Gascon, J.

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Status and prospects of the EDELWEISS direct WIMPsearchJ. GasconTo cite this version:J. Gascon. Status and prospects of the EDELWEISS direct WIMP search. XXIst Interna-tional Europhysics Conference on High Energy Physics - HEP2011, Jul 2011, Grenoble, France.pp.057, 2011. <in2p3-00700828>HAL Id: in2p3-00700828http://hal.in2p3.fr/in2p3-00700828Submitted on 24 May 2012HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.PoS(EPS-HEP2011)057Status and prospects of the EDELWEISS directWIMP searchJ. Gascon(for the EDELWEISS Collaboration)Institut de Physique Nucléaire de Lyon,Université de Lyon, Université Lyon 1, CNRS/IN2P3,4 rue E. Fermi 69622 Villeurbanne cedex, FranceE-mail: gascon@ipnl.in2p3.frEDELWEISS is a direct search for WIMP dark matter using cryogenic heat-and-ionization germa-nium detectors. We report the final results of the second stage of the experiment, EDELWEISS-II,obtained with an array of ten 400 g detectors equipped with interleaved electrodes for the rejectionof surface events. Limits on the elastic and inelastic cross-sections of spin-independent interac-tions of WIMPs were derived from a total exposure of 384 kg·day, obtained following fourteenmonths of continuous operation at the Laboratoire Souterrain de Modane. We also present theprospects of EDELWEISS-III, which plans to accumulate more than 3000 kg·day of data withfourty new 800 g detectors. All the surfaces of these new detectors are fully covered with inter-leaved electrodes, resulting in an increase of fiducial mass and of the rejection capabilities.The 2011 Europhysics Conference on High Energy Physics-HEP 2011,July 21-27, 2011Grenoble, Rhône-Alpes Francec© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(EPS-HEP2011)057EDELWEISS statusThe EDELWEISS Collaboration performs direct searches for WIMPs [1] using cryogenic ger-manium detectors [2] to identify rare events where the collision of these elusive particles inducenuclear recoils with kinetic energy in the ten’s of keV range. In these detectors, kept at a temper-ature of 18 mK, the nuclear recoils induce two signals: an elevation of temperature of a few µK,measured with a Ge-NTD heat sensor glued to the surface of the crystal providing a truly calorimet-ric measure of the total deposited energy, and an ionization signal, collected on electrodes coveringall surfaces. The dominant background of events due to β and γ radioactivity can be reduced byfactors exceeding 104 by comparing the size of the ionization signal to the recoil energy. For thesearch, the detector are operated in a radiopure underground environment located in the Labora-toire Souterrain de Modane (LSM) where the 4850 meter-water-equivalent rock cover reduces thecosmic-ray background by six order of magnitude. The collaboration has just concluded a searchusing an array of ten 400 g detectors with a novel electrode design ("ID" detectors), and is preparingthe deployment of an array of forty 800 g detectors of a new generation ("FID" detectors).1. EDELWEISS-II resultsThe experiment EDELWEISS [2] set its most recent limits using an array of ten 400 g germa-nium cryogenic detectors, installed in the Laboratoire Souterrain de Modane (LSM). The exper-iment is protected by the γ-ray background with a 20 cm lead shield and through a very carefulselection of all materials. A clean room surrounds the whole experiment and a class 100 laminarflow with deradonised air (< 0.1 Bq/m3) is used when mounting the detectors. Fast neutrons area particular background as their scattering in the detector can induce germanium recoil similar tothose expected from WIMPs. The fast neutron flux originating from the surrounding rock is moder-ated through a 50 cm polyethylene shield surrounding the lead shield. The Monte Carlo simulationof the efficiency of that shield has been tested by comparing the predicted attenuation of a strongAmBe source with the observed yields. Given the measured natural neutron background in theLSM [3], less than 0.1 nuclear are expected in an exposure of 400 kg·d. A muon veto with a 98%geometric efficiency surrounds the polyethylene shield, in order to tag neutrons created by muoninteractions inside the lead shield. The principle of operation and the performance of the germa-nium detectors are described in Ref. [4]. The originality of the detector design is the efficient use ofconcentric electrodes on the flat surfaces of the cylindrical crystal surfaces to tag events occurringnear them. This is of particular importance, since electron from β and γ interactions near the de-tector surface may suffer from bad charge collection, and thus mimic nuclear recoil interaction. Toidentify surface events, the electrodes on the flat surfaces are concentric annular interleaved elec-trodes, with a pitch of 2 mm. In this "InterDigit" electrode design (ID), surface events are tagged bythe presence of charge on two electrodes on the same side of the detector, resulting in a measuredrejection factor for events due to the surface contamination by 210Pb of ∼105 [4]. The cylindricalsides of the detectors are fully covered with plain electrodes, operated at lower voltages. No sur-face rejection can be done