Prospects for Probing Sub-GeV Leptophilic Dark Matter with the Future VLAST

TL;DR

The paper addresses the viability and detectability of sub-GeV leptophilic dark matter within the thermal relic paradigm, focusing on the MeV–GeV gamma-ray gap and the resonance region. It assesses VLAST's prospective sensitivity to DM annihilation in Draco for three benchmark models corresponding to $s$-wave, $p$-wave, and $(s+p)$-wave processes, using a comprehensive MCMC framework that includes relic density, Planck CMB, BBN, self-interactions, direct detection, and beam-dump constraints, as well as mediator-mass resonance effects. A key result is that $s$-wave annihilation is tightly constrained, while the resonance region enables viable $p$-wave and $(s+p)$-wave annihilation, yielding testable signals for VLAST, especially in the electrophilic and muonophilic channels; the resonance parameter $\xi$ maps the proximity to $m_{ m MED}^2 \approx 4 m_{ m DM}^2$. The projections show VLAST achieving superior sensitivity to these resonant scenarios compared to e-ASTROGAM and current instruments, suggesting a pivotal role in confirming the thermal-relic nature of sub-GeV DM. Overall, the work demonstrates that VLAST can fill the MeV gap and uniquely probe previously inaccessible regions of leptophilic DM parameter space, advancing indirect detection prospects for light DM.

Abstract

The proposed Very Large Area Space Telescope (VLAST), with its expected unprecedented sensitivity in the MeV-GeV range, can also address the longstanding "MeV Gap" in gamma-ray observations. We explore the capability of VLAST to detect sub-GeV leptophilic dark matter (DM) annihilation, focusing on scalar and vector mediators and emphasizing the resonance region where the mediator mass is approximately twice the DM mass. While $s$-wave annihilation is tightly constrained by relic density and cosmic microwave background observations, $p$-wave and mixed $(s+p)$-wave scenarios remain viable, particularly near resonance. Additionally, direct detection experiments, especially those probing DM-electron scattering, significantly constrain nonresonance parameter space but are less effective in the resonance regime. VLAST can uniquely probe this surviving region, outperforming existing and planned instruments, and establishing itself as a crucial tool for indirect detection of thermal relic DM.

Figures (5)

• Figure 1: Left panel: The sensitivity of VLAST for one year (red dashed line) to gamma-ray emission from Draco dwarf galaxy, compared with projected sensitivities of e-ASTROGAM for one year (black dashed line) e-ASTROGAM:2017pxr and 6-year Fermi-LAT data (blue dashed line) Fermi-LAT:2015attFermi-LAT:2016uux. Solid lines show the prompt gamma-ray spectra from DM annihilation into lepton pairs assuming a thermal relic cross section of $\langle \sigma v \rangle = 2.2 \times 10^{-26}~\rm cm^3\,s^{-1}$Steigman:2012nb with $\mathrm{DM}+\mathrm{DM} \to e^+e^-$ for two benchmark masses $m_{\rm DM} = 10~\,\mathrm{MeV}$ (green) and $800~\,\mathrm{MeV}$ (red), and $\mathrm{DM}+\mathrm{DM} \to \mu^+\mu^-$ for $m_{\rm DM} = 800~\,\mathrm{MeV}$ (blue). Right panel: Projected sensitivities to the DM annihilation cross section. Red lines correspond to VLAST sensitivities for $e^+e^-$ (solid) and $\mu^+\mu^-$ (dash-dotted) channels, while black lines show the corresponding sensitivities for e-ASTROGAM.
• Figure 2: The $2\sigma$ allowed regions ($\delta\chi^2 < 5.99$ for two degrees of freedom) in the $(m_\text{DM},m_\text{MED})$ plane for the three benchmark interactions. The upper panels correspond to the electrophilic DM scenario, while the lower panels correspond to the muonophilic DM scenario. Colored dots represent surviving parameter points from the MCMC scan. Red/orange indicate resonance regions, gray corresponds to forbidden annihilation, and blue/cyan mark the parameter space where $m_{\rm DM} > m_{\rm MED}$.
• Figure 3: The $2\sigma$ allowed region for the DM-electron scattering cross section $\bar{\sigma}_e$ as a function of $m_\text{DM}$. The region labeled as non-resonance region includes the forbidden and $m_{\rm DM}>m_{\rm MED}$ scenarios, as defined in Fig. \ref{['Fig:mx_mphi']}. The form factor is taken as heavy mediator limit, $F_{\rm DM}(q)=1$. The region above the solid lines is excluded by the null results from the XENON1T XENON:2019gfn, DarkSide-50 DarkSide:2022knj, and DAMIC-M DAMIC-M:2023gxo DM search experiments.
• Figure 4: The $2\sigma$ allowed regions for the resonance regions. The coupling product $M_D \times g_{e\,(\mu)}$ (left column) and $g_D \times g_{e\,(\mu)}$ (middle and right columns) are the function of the resonance parameter $\xi$ in the resonance regime. The resonance parameter is defined as $\xi=\sqrt{(m_{\rm{MED}}^2-4m_{\rm{DM}}^2)/4m_{\rm{DM}}^2}$.
• Figure 5: Expected sensitivity of VLAST to DM annihilation via mediator resonance in Draco dwarf galaxy over a 5-year observation period. The red and orange regions denote the parameter space accessible to VLAST for the electrophilic (upper panels) and muonphilic (lower panels) DM, respectively, while the gray region represents the parameter space that remains undetectable.