Dark Matter Results from 225 Live Days of XENON100 Data

We report on a search for particle dark matter with the XENON100 experiment, operated at the Laboratori Nazionali del Gran Sasso for 13 months during 2011 and 2012. XENON100 features an ultralow electromagnetic background of $(5.3\pm 0.6)\times {10}^{-3}\mathrm{events}/({\mathrm{keV}}_{\mathrm{ee}}\times \mathrm{kg}\times \mathrm{day})$ in the energy region of interest. A blind analysis of $224.6\mathrm{\text{live days}}\times 34\mathrm{kg}$ exposure has yielded no evidence for dark matter interactions. The two candidate events observed in the predefined nuclear recoil energy range of $6.6–30.5{\mathrm{keV}}_{\mathrm{nr}}$ are consistent with the background expectation of ($1.0\pm 0.2$) events. A profile likelihood analysis using a $6.6–43.3{\mathrm{keV}}_{\mathrm{nr}}$ energy range sets the most stringent limit on the spin-independent elastic weakly interacting massive particle–nucleon scattering cross section for weakly interacting massive particle masses above $8\mathrm{GeV}/c^2$, with a minimum of $2\times {10}^{-45}{\mathrm{cm}}^2$ at $55\mathrm{GeV}/c^2$ and 90% confidence level.

Figure 1

Combined cut acceptance (solid blue line). The $S2$ threshold cut $S2>150\mathrm{PE}$ (dashed red line) is independent of possible fluctuations in $S1$ and has to be applied to the $S1$ spectrum before taking into account the $S1$ resolution. It is conservatively set to zero below 1 PE. For the cross-check with the maximum gap method [13], a hard discrimination cut is used. Its acceptance to NRs is shown by the dotted green line. The lower analysis threshold is $6.6{\mathrm{keV}}_{\mathrm{nr}}$ (3 PE) and extends to $43.3{\mathrm{keV}}_{\mathrm{nr}}$, whereas the cross-check is restricted to $30.5{\mathrm{keV}}_{\mathrm{nr}}$ (dashed and dotted vertical blue lines).

Figure 2

(Top) Event distribution in the discrimination parameter space ${log}_{10}(S{2}_b/S1)$, flattened by subtracting the distribution’s mean, as observed after unblinding using all analysis cuts and a 34 kg fiducial volume (black squares). A lower analysis threshold of $6.6{\mathrm{keV}}_{\mathrm{nr}}$ (NR equivalent energy scale) is employed. The PL analysis uses an upper energy threshold of $43.3{\mathrm{keV}}_{\mathrm{nr}}$ (3–30 PE), and the benchmark WIMP search region is limited to $30.5{\mathrm{keV}}_{\mathrm{nr}}$ (3–20 PE). The negligible impact of the $S2>150\mathrm{PE}$ threshold cut is indicated by the dashed-dotted blue line, and the signal region is restricted by a lower border running along the 97% NR quantile. An additional hard $S{2}_b/S1$ discrimination cut at 99.75% ER rejection defines the benchmark WIMP search region from above (dotted green line) but is only used to cross-check the PL inference. The histogram in red and gray indicates the NR band from the neutron calibration. Two events fall into the benchmark region where ($1.0\pm 0.2$) are expected from background. (Bottom) Spatial event distribution inside the TPC using a $6.6–43.3{\mathrm{keV}}_{\mathrm{nr}}$ energy window. The 34 kg fiducial volume is indicated by the red dashed line. Gray points are above the 99.75% rejection line, and black circles fall below.

Figure 3

Result on spin-independent WIMP-nucleon scattering from XENON100: The expected sensitivity of this run is shown by the dark (green) and light (yellow) band [$1\sigma$ ($2\sigma$)] and the resulting exclusion limit (90% C.L.) by the solid blue line. For comparison, other experimental limits (90% C.L.) and detection claims ($2\sigma$) are also shown [19, 20, 21, 22], together with the regions ($1\sigma /2\sigma$) preferred by supersymmetric models [18].