in the corresponding volume. Events with charge on these electrodesare therefore rejected. With the additional cut on the interleaved electrodes, corresponding to depthdown to 1mm below the flat surfaces, the resulting fiducial volume per detector is 160 g [2].Fourteen months of data recored in low-background conditions have been collected with theten 400 g detectors between March 2009 and May 2010 [2]. A common analysis recoil energy2PoS(EPS-HEP2011)057EDELWEISS statusRecoil energy [keV]0 20 40 60 80 100 120 140 160 180 200Ionization yield00.20.40.60.811.21.4Figure 1: Left pannel: Ionization yield as a function of recoil energy for the 384 kg·d EDELWEISS data set.Right pannel: Limits on the cross-section for spin-independent scattering of WIMPs as a function of mass,derived from the present work, together with the limits from CDMS [5], ZEPLIN [6] and XENON100 [7].The shaded area correspond to the 68% and 95% probability regions of the cMSSM scan from Ref. [9].threshold of 20 keV has been used for all detectors, corresponding to the value at which the effi-ciency is independent of energy and the rejection of the gamma-ray background deduced from theionization and heat resolutions is above 99.99%. The total fiducial exposure for nuclear recoils is384 kg·d. The data are shown in the left panel of Fig. 1. Four nuclear recoil candidates are ob-served between 20.2 and 22.5 keV, and additional one is observed at 172 keV. The resulting limitsare shown in the right pannel of Fig. 1, compared with those of other direct WIMP searches [5, 6, 7].The data has a significant impact on the total sensitivity obtained when combining them with theresults of the other search using cryogenic germanium detectors, CDMS [8]. While this relativelysmall (1.6 kg fiducial) array achieved a sensitivity comparable to other leading searches, its resultswere limited by the appearance of background. The predicted background in the 384 kg·d exposureis 3 events, the largest uncertainty on this number being associated with the presence of the largelow-field region below the guard electrodes, and the possible production of neutron via (α ,n) reac-tion in the volume inside the lead shield [2]. This lead to plans to improve the detector design byremoving as much as possible the surfaces covered by conventional flat electrodes, and to providefor an internal neutron polyethylene shield.2. EDELWEISS-III status and prospectsFollowing these observations, the detector design has been improved and it was found possibleto covers the entire surface of the detectors with the interleaved electrode pattern, as shown in theleft pannel on Fig. 2. With this new design without a low-field guard region, it was also possibleto increase the mass of the detector to ∼800 g. With these two modifications, the fiducial mass perdetector is nearly 600 g. A second thermal measurement has been added to remove any ambiguityin the interpretation of the thermal signal and improve the resolution of the heat measurement. Testsof these new detectors are under way, with encouraging results. In particular, the γ-ray rejectionperformance was found to be improved relative to the ID design. The right pannel of Fig. 2 shows3PoS(EPS-HEP2011)057EDELWEISS statusFigure 2: Left: FID detector just after the evaporation of its interleaved electrodes. Right: Ionization yieldas a function of recoil energy obtained by exposing FID detectors to more than 400,000 fiducial γ-ray events.the results of a γ-ray calibration of two FID detectors with a 133Ba source. No nuclear recoilcandidates are observed after an exposure to more than 400,000 electron recoils.The construction of an array of forty 800 g detector, to be completed by 2012, is in progress,with a projected sensitivity of WIMP scattering cross-section of 5×10−9 pb. This third stage ofEDELWEISS will also see its cryogenics and electronics being upgraded for achieving a recoilenergy analysis threshold below 15 keV. Further developments are being studied in the frameworkof the EURECA [10] Collaboration, aiming for an experiment in the future extension of the un-derground laboratory at LSM, with a sensitivity goal of 10−11pb . This project brings togetherthe European efforts of EDELWEISS and also of the CRESST and ROSEBUD teams working onheat-and-scintillation detectors, to built a ton-scale multi-target array of cryogenic detectors.References[1] G. Jungman, M. Kamionkowski, and K. Griest, Phys. Rep. 267, 195 (1996); G. Bertone, D. Hooperand J. Silk, Phys. Rep. 405, 279 (2005).[2] E. Armengaud et al. (EDELWEISS Collaboration), Phys. Lett. B 702 (2011) 329.[3] R. Lemrani et al., J. Phys. Conf. Ser. 39 145 (2006) 145.[4] A. Broniatowski et al. (EDELWEISS Collaboration), PLB 681 (2009) 305.[5] Z. Ahmed et al., Science 327 (2010) 1619.[6] V.N. Lebedenko et al., Phys. Rev. D 80 (2009) 052010.[7] E. Aprile et al. (XENON Collaboration), Phys. Rev. Lett. 107 (2011) 131302.[8] Z. Ahmed et al. (CDMS and EDELWEISS Coll.), Phys. Review D 84 (2011) 011102.[9] L. Roszkowski, R. Ruiz de Autri and R. Trotta, JHEP 07 (2007) 075.[10] H. Kraus et al., J. Phys. Conf. Ser. 39 (2006) 139.4

Status and prospects of the EDELWEISS direct WIMP search

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