Dark matter search results from the commissioning run of PandaX-II

We present the results of a search for weakly interacting massive particles (WIMPs) from the commissioning run of the PandaX-II experiment located at the China Jinping Underground Laboratory. A WIMP search data set with an exposure of $306\times 19.1\mathrm{kg}\text{−}\text{day}$ was taken, while its dominant ${}^{85}\mathrm{Kr}$ background was used as the electron recoil calibration. No WIMP candidates are identified, and a 90% upper limit is set on the spin-independent elastic WIMP-nucleon cross section with a lowest excluded cross section of $2.97\times {10}^{-45}{\mathrm{cm}}^2$ at a WIMP mass of $44.7\mathrm{GeV}/{\mathrm{c}}^2$.

Figure 1

Design drawings of PandaX-II TPC, left: cross sectional view with both inner and outer PTFE panels, right: full view with skin volume exposed. See the text for details.

Figure 2

Linear fit in $\mathrm{S}2/E_{ee}$ vs $\mathrm{S}1/E_{ee}$ for all $\gamma$ peaks. Each $\gamma$ peak was fit with a 2D Gaussian in the (S1, S2) plane with anticorrelation taken into account; only fit uncertainties are reflected on the data points.

Figure 3

The comparison of ${}^{252}\mathrm{Cf}$ neutron calibration data with the median from the original NEST model (blue line) and the tuned model (green line). A fit to the median of the data is given by the red line. The dashed red line is the NR 99.99% acceptance line based on the tuned MC model (without cuts and efficiency applied) to remove spurious “neutron-X” events with suppressed S2s due to charge loss in the inactive region.

Figure 4

Position distribution of events that pass all selection cuts (gray points) and those below the NR median (outside FV: red points; inside FV: green stars), with FV cuts indicated as the black dashed box. The red points clustered at the top, bottom, and upper right corner correspond to events in these locations losing electrons on the electrodes or PTFE wall, leading to a suppression of S2. The severe loss of S2 close to the bottom wall leads to a significant event inefficiency indicated by the lack of events in the lower right corner in the figure.

Figure 5

The distribution of ${\log }_{10}(\mathrm{S}2/\mathrm{S}1)$ vs S1 for dark matter search data. The median of the NR calibration band is indicated as the red curve. The dashed magenta curve is the equivalent 100 PE cut on S2. The solid magenta and blue curves are the 99.99% and 33.3% NR acceptance curves, respectively. The gray dashed curves are the equal energy curves with NR energy indicated in the figures. The two data points located below the NR median curve are highlighted in green stars.

Figure 6

The 90% C.L. upper limit for the spin-independent isoscalar WIMP-nucleon cross section from the PandaX-II commissioning run (red). A selected set of recent world results are plotted for comparison: PandaX-I final results [10] (magenta), XENON100 225 day results [3] (black), LUX 2015 results [5](blue), SuperCDMS 2014 [27](orange), and DarkSide 2015 [28] results (brown). The $\pm 1\sigma$ and $2\sigma$ sensitivity bands are shown in green and yellow, respectively.