First Dark Matter Results from the XENON100 Experiment

The XENON100 experiment, in operation at the Laboratori Nazionali del Gran Sasso in Italy, is designed to search for dark matter weakly interacting massive particles (WIMPs) scattering off 62 kg of liquid xenon in an ultralow background dual-phase time projection chamber. In this Letter, we present first dark matter results from the analysis of 11.17 live days of nonblind data, acquired in October and November 2009. In the selected fiducial target of 40 kg, and within the predefined signal region, we observe no events and hence exclude spin-independent WIMP-nucleon elastic scattering cross sections above $3.4\times {10}^{-44}{\mathrm{cm}}^2$ for $55\mathrm{GeV}/c^2$ WIMPs at 90% confidence level. Below $20\mathrm{GeV}/c^2$, this result constrains the interpretation of the CoGeNT and DAMA signals as being due to spin-independent, elastic, light mass WIMP interactions.

Figure 1

Global fit to all ${\mathcal{L}}_{\mathrm{eff}}$ measurements with fixed neutron energies between $5{\mathrm{keV}}_{\mathrm{nr}}$ and $100{\mathrm{keV}}_{\mathrm{nr}}$, together with 90% confidence contours (solid lines) and extrapolations to lower energies (dashed lines).

Figure 2

Electronic (top) and nuclear (bottom) recoil bands from ${}^{60}\mathrm{Co}$ and ${}^{241}\mathrm{AmBe}$ calibration data, respectively, after data selection and the 40 kg fiducial volume cut. Colored lines correspond to the median ${log}_{10}(S2/S1)$ values of the electronic (blue [dark grey]) and nuclear (red [light grey]) recoil bands. The WIMP search energy window $8.7–32.6{\mathrm{keV}}_{\mathrm{nr}}$ (vertical, dashed line) and $S2$ software threshold of 300 PE (long dashed line) are shown.

Figure 3

Cut acceptance (top, not including 50% acceptance from $S2/S1$ discrimination) and ${log}_{10}(S2/S1)$ (bottom) as functions of nuclear-recoil energy for events observed in the 40 kg fiducial volume during 11.17 live days. Lines as in Fig. 2.

Figure 4

Distribution of all events (dots) and events below the nuclear-recoil median (red circles) in the TPC (grey line) observed in the $8.7–32.6{\mathrm{keV}}_{\mathrm{nr}}$ energy range during 11.17 live days. No events below the nuclear-recoil median are observed within the 40 kg fiducial volume (dashed line).

Figure 5

90% confidence limit on the spin-independent elastic WIMP-nucleon cross section (solid and long dashed lines), together with the best limit to date from CDMS (dotted line) [15], recalculated assuming an escape velocity of $544\mathrm{km}/\mathrm{s}$ and $v_0=220\mathrm{km}/\mathrm{s}$. Expectations from a theoretical model [16], and the areas (90% C.L.) favored by CoGeNT (green) [13] and DAMA (red/orange) [14] are also shown.