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Final results of the EDELWEISS-II WIMP search using a 4-kg array of cryogenic germanium detectors with interleaved electrodes

EDELWEISS Collaboration, E. Armengaud, C. Augier, A. Benoît, L. Bergé, J. Blümer, A. Broniatowski, V. Brudanin, B. Censier, G. Chardin, M. Chapellier, F. Charlieux, P. Coulter, G. A. Cox, X. Defay, M. De Jesus, Y. Dolgorouki, J. Domange, L. Dumoulin, K. Eitel, D. Filosofov, N. Fourches, J. Gascon, G. Gerbier, J. Gironnet, M. Gros, S. Henry, S. Hervé, A. Juillard, H. Kluck, V. Kozlov, H. Kraus, V. A. Kudryavtsev, P. Loaiza, S. Marnieros, X-F. Navick, C. Nones, E. Olivieri, P. Pari, L. Pattavina, B. Paul, M. Robinson, S. Rozov, V. Sanglard, B. Schmidt, S. Scorza, S. Semikh, A-S. Torrento-Coello, L. Vagneron, M-A. Verdier, R. J. Walker, E. Yakushev

TL;DR

This study reports final results from the EDELWEISS-II WIMP search using ten interleaved-electrode Ge detectors (Ge-ID) to suppress gamma and surface backgrounds. By accumulating a large exposure of $\sim384\ \mathrm{kg\cdot d}$ in a low-background underground setting, the collaboration observes five nuclear-recoil candidates above 20 keV and derives a spin-independent WIMP–nucleon cross-section limit of $\sigma_{\mathrm{SI}} = 4.4\times10^{-8}$ pb at $m_{\chi}=85$ GeV, with background accounting constrained to $<3$ events at 90% CL. The results also constrain inelastic scattering with a mass splitting of $\delta\approx120$ keV, excluding part of the DAMA-allowed region for $m_{\chi}\gtrsim90$ GeV/$c^2$ and strengthening limits above $m_{\chi}\gtrsim200$ GeV/$c^2$ relative to CDMS. Overall, the work demonstrates the efficacy of Ge-ID technology, informs background-reduction strategies for future phases, and sets the stage for a 40-kg array targeting sensitivities near $10^{-9}$ pb.

Abstract

The EDELWEISS-II collaboration has completed a direct search for WIMP dark matter with an array of ten 400-g cryogenic germanium detectors in operation at the Laboratoire Souterrain de Modane. The combined use of thermal phonon sensors and charge collection electrodes with an interleaved geometry enables the efficient rejection of gamma-induced radioactivity as well as near-surface interactions. A total effective exposure of 384 kg.d has been achieved, mostly coming from fourteen months of continuous operation. Five nuclear recoil candidates are observed above 20 keV, while the estimated background is 3.0 events. The result is interpreted in terms of limits on the cross-section of spin-independent interactions of WIMPs and nucleons. A cross-section of 4.4x10^-8 pb is excluded at 90%CL for a WIMP mass of 85 GeV. New constraints are also set on models where the WIMP-nucleon scattering is inelastic.

Final results of the EDELWEISS-II WIMP search using a 4-kg array of cryogenic germanium detectors with interleaved electrodes

TL;DR

This study reports final results from the EDELWEISS-II WIMP search using ten interleaved-electrode Ge detectors (Ge-ID) to suppress gamma and surface backgrounds. By accumulating a large exposure of in a low-background underground setting, the collaboration observes five nuclear-recoil candidates above 20 keV and derives a spin-independent WIMP–nucleon cross-section limit of pb at GeV, with background accounting constrained to events at 90% CL. The results also constrain inelastic scattering with a mass splitting of keV, excluding part of the DAMA-allowed region for GeV/ and strengthening limits above GeV/ relative to CDMS. Overall, the work demonstrates the efficacy of Ge-ID technology, informs background-reduction strategies for future phases, and sets the stage for a 40-kg array targeting sensitivities near pb.

Abstract

The EDELWEISS-II collaboration has completed a direct search for WIMP dark matter with an array of ten 400-g cryogenic germanium detectors in operation at the Laboratoire Souterrain de Modane. The combined use of thermal phonon sensors and charge collection electrodes with an interleaved geometry enables the efficient rejection of gamma-induced radioactivity as well as near-surface interactions. A total effective exposure of 384 kg.d has been achieved, mostly coming from fourteen months of continuous operation. Five nuclear recoil candidates are observed above 20 keV, while the estimated background is 3.0 events. The result is interpreted in terms of limits on the cross-section of spin-independent interactions of WIMPs and nucleons. A cross-section of 4.4x10^-8 pb is excluded at 90%CL for a WIMP mass of 85 GeV. New constraints are also set on models where the WIMP-nucleon scattering is inelastic.

Paper Structure

This paper contains 15 sections, 1 equation, 5 figures.

Table of Contents

  1. Introduction
  2. Experimental setup and data sets
  3. Detector calibration
  4. Fiducial volume
  5. Nuclear recoil selection
  6. Gamma-ray rejection
  7. WIMP search data
  8. Data selection and exposure
  9. Observed distributions
  10. Background estimates
  11. Limit on spin-independent cross-sections for elastic and inelastic models
  12. Elastic cross-section
  13. Inelastic cross-section
  14. Conclusion
  15. Acknowledgments

Figures (5)

  • Figure 1: Distribution of the ionization yield versus recoil energy for fiducial events recorded during neutron calibrations for all Ge-ID detectors. The full lines represent the parametrization of Ref. martineau for nuclear recoils and the 90%CL nuclear recoil band. In addition to pure electron and nuclear recoils, inelastic nuclear recoils are visible with associated electromagnetic energies of 13.26 and 68.75 keV, due to the desexcitation of short-lived states of $^{73}$Ge created by neutron diffusion (dashed lines).
  • Figure 2: Distribution of the ionization yield versus recoil energy for fiducial events recorded by Ge-ID detectors during all $\gamma$-ray calibrations regularly performed with $^{133}$Ba sources. The same period selection and quality cuts are applied than in WIMP search. The top line represents the 99.99% lower limit of the electron recoil band for typical noise conditions. The bottom (green) line is the typical ionization threshold, while the 90%CL nuclear recoil region is represented as a red band.
  • Figure 3: Ionization yield vs recoil energy of fiducial events recorded by EDELWEISS-II in an exposure of 427 kg$\cdot$d. The WIMP search region is defined by recoil energies between 20 and 200 keV, and an ionization yield inside the 90% acceptance band (full red lines, corresponding to an effective exposure of 384 kg$\cdot$d). WIMP candidates are highlighted in red. The average (resp. worst) one-sided 99.99% rejection limits for electron recoils are represented with a continuous (resp. dashed) blue line. The average (resp. worst) ionization thresholds are represented with a continuous (resp. dashed) green line.
  • Figure 4: Limits on the cross-section for spin-independent scattering of WIMPs on the nucleon as a function of WIMP mass, derived from the present work, together with the limits from CDMS cdms, ZEPLIN zeplin and XENON100 xenon100. The shaded area correspond to the 68% and 95% probability regions of the cMSSM scan from Ref. theo.
  • Figure 5: Inelastic WIMP-nucleon cross-section limits at 90%CL as a function of WIMP mass, for a mass splitting value $\delta =120$ keV. Also shown are the limits from XENON10 xenon_idm (with a conservative $L_{\rm eff}$), ZEPLIN-III zeplin_idm and CDMS arrenberg2010 (from a dedicated analysis). The 95% allowed DAMA contour, as estimated in arrenberg2010 from dama, is shown in light gray.