First Dark Matter Search Results from the LUX-ZEPLIN (LZ) Experiment

The LUX-ZEPLIN experiment is a dark matter detector centered on a dual-phase xenon time projection chamber operating at the Sanford Underground Research Facility in Lead, South Dakota, USA. This Letter reports results from LUX-ZEPLIN’s first search for weakly interacting massive particles (WIMPs) with an exposure of 60 live days using a fiducial mass of 5.5 t. A profile-likelihood ratio analysis shows the data to be consistent with a background-only hypothesis, setting new limits on spin-independent WIMP-nucleon, spin-dependent WIMP-neutron, and spin-dependent WIMP-proton cross sections for WIMP masses above $9\mathrm{GeV}/{\mathrm{c}}^2$. The most stringent limit is set for spin-independent scattering at $36\mathrm{GeV}/{\mathrm{c}}^2$, rejecting cross sections above $9.2\times {10}^{-48}{\mathrm{cm}}^2$ at the 90% confidence level.

Figure 1

Calibration events in ${\log }_{10}\mathrm{S}2c\text{−}\mathrm{S}1c$ for the tritium source (dark blue points, 5343 events) and the DD neutron source (orange points, 6324 events). Solid blue (red) lines indicate the median of the ER (NR) simulated distributions, and the dotted lines indicate the 10% and 90% quantiles. Thin gray lines show contours of constant electron-equivalent energy (${\mathrm{keV}}_{\mathrm{ee}}$) and nuclear recoil energy (${\mathrm{keV}}_{\mathrm{nr}}$).

Figure 2

Signal efficiency as a function of NR energy for the trigger (blue), the threefold coincidence and $>3\mathrm{phd}$ threshold on S1c (orange), single-scatter (SS) reconstruction and analysis cuts (green), and the search ROI in S1 and S2 (black). The inset shows the low-energy behavior, with the dotted line at $5.5{\mathrm{keV}}_{\mathrm{nr}}$ marking 50% efficiency. The error band (gray) is assessed using AmLi and tritium data as discussed in the text.

Figure 3

Data in reconstructed $r^2$ and $z$ after all analysis cuts. Black (gray) points show the data inside (outside) the FV. Red crosses and blue circles show events vetoed by a prompt LXe skin or OD signal, respectively. The solid line shows the FV definition, and the dashed line shows the extent of the active TPC. Field nonuniformities cause the reconstructed $r$ position of the active volume boundary to vary as a function of z. Events with drift time of approximately $50\mathrm{\mu }\mathrm{s}$ are from recoils in the gas that produce S1 and S2 pulses with a fixed time separation.

Figure 4

WIMP-search data (black points) after all cuts in ${\log }_{10}\mathrm{S}2c\text{−}\mathrm{S}1c$ space. Contours enclose $1\sigma$ and $2\sigma$ of the following models: the best-fit background model (shaded gray regions), the ${}^{37}\mathrm{Ar}$ component (orange ellipses), a $30\mathrm{GeV}/{\mathrm{c}}^2$ WIMP (purple dashed lines), and ${}^8\mathrm{B}$ solar neutrinos (shaded green regions). The red solid line indicates the NR median, and the red dotted lines indicate the 10% and 90% quantiles. Model contours incorporate all efficiencies used in the analysis. Thin gray lines indicate contours of constant energy.

Figure 5

The 90% confidence limit (black line) for the spin-independent WIMP cross section vs WIMP mass. The gray dot-dash line shows the limit before applying the power constraint described in the text. The green and yellow bands are the $1\sigma$ and $2\sigma$ sensitivity bands. The dotted line shows the median of the sensitivity projection. Also shown are the PandaX-4T [26], XENON1T [25], LUX [28], and DEAP-3600 [78] limits.

Figure 6

Reconstructed energy spectrum of the best-fit model. Data points are shown in black. The blue line shows total summed background. The darker blue band shows the model uncertainty and the lighter blue band the combined model and statistical uncertainty. Background components are shown in colors as given in the legend. Background components from ${}^8\mathrm{B}$ solar neutrinos and accidentals are included in the fit but are too small to be visible in the plot.

Figure 7

The 90% confidence limit (black line) and uncertainty bands (gray) coming from xenon nuclear correction factors for the spin-dependent WIMP-neutron cross section vs WIMP mass using the mean of the nuclear structure factors from [89] and range across [87, 89, 90]. Also shown are the PandaX-II [91], LUX [92], and XENON1T [93] limits. A similar uncertainty band as shown on this result applies to the other Xe-based limits.

Figure 8

The 90% confidence limit (black line) and uncertainty bands (gray) coming from xenon nuclear correction factors for the spin-dependent WIMP-proton cross section vs WIMP mass using the mean of the nuclear structure factors from [89] and range across [87, 89, 90]. Also shown are the PICO-60 [24], PandaX-II [91], LUX [92], and XENON1T [93] limits. A similar uncertainty band as shown on this result applies to the other Xe-based limits. The PICO-60 result relies on WIMP-scattering on the spin of the unpaired proton of ${}^{19}\mathrm{F}$ with minimal uncertainty.