Improved Exclusion Limits from the EDELWEISS WIMP Search

Abstract

The EDELWEISS experiment has improved its sensitivity for the direct search for WIMP dark matter. In the recoil energy range relevant for WIMP masses below 10 TeV/c2, no nuclear recoils were observed in the fiducial volume of a heat-and-ionization cryogenic Ge detector operated in the low-background environment of the Laboratoire Souterrain de Modane in the Frejus Tunnel, during an effective exposure of 7.4 kg.days. This result is combined with the previous EDELWEISS data to derive a limit on the cross-section for spin-independent interaction of WIMPs and nucleons as a function of WIMP mass, using standard nuclear physics and astrophysical assumptions. This limit excludes at more than 99.8%CL a WIMP candidate with a mass of 44 GeV/c2 and a cross-section of 5.4 x 10-6 pb, as reported by the DAMA collaboration. A first sample of supersymmetric models are also excluded at 90%CL.

Figures (5)

• Figure 1: Energy pulse height spectra for low-energy gammas (sum of the ionization and heat channels, weighted by their resolution squared) in the fiducial volume of the EDELWEISS detector, for the low-background physics runs: (a) sum of the distributions of the detectors GeAl9 and GeAl10; (b) distribution in the detector GGA1. The arrows indicate the peaks at 8.98 and 10.37 keV, corresponding to the de-excitation of the cosmogenic activation of $^{65}$Zn and $^{68}$Ge in the detector, and the $^{71}$Ge activation that follows neutron calibrations.
• Figure 2: Distribution of the ratio of the ionization pulse height to the recoil energy (quenching factor Q) obtained by exposing the detectors to $^{57}$Co and $^{60}$Co $\gamma$-ray sources. The ratio is normalized to 1 for electron recoils using the photopeaks of the $^{57}$Co source. Shaded histogram: detector GGA1. Line histogram: sum of the detector GeAl9 and GeAl10, normalized to the same number of entries of GGA1.
• Figure 3: Distribution of the quenching factor (ratio of the ionization signal to the recoil energy) as a function of the recoil energy from the data collected in the center fiducial volume of the 320 g EDELWEISS detector GGA1. The exposure of the fiducial volume corresponds to 8.6 kg$\cdot$d. Also plotted as full lines are the $\pm$1.645$\sigma$ bands (90% efficiency) for photons and for nuclear recoils. The 99.9% efficiency region for photons is also shown (dotted line). The hyperbolic dashed curve corresponds to 3.5 keV ionization energy and the vertical dashed line to 20 keV recoil energy.
• Figure 4: Spin-independent exclusion limits (dark solid curve) obtained by combining our 2000 data from Ref. bib-edw2000 with the present data, for a total exposure of 11.7 kg$\cdot$d. Dashed curve: previous EDELWEISS data bib-edw2000 re-analyzed using the new definition of the upper bound of the recoil energy range (acceptance of 95%). Dash-dotted curve: present 2002 data. Closed contour: allowed region at 3$\sigma$ CL from the DAMA1-4 annual modulation data bib-dama. The full circle and triangle within this contour are defined in the text.
• Figure 5: Combined EDELWEISS spin-independent exclusion limits (dark solid curve) compared with published limits from other experiments and theoretical calculations. Dashed curve: Ge diode limit from IGEX bib-igex. Dash-dotted curve: CDMS limit with statistical subtraction of the neutron background bib-cdms2. Closed contour: allowed region at 3$\sigma$ CL from the DAMA1-4 annual modulation data bib-dama. Two regions spanned by some of the supersymmetric model calculations of Refs. bib-bottinobib-gondolo are also shown.