Self-Interacting Sub-GeV Dark Matter with Strong MeV Gamma-ray

TL;DR

This work addresses the tension in sub-GeV dark matter models where an $s$-channel resonance can both enhance self-interactions and deplete relic abundance via annihilation, risking conflict with γ-ray observations. It introduces a two-mediator framework in a gauged U(1)$_{ m B}$-extended SM with a singlet scalar DM: a vector mediator governs self-scattering and resonant annihilations, while a light dark Higgs mediator controls relic abundance through forbidden annihilation channels. A full UV-complete Lagrangian is analyzed, and a comprehensive parameter scan identifies viable regions that satisfy cosmological, experimental, and theoretical constraints, while predicting distinctive MeV γ-ray signatures testable by next-generation telescopes such as COSI. The study finds that decoupling the mediators allows the relic density to be set without violating γ-ray bounds, while maintaining strong, testable MeV γ-ray signals; COSI is projected to probe a substantial portion of the viable parameter space. The framework is extensible to alternative gauge charges and mediator spins, offering a versatile path to reconciling small-scale structure issues with observed relic abundance and γ-ray constraints.

Abstract

Sub-GeV dark matter (DM) with $s$-channel resonant self-scattering provides a promising framework for addressing small-scale structure problems. However, models that also account for the observed relic abundance through the same resonance are strongly constrained by current $γ$-ray observations, since the associated signals are significantly enhanced. To overcome this limitation, we propose a framework in which the relic abundance and self-scattering are governed independently by two distinct mediators. As a concrete realization, we present a singlet scalar DM model in which self-scattering is mediated by a vector boson associated with a gauged baryon number, while the relic density is determined by forbidden annihilation into dark Higgs bosons that generate the gauge boson mass. By imposing cosmological, experimental, and theoretical constraints, We identify viable parameter regions that reproduce the observed relic density, alleviate small-scale problems, and remain consistent with current bounds. Notably, the model predicts multiple distinctive MeV $γ$-ray signals, a significant fraction of which will be testable with next-generation MeV $γ$-ray telescopes, including the Compton Spectrometer and Imager (COSI).

Figures (9)

• Figure 1: The velocity dependence of DM annihilation cross sections into various final states is shown for the parameter set $(m_\varphi, v_{\rm R}, v_{\rm th}, \xi, g_{\rm B}, C_{\varsigma\varphi\varphi}) = (69~\mathrm{MeV}, 4\times 10^{-4}, 2\times 10^{-3}, 10^{-11}, 2\times 10^{-8})$ as an illustrative example, with other parameters having no impact on this behavior. Here, $v_{\rm FO} \sim \mathcal{O}(0.1)$ denotes the DM relative velocity at freeze-out, and $v_{\rm GC} \sim 400$ km/s corresponds to the DM velocity at the GC Lacroix:2020lhn. The DM velocity at the recombination era, $v_{\rm CMB}$, is extremely small and lies outside the range shown in the figure.
• Figure 2: The velocity dependence of the DM self-scattering cross section for the parameter set $(m_\varphi, v_R, g_\varphi, \sigma_0/m_\varphi) = (100~\mathrm{MeV}, 4\times 10^{-4}, 4 \times 10^{-3}, 0.067~\mathrm{cm}^2/\mathrm{g})$, with other parameters having no impact on this behavior, is shown as a black solid line. The cross sections required to alleviate small-scale structure issues, inferred from kinematical observations, are indicated by black dots with error bars Kaplinghat:2015aga.
• Figure 3: Constraints on DM elastic scattering off electrons at 95% C.L. The shaded regions are excluded by existing direct detection experiments, while the colored lines indicate projected sensitivities of future experiments. See the main text for details.
• Figure 4: Constraints on mediator particles at 90% C.L. from various accelerator experiments. The upper-left panel shows the bounds for the scalar mediator. The bottom panels correspond to the vector mediator: the left panel shows constraints from production via kinetic mixing with neutral gauge bosons, while the right panel shows constraints from baryon-number interactions. See text for details.
• Figure 5: Present constraints and projected COSI sensitivities on the DM annihilation cross section into $e^+e^-(\gamma)$ (left) and $\gamma\gamma$ (right). Shaded regions indicate current experimental exclusions. The red solid line shows the COSI sensitivity assuming the median J-factor,deSalas:2019pee, while the blue dashed lines illustrate the corresponding $\pm 2\sigma$ uncertainty range.