Spin-Dependent Weakly-Interacting-Massive-Particle–Nucleon Cross Section Limits from First Data of PandaX-II Experiment

New constraints are presented on the spin-dependent weakly-interacting-massive-particle–– (WIMP-)nucleon interaction from the PandaX-II experiment, using a data set corresponding to a total exposure of $3.3\times {10}^4\mathrm{kg}\mathrm{day}$. Assuming a standard axial-vector spin-dependent WIMP interaction with ${}^{129}\mathrm{Xe}$ and ${}^{131}\mathrm{Xe}$ nuclei, the most stringent upper limits on WIMP-neutron cross sections for WIMPs with masses above $10\mathrm{GeV}/c^2$ are set in all dark matter direct detection experiments. The minimum upper limit of $4.1\times {10}^{-41}{\mathrm{cm}}^2$ at 90% confidence level is obtained for a WIMP mass of $40\mathrm{GeV}/c^2$. This represents more than a factor of 2 improvement on the best available limits at this and higher masses. These improved cross-section limits provide more stringent constraints on the effective WIMP-proton and WIMP-neutron couplings.

Figure 1

Structure factors as a function of nuclear recoil energy for neutron-only (plain) and proton-only (dashed) couplings for ${}^{129}\mathrm{Xe}$ (red) and ${}^{131}\mathrm{Xe}$ (blue), using the calculations from Ref. [11].

Figure 2

Nuclear recoil-energy distributions without detector effects for two WIMP mass points 40 (blue) and $400\mathrm{GeV}/c^2$ (red), for neutron-only (plain) and proton-only (dashed) couplings. Here we use $dR/dE$ calculations from Ref. [9] and structure factor calculations from Ref. [11]. The WIMP-neutron and WIMP-proton cross sections are assumed to be ${\sigma }_n={10}^{-40}$ and ${\sigma }_p=3\times {10}^{-39}{\mathrm{cm}}^2$, respectively, for visual clarity.

Figure 3

Detection efficiency per interaction as a function of the WIMP mass for neutron-only (blue) and proton-only (red) SD interactions. Efficiency for SI interaction (black) is also plotted for comparison.

Figure 4

PandaX-II 90% C.L. upper limits for the SD WIMP-neutron (top) and WIMP-proton (bottom) cross sections. Selected recent world results are plotted for comparison: LUX [7], XENON100 [8], CMS mono-jet [20, 22], ATLAS mono-jet [21], PICO-2L [25], PICO-60 [26], IceCube [27], and Super-K [28]. The 1 and $2\sigma$ sensitivity bands are shown in green and yellow, respectively.

Figure 5

PandaX-II constraints on the effective WIMP-proton and WIMP-neutron couplings, $a_p$ and $a_n$, for two WIMP masses (50 and $1000\mathrm{GeV}/c^2$). Also shown are results from the LUX, PICO, and CDMS experiments (all from Ref. [7]).