Search for Light Dark Matter–Electron Scattering in the PandaX-II Experiment

We report constraints on light dark matter through its interactions with shell electrons in the PandaX-II liquid xenon detector with a total $46.9\mathrm{tonnes}/\mathrm{day}$ exposure. To effectively search for these very low energy electron recoils, ionization-only signals are selected from the data. 1821 candidates are identified within an ionization signal range between 50 and 75 photoelectrons, corresponding to a mean electronic recoil energy from 0.08 to 0.15 keV. The 90% C.L. exclusion limit on the scattering cross section between the dark matter and electron is calculated with systematic uncertainties properly taken into account. Under the assumption of point interaction, we provide the world’s most stringent limit within the dark matter mass range from 15 to $30\mathrm{MeV}/c^2$, with the corresponding cross section from $2.5\times {10}^{-37}$ to $3.1\times {10}^{-38}{\mathrm{cm}}^2$.

Figure 1

Event rate of the US2 signals with charge range from 50 to 200 photoelectrons.

Figure 2

Data quality cut efficiency (red), trigger efficiency (blue), and combined efficiency (black) vs detected ionization signals in run 9 (top) and runs 10 and 11 (bottom). Details about the uncertainties are described in the Supplemental Material [30]. The quality cut efficiencies (dashed) from ${}^{241}\mathrm{Am}\text{−}\mathrm{Be}$ and ${}^{220}\mathrm{Rn}$ calibration runs are used to validate that from tritium. The upper axes correspond to mean ER energy computed using the constant model ($f_e=0.83$ and $E=13.7\mathrm{eV}\times S2/(\mathrm{SEG}\times \mathrm{EEE}\times f_e)$; see Fig. 3).

Figure 3

Charge yield vs ER energy for three DM search runs for NEST 2.0 [33, 34], the constant model [35], and the PandaX-II model [36], with uncertainty bands obtained from the original publications. The lowest energy measurement at 186 eV [37] is also shown. The shaded vertical band indicates the mean energy range corresponding to the region of interest (50–75 PE).

Figure 4

Detected ionization signals (US2, black histograms) and expected signals from DM-electron scattering with $F_{\mathrm{DM}}=1$ (upper) and ${\alpha }^2{m}_e^2/q^2$ (lower), with blue (red) histogram corresponding to a DM mass of $20\mathrm{MeV}/c^2$ $200\mathrm{MeV}/c^2$). The gray shadow shows the ROI of this analysis. The excess in the data peaking at $\sim 25\mathrm{PE}$ comprises single-electron events, likely due to stray electrons in LXe.

Figure 5

90% C.L. upper limits (solid, the constant model; dashed, NEST 2.0) on light DM-electron scattering cross section from PandaX-II data for $F_{\mathrm{DM}}=1$ (upper) and $F_{\mathrm{DM}}={\alpha }^2{m}_e^2/q^2$ (lower). Results from XENON1T [15], XENON10 [40], DarkSide-50 [16], SENSEI [18], DAMIC [17], and EDELWEISS [42] are also shown for comparison.