Light thermal dark matter models in the light of DAMIC-M 2025 constraints

Abstract

We study the viability of light thermal dark matter (DM) in sub-GeV mass range in view of the stringent new DAMIC-M limits on DM-electron scattering. Considering a Dirac fermion singlet DM charged under a new Abelian gauge symmetry $U(1)$, we outline two possibilities: (i) family non-universal $U(1)$ gauge coupling with resonantly enhanced DM annihilation into standard model (SM) fermions and (ii) family universal dark $U(1)$ gauge symmetry where relic is set by DM annihilation into light gauge bosons. As an illustrative example of the first class of models, we consider a gauged $L_μ-L_τ$ extension of the SM having interesting detection prospects at several experiments. While both of these class of models lead to observed DM relic and consistency with DAMIC-M together with other experimental limits, the second class of models also lead to strong DM self-interactions, potentially solving the small-scale structure issues of cold dark matter. While a vast part of the parameter space in both the models is already ruled out, the current allowed region of parameter space can be further probed at ongoing or future experiments keeping the models testable.

Figures (8)

• Figure 1: Relic density favored parameter space in the $(g_{\mu\tau}, m_\chi)$ plane for $M_{Z_{\mu\tau}} = 10$ MeV (orange) and $100$ MeV (gray). Direct-detection constraints from DAMIC-M and DarkSide-50, as well as upper limits from $(g-2)_\mu$, are overlaid.
• Figure 2: Indirect-detection limits from X-ray observations constrain the annihilation cross-section $\langle \sigma v \rangle_{\chi \bar{\chi} \to e^+ e^-}$. Complementary bounds on the annihilation rate to $e^+ e^-$ are plotted by computing the cross-sections for (R=2.01) and (R=2.1) using constraints from fixed-target experiments, specifically CCFR and NA64$\mu$.
• Figure 3: Constraints in the $(g_{\mu\tau},M_{Z_{\mu\tau}})$ plane for two representative choices of the mass ratio $R \equiv M_{Z_{\mu\tau}}/m_\chi = 2.01$ (dashed lines) and $R=2.1$ (dotted lines). The blue (DAMIC-M) and orange (DarkSide-50) curves show direct-detection limits from DM--electron scattering. Additional exclusions from NA64$\mu$, Borexino, CCFR, COHERENT, BABAR and $\Delta N_{\rm eff}$ (PLANCK) are overlaid, along with the CMB bound (brown) shown for both values of $R$ using the corresponding line styles. The dark-green curves denote the parameter space yielding the observed relic abundance for $R=2.01$ (dashed) and $R=2.1$ (dotted). As $R$ is increased away from the resonant value $R=2$, the relic-density contour shifts to larger values of $g_{\mu\tau}$ due to the reduced resonant enhancement. The figure highlights that only a narrow window of $M_{Z_{\mu\tau}}$ remains viable, which will be probed by upcoming NA64 searches.
• Figure 4: Parameter space in the $g_d$–$m_\chi$ plane illustrating the region consistent with self-interaction criteria for $M_X = 1 \,\text{MeV}$, together with the correct relic density constraint (magenta line) in the minimal setup. Parameter space above the magenta line give an under-abundant relic density, while points below the line are over-abundant. The large blue region indicates the Born-regime of self-interactions while the thin lines in the upper-right corner depicts the transition to the resonant regime (see Appendix \ref{['app::sidm_cross-section']}).
• Figure 5: Parameter space in $\epsilon-M_X$ plane of the $U(1)_X$ model after imposing various constraints. The most stringent bounds from DAMIC-M ($m_\chi$=10 MeV) and DarkSide-50 ($m_\chi$=100 MeV) for two different values of gauge coupling $g_d$= are shown by solid and dashed lines, respectively. Other shaded regions and contours correspond to different bounds or future sensitivities, see main text for details.