Direct Detection of Sub-GeV Dark Matter

TL;DR

The paper addresses the challenge of directly detecting sub-GeV dark matter by exploiting DM–electron scattering to produce observable electron ionization, since nuclear recoils are typically too small to detect in this mass range. It develops a model-independent framework using a reference cross section $\overline{\sigma}_e$ and a DM form factor $F_{\rm DM}(q)$, derives ionization rates for atomic/molecular targets and crystals, and evaluates experimental sensitivities with realistic backgrounds. Key contributions include explicit rate formulas, treatment of atomic and crystal ionization form factors, and qualitative/quantitative sensitivity estimates across xenon, argon, helium, and germanium targets, including neutrino backgrounds and annual modulation as a signal discriminator. The results indicate that existing and near-future direct-detection experiments could probe new regions of light DM parameter space, potentially discovering or constraining hidden-sector scenarios such as kinetic-mmixing mediated interactions and Freeze-In production mechanisms, with dedicated detectors offering significant gains.

Abstract

Direct detection strategies are proposed for dark matter particles with MeV to GeV mass. In this largely unexplored mass range, dark matter scattering with electrons can cause single-electron ionization signals, which are detectable with current technology. Ultraviolet photons, individual ions, and heat are interesting alternative signals. Focusing on ionization, we calculate the expected dark matter scattering rates and estimate the sensitivity of possible experiments. Backgrounds that may be relevant are discussed. Theoretically interesting models can be probed with existing technologies, and may even be within reach using ongoing direct detection experiments. Significant improvements in sensitivity should be possible with dedicated experiments, opening up a window to new regions in dark matter parameter space.

Figures (4)

• Figure 1: Background solar neutrino rates per kg$\cdot$year. Solid lines show nuclear recoil spectra for neutrinos scattering with xenon (blue), germanium (brown), argon (red), and helium (green). These are not expected to significantly contribute to the ionized electron signal from LDM-electron scattering. Dotted lines, with same color coding as above, show rates for neutrino scattering off electrons. These rates are small and peak at higher energies than LDM-electron scattering.
• Figure 2: The cross section exclusion reach (left axis) at 95% confidence level for 1 kg$\cdot$year of exposure, assuming only the irreducible neutrino background (note that additional unknown backgrounds are likely to exist, which would weaken the sensitivity --- see Fig. \ref{['fig:sigmsVSbackground']}). This corresponds to the cross section for which 3.6 events are expected after 1 kg$\cdot$year. The right axis shows the event rate assuming a cross section of $\overline{\sigma}_e=10^{-37}$ cm$^2$. Results are shown for xenon (blue), argon (red), germanium (brown), and helium (green) targets. Left: Models with no DM form-factor. The green shaded area indicates the allowed region for $U(1)_D$ (hidden photon) models with $m_{A_D} \mathrel{\hbox{$>$$\sim$}} 10$ MeV. The orange shaded area is the region in which a particular model of "MeV" DM can explain the INTEGRAL 511 keV $\gamma$-rays from the galactic bulge Borodatchenkova:2005ct. Right: Models with a very light scalar or vector mediator, for which $F_{\rm DM} = \alpha^2 m_e^2/q^2$. The blue region indicates the allowed parameter space for a hidden $U(1)_D$ model with a very light ($\ll$ keV) hidden photon. The darker blue band corresponds to the "Freeze-In" region. For illustration, constant $g_D$ contours are shown with dashed lines, assuming $m_{A_D}=8\mathrm{\ MeV}$ and $\varepsilon=2\times10^{-3}$ (left plot) and $m_{A_D}=1\mathrm{\ meV}$ and $\varepsilon=3\times10^{-6}$ (right plot). For more details see the text and the Appendix.
• Figure 3: The differential rates of LDM-induced ionization versus electron recoil energy, for a cross section of $\bar{\sigma}_{e} = 10^{-37}$ cm$^2$. Results are shown for xenon (blue), argon (red), and helium (green) targets, and a DM mass of 10 MeV (solid lines) and 1 GeV (dashed lines). The two plots show results for scattering with no DM form-factor ( top) and with $F_{\rm DM} = \alpha^2 m_e^2/q^2$ ( bottom). The dotted lines in the bottom right corner show the irreducible solar-neutrino--electron scattering backgrounds. We emphasize that other backgrounds of an unknown size can be expected at all energies, and will require a dedicated study to be measured and understood.
• Figure 4: The discovery reach using annual modulation, as a function of the background event rate, for $m_{\rm DM} = 30$ MeV and 1 kg$\cdot$year exposure. Results are shown for xenon (blue), argon (red), germanium (brown) and helium (green) targets, assuming either no DM interaction form-factor (solid lines) or $F_{\rm DM} = \alpha^2 m_e^2/q^2$ (dashed lines). The annual modulation is $\mathcal{O}(10\%)$ in all cases. The reach scales as $\sqrt{\text{exposure}}$ (exposure) for large (small) background rates.