First Dark Matter Search Results from the XENON1T Experiment

We report the first dark matter search results from XENON1T, a $\sim 2000\text{−}\mathrm{kg}$-target-mass dual-phase (liquid-gas) xenon time projection chamber in operation at the Laboratori Nazionali del Gran Sasso in Italy and the first ton-scale detector of this kind. The blinded search used 34.2 live days of data acquired between November 2016 and January 2017. Inside the $(1042\pm 12)\text{−}\mathrm{kg}$ fiducial mass and in the $[5,40]{\mathrm{keV}}_{\mathrm{nr}}$ energy range of interest for weakly interacting massive particle (WIMP) dark matter searches, the electronic recoil background was $(1.93\pm 0.25)\times {10}^{-4}\mathrm{events}/(\mathrm{kg}\times \mathrm{day}\times {\mathrm{keV}}_{\mathrm{ee}})$, the lowest ever achieved in such a dark matter detector. A profile likelihood analysis shows that the data are consistent with the background-only hypothesis. We derive the most stringent exclusion limits on the spin-independent WIMP-nucleon interaction cross section for WIMP masses above $10\mathrm{GeV}/c^2$, with a minimum of $7.7\times {10}^{-47}{\mathrm{cm}}^2$ for $35\text{−}\mathrm{GeV}/c^2$ WIMPs at 90% C.L.

Figure 1

NR detection efficiency in the fiducial mass at successive analysis stages as a function of recoil energy. At low energy, the detection efficiency (blue line) dominates. At 20 keV, the efficiency is 82%, primarily due to event selection losses (green line). At high energies, the effect of restricting our data to the search region described in the text (black line) is dominant. The black line is our final NR efficiency, with uncertainties shown in gray. The NR energy spectrum shape of a $50\text{−}\mathrm{GeV}/c^2$ WIMP (in a.u.) is shown in red for reference.

Figure 2

Observed data in $cS{2}_b$ vs $cS1$ for (a) ${}^{220}\mathrm{Rn}$ ER calibration, (b) ${}^{241}\mathrm{Am}\mathrm{Be}$ NR calibration, and (c) the 34.2-day dark matter search. Solid and dotted lines indicate the median and $\pm 2\sigma$ quantiles, respectively, of simulated event distributions (with the simulation fitted to calibration data). Red lines show NR (fitted to ${}^{241}\mathrm{Am}\mathrm{Be}$) and blue ER (fitted to ${}^{220}\mathrm{Rn}$). In (c), the purple distribution indicates the signal model of a $50\text{−}\mathrm{GeV}/c^2$ WIMP. Thin gray lines and labels indicate contours of the constant combined energy scale in keV for (a) ER and (b),(c) NR. Data below $cS1=3\mathrm{PE}$ (the gray region) are not in our analysis region of interest and are shown only for completeness.

Figure 3

Background model in the fiducial mass in a reference region between the NR median and the $-2\sigma$ quantile in $cS{2}_b$, projected onto $cS1$. Solid lines show that the expected number of events from individual components listed in Table 1; the labels match the abbreviations shown in the table. The dotted black line “Total” shows the total background model, while the dotted red line “WIMP” shows an $m=50\mathrm{GeV}/c^2$, $\sigma ={10}^{-46}{\mathrm{cm}}^2$ WIMP signal for comparison.

Figure 4

The spin-independent WIMP-nucleon cross section limits as a function of the WIMP mass at 90% confidence level (black line) for this run of XENON1T. In green and yellow are the $1\sigma$ and $2\sigma$ sensitivity bands. Results from LUX [27] (the red line), PandaX-II [28] (the brown line), and XENON100 [23] (the gray line) are shown for reference.