Dark Matter Results from 54-Ton-Day Exposure of PandaX-II Experiment

We report a new search for weakly interacting massive particles (WIMPs) using the combined low background data sets acquired in 2016 and 2017 from the PandaX-II experiment in China. The latest data set contains a new exposure of 77.1 live days, with the background reduced to a level of $0.8\times {10}^{-3}\mathrm{evt}/\mathrm{kg}/\mathrm{day}$, improved by a factor of 2.5 in comparison to the previous run in 2016. No excess events are found above the expected background. With a total exposure of $5.4\times {10}^4\mathrm{kg}\mathrm{day}$, the most stringent upper limit on the spin-independent WIMP-nucleon cross section is set for a WIMP with mass larger than $100\mathrm{GeV}/c^2$, with the lowest 90% C.L. exclusion at $8.6\times {10}^{-47}{\mathrm{cm}}^2$ at $40\mathrm{GeV}/c^2$.

Figure 1

Fractional difference between the reconstructed energy $E_{\mathrm{comb}}$ and the expected energy $E_{\text{expect}}$ vs $E_{\text{expect}}$ for ER peaks in Run 9 (black circles) and Run 10 (red squares), using the best fit PDE and EEE values described in the text. Uncertainties include both statistical and systematic components.

Figure 2

Tritium (solid black dots) and AmBe data (open red circles) in ${\log }_{10}(S2/S1)$ vs $S1$. For comparison, the median (Runs 9 and 10 averaged, solid blue), 10% quantile, and 90% quantile (Run 9, dashed blue, Run 10, light dashed blue) of the ER background PDFs are overlaid. The solid red line is the median of the AmBe events. The dashed and solid magenta curves are the 100 PE selection cut for $S2$, and the 99.99% NR acceptance curve from the MC calculation, respectively. The gray curves represent the equal energy curves in nuclear recoil energy (${\mathrm{keV}}_{\mathrm{nr}}$).

Figure 3

The position distributions for events within the $S1$ and $S2$ range cuts. (a) $z$ vs $r^2$ and (b) $y$ vs $x$. The drift time cut between 20 and $350\mu \mathrm{s}$ is applied for (b). The gray and red points are events above and below the NR median, respectively. The dashed box (a) and circle (b) represent the FV cut. The gray background circles in (b) indicate locations of the top PMTs.

Figure 4

The distribution of ${\log }_{10}(S2/S1)$ vs $S1$ for the DM search data in Run 10, overlaid with the corresponding median, 10% quantile, and 90% quantile of the ER background PDFs. The red curve is the NR median from the AmBe calibration.

Figure 5

The 90% C.L. upper limits versus $m_{\chi }$ [(a) log scale, (b) linear scale between $40\mathrm{GeV}/c^2$ and $10\mathrm{TeV}/c^2$] for the spin independent WIMP-nucleon elastic cross sections from the combined PandaX-II Run 9 and Run 10 data (red), overlaid with that from PandaX-II 2016 [3] (blue), LUX 2017 [2] (magenta), and XENON1T 2017 [4] (black). The green band represents the $\pm 1\sigma$ sensitivity band.