First Dark Matter Search with Nuclear Recoils from the XENONnT Experiment

We report on the first search for nuclear recoils from dark matter in the form of weakly interacting massive particles (WIMPs) with the XENONnT experiment, which is based on a two-phase time projection chamber with a sensitive liquid xenon mass of 5.9 ton. During the $(1.09\pm 0.03)\mathrm{ton}\mathrm{yr}$ exposure used for this search, the intrinsic ${}^{85}\mathrm{Kr}$ and ${}^{222}\mathrm{Rn}$ concentrations in the liquid target are reduced to unprecedentedly low levels, giving an electronic recoil background rate of $(15.8\pm 1.3)\text{events}/\mathrm{ton}\mathrm{yr}\mathrm{keV}$ in the region of interest. A blind analysis of nuclear recoil events with energies between 3.3 and 60.5 keV finds no significant excess. This leads to a minimum upper limit on the spin-independent WIMP-nucleon cross section of $2.58\times {10}^{-47}{\mathrm{cm}}^2$ for a WIMP mass of $28\mathrm{GeV}/c^2$ at 90% confidence level. Limits for spin-dependent interactions are also provided. Both the limit and the sensitivity for the full range of WIMP masses analyzed here improve on previous results obtained with the XENON1T experiment for the same exposure.

Figure 1

NR and ER calibration data from ${}^{241}\mathrm{AmBe}$ (orange), ${}^{222}\mathrm{Rn}$ (blue), and ${}^{37}\mathrm{Ar}$ (black). The median and the $\pm 2\sigma$ contours of the NR and ER model are shown in blue and red, respectively. The gray dash-dotted contour lines show the reconstructed NR energy (${\mathrm{keV}}_{\mathrm{NR}}$). Only not shaded events up to a cS1 of 100 PE are considered in the response model fits.

Figure 2

Detection and selection efficiency for NR events in this search as a function of the NR recoil energy. The total efficiency in the WIMP search region (black) is dominated by the detection efficiency (green) at low energies and event selections (blue) at higher energies until the edge of the ROI. Normalized recoil spectra for WIMPs with masses of 10, 50, and $200\mathrm{GeV}/c^2$ are shown with orange dashed lines for reference.

Figure 3

DM search data in the cS1-cS2 space. Each event is represented with a pie chart showing the fraction of the best-fit model, including the expected number of $200\mathrm{GeV}/c^2$ WIMPs (orange) evaluated at the position of the event. The size of the pie charts is proportional to the signal model at that position. Background probability density distributions are shown as $1\sigma$ (dark) and $2\sigma$ (light) regions as indicated in the legend for ER (blue), AC (purple), and surface (green, “wall”). The neutron background (yellow in pies) has a similar distribution to the WIMP (orange-filled area showing the $2\sigma$ region). The orange dashed contour contains a signal-like region which is constructed to contain 50% of a $200\mathrm{GeV}/c^2$ WIMP signal with the highest possible signal-to-noise ratio.

Figure 4

Upper limit on spin-independent WIMP-nucleon cross section at 90% confidence level (full black line) as a function of the WIMP mass. A power constraint is applied to the limit to restrict it at or above the median unconstrained upper limit. The dashed lines show the upper limit without a power constraint applied. The $1\sigma$ (green) and $2\sigma$ (yellow) sensitivity bands are shown as shaded regions, with lighter colors indicating the range of possible downward fluctuations. The result from XENON1T [3] is shown in blue with the same power constraint applied. At masses above $100\mathrm{GeV}/c^2$, the limit scales with mass as indicated with the extrapolation formula.