Search for Dark-Matter–Nucleon Interactions via Migdal Effect with DarkSide-50

Dark matter elastic scattering off nuclei can result in the excitation and ionization of the recoiling atom through the so-called Migdal effect. The energy deposition from the ionization electron adds to the energy deposited by the recoiling nuclear system and allows for the detection of interactions of sub-$\mathrm{GeV}/{\mathrm{c}}^2$ mass dark matter. We present new constraints for sub-$\mathrm{GeV}/{\mathrm{c}}^2$ dark matter using the dual-phase liquid argon time projection chamber of the DarkSide-50 experiment with an exposure of $(12306\pm 184)\mathrm{kg}\mathrm{d}$. The analysis is based on the ionization signal alone and significantly enhances the sensitivity of DarkSide-50, enabling sensitivity to dark matter with masses down to $40\mathrm{MeV}/c^2$. Furthermore, it sets the most stringent upper limit on the spin independent dark matter nucleon cross section for masses below $3.6\mathrm{GeV}/c^2$.

Figure 1

2D differential ionization rate as a function of the nuclear recoil energy ($E_{\mathrm{nr}}$) and electromagnetic channel energy ($E_{\mathrm{EM}}$) is shown for two representative DM masses, $0.36\mathrm{GeV}/c^2$ (top) and $1.49\mathrm{GeV}/c^2$ (bottom). The rate is given in $\text{events}/({\mathrm{keV}}^2\mathrm{kg}\mathrm{d})$ and covers $E_{\mathrm{EM}}$ from 0.01 to 3 keV and $E_{\mathrm{nr}}$ from $4\times {10}^{-4}$ to $4\times {10}^{-2}\mathrm{keV}$ for a DM mass of $0.36\mathrm{GeV}/{\mathrm{c}}^2$ and $E_{\mathrm{nr}}$ from $2.5\times {10}^{-2}$ to $7.5\times {10}^{-1}\mathrm{keV}$ for a particle of mass $1.49\mathrm{GeV}/c^2$. The top and side panels of each figure depict the corresponding integrated distributions in the ME electron (top panels) and the NR (left panels) channels.

Figure 2

Data (black) and background model (red) after the selection and fit described in [40]. The expected spectra including the Migdal effect assuming a spin independent DM-nucleon scattering cross section of ${10}^{-35}{\mathrm{cm}}^2$ and DM masses of 0.1, 0.5, and $0.9\mathrm{GeV}/c^2$ are shown in orange, blue, and green. The gray shaded band shows the S2 threshold used in the analysis.

Figure 3

Upper limits on the spin independent DM-nucleon cross section at 90% C.L. obtained with a signal including the Migdal effect, together with the corresponding $\pm 1\sigma$ (green shaded area) and $\pm 2\sigma$ (yellow shaded area) expected limits. NQ is red solid, QF is dashed, and the ER contribution from the ME is dash-dotted. Also shown are limits Cresst-III (green) [49], Xenon1T (light and dark blue) [33, 50], PandaX-4T [51] and DS-50 (dark red) [36]. Other weaker limits [6, 8, 51, 52, 53, 54, 55, 56, 57] and claimed discovery [58, 59, 60, 61] are not shown.