Indirect searches for realistic sub-GeV Dark Matter models

TL;DR

This work investigates indirect detection prospects for sub-GeV dark matter (1 MeV–1 GeV) in two realistic portal models: vector-portal (kinetic mixing) and higgs-portal. It computes both prompt and secondary photon signals, including inverse Compton scattering, bremsstrahlung, and in-flight annihilation, using spectra from the HAZMA/HAZMA2 tools and full galactic propagation with DRAGON2 for comparison. By confronting these signals with existing X-ray/gamma-ray data (Integral, Comptel, Fermi-LAT) and Voyager-1 cosmic-ray data, it derives current upper limits on the annihilation cross-section ⟨σv⟩ and demonstrates that secondary photons often dominate the constraints, especially in the MeV–GeV range. The study also forecasts COSI’s 3σ sensitivity to the total photon signal from a GC-focused observation, highlighting that COSI can probe substantial new regions of parameter space beyond current bounds, with the impact strongest for models where ICS is significant. Overall, the results emphasize the importance of including secondary emissions in sub-GeV DM analyses and show that future MeV missions like COSI will play a crucial role in constraining or discovering such DM candidates.

Abstract

Indirect searches for Dark Matter (DM) particles with mass in the MeV -- GeV scale have received significant attention lately. Pair-annihilations of such DM particles in the Galaxy can give rise to (at the same time) MeV to GeV $γ$-rays via prompt emission, sub-GeV $e^\pm$ in cosmic-rays, as well as a broad photon spectrum ranging from $X$-rays to soft $γ$-rays, produced by the DM induced $e^\pm$ via inverse Compton scattering, bremsstrahlung and in-flight annihilation processes (collectively called `secondary emissions'). We focus on two representative realistic sub-GeV DM models, namely, the vector-portal kinetic-mixing model and the higgs-portal model, and perform a detailed study of the indirect detection constraints from existing $X$-rays, $γ$-rays and cosmic-ray observations, based on all of the above-mentioned signals. We also estimate the future prospects from the upcoming MeV photon telescope COSI, including all possible types of prompt and secondary emission signals. We compare our results with the constraints and (or) projections from cosmological and terrestrial observations. We find that, for both the sub-GeV DM models, the current observations constrain the annihilation cross-section at the level of $\langle σv \rangle \lesssim 10^{-27} {\rm cm}^3/{\rm s}$, or lower for some specific mass ranges or under optimistic assumptions. Moreover, new unconstrained DM parameter space can be probed at the upcoming instruments like COSI, thanks to the inclusion of secondary photons which in many cases provide the dominant signal.

Figures (8)

• Figure 1: Total electron or positron spectra$\left.\frac{dN_e}{dE_e}\right\vert_{\rm tot}$ as a function of $E_e$ for different values of $m_{\rm DM}$, considering the kinetic-mixing model with $m_V = 3\,m_{\rm DM}$ (left panel) and higgs-portal model with $m_S = 0.5\,m_{\rm DM}$ (right panel). In each case, we add a 5% resolution in the $e^\pm$ energy.
• Figure 2: Illustration of the different components (prompt and secondaries) of the total photon flux (solid black line) produced due to the annihilation of DM particles (of mass $m_{\rm DM} = 100$ MeV) in the Galaxy are shown for the considered photon energy range. The DM interaction is driven by the kinetic-mixing model with $m_V = 3 \, m_{\rm DM}$ and the total annihilation cross-section $\langle \sigma v \rangle = 2\times10^{-26}$$\rm cm^3 s^{-1}$. The distribution of DM-induced $e^\pm$ (that give rise to the secondaries) in the Galaxy is obtained considering the approach discussed in sec. \ref{['sec:semi-analytic']}. All DM signals (including the total one) are estimated from the region $4^{\circ} < |b| < 20^{\circ} , |l| < 60^{\circ}$, a region similar to the one covered by Comptel observation, but masking $|b| \leq 4^{\circ}$. The red, green, blue and cyan data points are from the observations of Integral ($|b| < 15^{\circ} , |l| < 30^{\circ}$), Comptel ($|b| < 20^{\circ} , |l| < 60^{\circ}$), Egret ($|b| < 10^{\circ} , |l| < 60^{\circ}$) and Fermi-Lat ($\theta<10^{\circ}$), respectively (as discussed in sec. \ref{['sec:obs_data_analysis']}). All data presented in the figure are rescaled to the observation region of Comptel. The light orange band indicates the energy range covered by upcoming MeV telescope Cosi (see sec. \ref{['sec:MeV_telescope']}).
• Figure 3: Illustration of the effects of considering the full propagation of the DM-induced $e^\pm$ in the Galaxy for three different DM masses: 10, 100 and 1000 MeV. The model is the same one used in figure \ref{['fig:DM_fluxes']}. The galactic propagation of $e^\pm$ is solved numerically using the package DRAGON2, with the propagation model 'prop. a' discussed in the text. For each DM mass, the total photon flux obtained (from a region $|b| < 20^{\circ} , |l| < 60^{\circ}$) in this way is denoted with a black dotted curve. This flux is compared to that obtained using the method used in figure \ref{['fig:DM_fluxes']} (the black solid curve), i.e. using the semi-analytic approximated approach for the galactic propagation discussed in section \ref{['sec:semi-analytic']}, and masking the region $|b| \leq 4^{\circ}$.
• Figure 4: The DM-induced $\bm{e^\pm}$ flux compared to the measurement of Voyager-1. The two galactic propagation models 'prop. a' and 'prop. c' discussed in the text are considered. The DM model is the same vector portal one considered in fig. \ref{['fig:DM_fluxes']}. The Voyager-1 2013 data Boudaud:2016mosdoi:10.1126/science.1236408 and Voyager-1 2016 data 2016ApJ...831...18C are shown by blue and green points, respectively.
• Figure 5: Indirect detection upper-limits obtained on the total $\langle \sigma v \rangle$ of the two considered DM models (vector portal and scalar portal, left and right panels respectively) based on existing astrophysical observations. Limits obtained by comparing the DM-induced total photon signal (primary + secondaries) with data from Comptel, Integral and Fermi-Lat are presented in the upper panels by green, red and cyan curves, respectively. The solid curves are obtained under the conservative analysis, while the dashed curves are obtained using the optimistic analysis. Limits obtained by comparing the DM-induced $e^\pm$ flux with Voyager-1 data (under the conservative analysis) are shown in the lower panels by the orange and pink solid curves, which correspond to the galactic propagation models 'prop. a' and 'prop. c', respectively (see the text). We also show the thermal relic DM line (gray dotted lines) and the CMB bounds (gray dashed-dotted lines), as well as the combined exclusions (dark gray region) and the combined projections (light gray region) from terrestrial experiments Krnjaic:2022ozp. The contours corresponding to different values of the coupling $g_{S{\rm DM}}$ are indicated by the purple dotted lines on the right.