Dark Matter Results from 100 Live Days of XENON100 Data

We present results from the direct search for dark matter with the XENON100 detector, installed underground at the Laboratori Nazionali del Gran Sasso of INFN, Italy. XENON100 is a two-phase time-projection chamber with a 62 kg liquid xenon target. Interaction vertex reconstruction in three dimensions with millimeter precision allows the selection of only the innermost 48 kg as the ultralow background fiducial target. In 100.9 live days of data, acquired between January and June 2010, no evidence for dark matter is found. Three candidate events were observed in the signal region with an expected background of ($1.8\pm 0.6$) events. This leads to the most stringent limit on dark matter interactions today, excluding spin-independent elastic weakly interacting massive particle (WIMP) nucleon scattering cross sections above $7.0\times {10}^{-45}{\mathrm{cm}}^2$ for a WIMP mass of $50\mathrm{GeV}/c^2$ at 90% confidence level.

Figure 1

All direct measurements of ${\mathcal{L}}_{\mathrm{eff}}$ [12, 13] described by a Gaussian distribution to obtain the mean (solid line) and the uncertainty band ($1\sigma$ and $2\sigma$). Below $3{\mathrm{keV}}_{\mathrm{nr}}$ the trend is logarithmically extrapolated to ${\mathcal{L}}_{\mathrm{eff}}=0$ at $1{\mathrm{keV}}_{\mathrm{nr}}$.

Figure 2

Acceptance of all data quality cuts used for the analysis for $m_{\chi }\ge 50\mathrm{GeV}/c^2$ (solid red line), $m_{\chi }=10\mathrm{GeV}/c^2$ (dotted green line), $m_{\chi }=7\mathrm{GeV}/c^2$ (dash-dotted black line). The optimum interval analysis additionally uses a $S2/S1$ ER discrimination cut. Its NR acceptance is also shown (dashed blue line).

Figure 3

Observed event distribution using the discrimination parameter ${log}_{10}(S{2}_b/S1)$, flattened by subtracting the ER band mean, as a function of NR equivalent energy (${\mathrm{keV}}_{\mathrm{nr}}$). All quality cuts, including those defined after unblinding, are used. Gray points indicate the NR distribution as measured with a ${}^{241}\mathrm{AmBe}$ neutron source. The WIMP search region is defined by the energy window $8.4–44.6{\mathrm{keV}}_{\mathrm{nr}}$ (4–30 PE) and the lower bound of the software threshold $S2>300\mathrm{PE}$ (blue dashed line). The optimum interval analysis additionally uses the 99.75% rejection line from above and the $3\sigma$ contour of the NR distribution from below (green dotted line). Three events fall into this WIMP search region (red circles), with $(1.8\pm 0.6)$ events expected from the background.

Figure 4

Distribution of all events (gray dots) and events below the 99.75% rejection line (black dots) in the TPC observed in the $8.4–44.6{\mathrm{keV}}_{\mathrm{nr}}$ energy range during 100.9 live days. All cuts are used here, including the ones introduced post-unblinding to remove a population due to electronic noise. The 48 kg fiducial volume (blue dashed line) and the TPC dimensions (gray line) are also indicated.

Figure 5

Spin-independent elastic WIMP-nucleon cross section $\sigma$ as a function of WIMP mass $m_{\chi }$. The new XENON100 limit at 90% C.L., as derived with the profile likelihood method taking into account all relevant systematic uncertainties, is shown as the thick (blue) line together with the expected sensitivity of this run (green-yellow band). The limits from XENON100 (2010) [7], EDELWEISS (2011) [6], CDMS (2009) [5] (recalculated with $v_{\mathrm{esc}}=544\mathrm{km}/\mathrm{s}$, $v_0=220\mathrm{km}/\mathrm{s}$), CDMS (2011) [22], and XENON10 (2011) [23] are also shown. Expectations from CMSSM are indicated at 68% and 95% C.L. (shaded gray [19], gray contour [24]), as well as the 90% C.L. areas favored by CoGeNT [21] and DAMA (no channeling) [20].