‑Gaussian and multi‑modal structure in the likelihood due to SDM cooling/heating transitions, suggesting future Bayesian methods for robust inference. Overall, the paper demonstrates Hongmeng’s potential as a clean, early‑Universe probe of new dark matter interactions, leveraging lunar shielding to mitigate terrestrial foregrounds.

Abstract

Scattering between dark matter and baryon can cool intergalactic medium temperature and deepen the 21cm signal. Such interactions have been proposed to explain the unusually deep 21cm absorption signal reported by EDGES in 2018. This paper explores the potential to detect dark matter - baryon scattering with the Hongmeng project, an upcoming lunar orbiting satellite experiment dedicated to measuring global 21cm signal between redshifts $11-46$. We self-consistently forward-model the simulated sky temperature data, jointly varying both the astrophysical and foreground models. We show that even with a very conservative observational strategy in which the experiment only takes data when both the Earth and the Sun are shielded by the Moon, Hongmeng can tighten the current constraints on dark matter - baryon scattering cross-section $σ_0$ by a factor of 21 after the full mission which lasts for five years. The prospective upper limit on $σ_0$ can reach $4 \times 10^{-43} {\rm cm^2}$ for dark matter masses between 0.1 MeV and 0.4 GeV. Even after only one month of observation, a factor of 3 improvement relative to current $σ_0$ limits can be expected.

Figures (3)

• Figure 1: 95% C.I. upper limits on DM-baryon scattering cross-section $\sigma_0$. The blue and red solid lines assume observation times of one month and one year respectively, the black solid line shows the results for full mission which lasts for five years. The black dashed line shows expected five-year limits varying only the parameters for Pop II galaxies. Current exclusion bound from CMB Nguyen:2021cnb is shown with red shaded regions.
• Figure 2: Prospective constraints on astrophysical parameters, FG parameters, and $\sigma_0$. We assumed five year observation time and $m_\chi$ is fixed to 0.1 MeV. The dark and light shaded regions indicate 68% and 95% confidence intervals, respectively. The green contours shows the case in which we jointly vary both Pop II and Pop III parameters (Pop II + Pop III), whereas in blue contours we fixed Pop III parameters ($f_{\star, 7}, f_{\rm esc,7}, \mathcal{L}_{\rm x, III}$ and $\alpha_{\star, {\rm III}}$) to their fiducial values. Note that constraints on Pop II, FG and $\sigma_0$ are all significantly tightened when ignoring Pop III parameters. The green dashed line shows the input fiducial values for each parameter, the titles show the 68% limits from the Pop II + Pop III joint analysis. In the upper right panels in which we show constraints for FG parameters, $p_{\rm FG0, fid} = 1697.81$K is the fiducial values for $p_{\rm FG, 0}$.
• Figure 3: Effects of SDM parameters on IGM temperature and on our likelihood. Color coding represents the value of $\chi^2 \equiv (T_{\rm sky, data} - T_{\rm sky, model})^2/\sigma_{\rm n}^2$. Vertical and horizontal hatches identify regions where SDM consistently cool or heat IGM during $11 \lesssim z \lesssim 46$.