Search for Cosmic-Ray Boosted Sub-GeV Dark Matter at the PandaX-II Experiment

We report a novel search for the cosmic-ray boosted dark matter using the $100\mathrm{tonne}·\mathrm{day}$ full dataset of the PandaX-II detector located at the China Jinping Underground Laboratory. With the extra energy gained from the cosmic rays, sub-GeV dark matter particles can produce visible recoil signals in the detector. The diurnal modulations in rate and energy spectrum are utilized to further enhance the signal sensitivity. Our result excludes the dark matter–nucleon elastic scattering cross section between ${10}^{-31}$ and ${10}^{-28}{\mathrm{cm}}^2$ for dark matter masses from $0.1\mathrm{MeV}/c^2$ to $0.1\mathrm{GeV}/c^2$, with a large parameter space previously unexplored by experimental collaborations.

Figure 1

Attenuated CRDM flux at CJPL at sidereal hours 6 (red lines) and 18 (blue lines), where the solid and dashed lines denote results obtained with the full MC and BT methods, respectively. Because of the DM reflection effect considered in the MC, the CRDM flux obtained from the full MC method is significantly lower, approximately 10% (50%) of that from the BT method for DM kinetic energy $T_{\chi }=1\mathrm{MeV}$ (100 MeV).

Figure 2

(a) ${\log }_{10}(S2/S1)$ vs $S1$ distribution for all 2111 events in PandaX-II ($\mathrm{black}=\text{run}$ 9, $\mathrm{brown}=\text{runs}$ 10 and 11) with equal-NR-energy lines (corresponding ER energy in parentheses) in run 11 indicated by the gray curves. The red and blue bands are the 90% contours of the ER and NR medians, respectively, in run 11. Larger circles are the final 25 candidates used in this search. (b) Energy distribution of the final candidates with statistical uncertainties, overlaid with the nominal background (blue line) and the CRDM signals at ($0.1\mathrm{GeV}/c^2$, ${10}^{-30}{\mathrm{cm}}^2$) (red dashed line) and (${10}^{-4}\mathrm{GeV}/c^2$, ${10}^{-31}{\mathrm{cm}}^2$) (purple dashed line); (c) $t_{\mathrm{sid}}$ distribution, overlaid with the same CRDM signals in (b) and the nominal background. (d) $E_{\mathrm{ee}}$ vs $t_{\mathrm{sid}}$ distribution for the 25 candidates, overlaid with the background distribution.

Figure 3

90% exclusion region with the full MC method (red region), together with constraints from PROSPECT [48] (yellow), XENON1T [28] (pink), CDEX [27] (orange), CRESST [18] (cyan), CMB [5] (gray), and cosmology [4, 6] (brown). The green band is the $\pm 1\sigma$ sensitivity band. The dashed blue line represents our result if interpreted using the PROSPECT treatment (see the text), whereas the dashed red line is our exclusion contour when the CRDM energy cutoff is relaxed to 1 GeV. Phenomenological interpretations of the experimental data [38, 39, 40, 41] are omitted from the figure, for visual clarify.