EDELWEISS and EURECA
EDELWEISS team in Sheffield
|Dr. Vitaly Kudryavtsev|
|Dr. Matthew Robinson|
EURECA team in Sheffield
|Dr. Vitaly Kudryavtsev|
|Dr. Matthew Robinson|
|Dr. Vito Tomasello|
|Dr. Ed Daw|
News from EDELWEISS-II
March 2011 - Final results of the EDELWEISS-II WIMP search with Ge-ID detectors (arXiv:1103.4070)
The EDELWEISS-II collaboration has completed a direct search for dark matter WIMPs (Weakly Interacting Massive Particles) with an array of ten 400 g germanium detectors operated at cryogenic temperatures of about 10 mK in the Modane Underground Laboratory (France). 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. After more than one year of operation, five events have been observed above 20 keV in the region where WIMP interactions are expected with the estimated background being less than 3.0 events. The Collaboration cannot exclude the possibility that the observed events are due to background radiation but some of these events can be due to dark matter particles. The result has been interpreted in terms of limits on the cross-section of WIMP interactions with matter. Cross-sections above 4.4×10-8 pb (4.4×10-44 cm2) are excluded at 90%CL for a WIMP mass of 85 GeV. This significant progress in detector sensitivity (a factor of 35 improvement over previous EDELWEISS results published in 2002) has been achieved thanks to the development of a new technique of background rejection using inter-digit (ID) electrodes. The EDELWEISS-II Collaboration is currently working on improving the background rejection techniques to be able to reach an order of magnitude better sensitivity in a couple of years.
The EDELWEISS-II Collaboration includes scientists from universities and research institutions from France, Germany, Russia and the United Kingdom (Oxford and Sheffield). Official EDELWEIS-II web-page: http://edelweiss2.in2p3.fr, web-page in Sheffield: http://www.hep.shef.ac.uk/research/dm/eureca.php
Brief introduction to dark matter search
Non-baryonic dark matter is believed to be responsible for about 85% of the total matter content and for about 23% of the total energy content of the Universe. The most likely dark matter candidate - Weakly Interacting Massive Particle (WIMP), is a natural product of Supersymmetric Theories of particle physics. A search for these particles with accelerators (particle physics) and using direct or indirect methods of WIMP detection (particle astrophysics) complement each other and, in case of discovery, will allow physicists to determine or to severely constrain parameters of supersymmetric models.
Dark matter searches with cryogenic bolometers
Several hundred of particle astrophysicists across the world are working in the area of direct dark matter searches using highly sensitive instruments. Among these instruments, cryogenic bolometric detectors have been found to provide the lowest possible levels of background radiation and very high discrimination power between nuclear recoil events produced by WIMPs and electron recoil events caused by gamma-rays from environmental radioactivity. The most sensitive existing experiments using this technology are CDMS (US), EDELWEISS (France, Germany, Russia) and CRESST (Germany, UK). They have already set limits on spin-independent WIMP-nucleon cross-section at the level below 10-7 pb (1 picobarn = 10-36 cm2) starting to probe the region of parameter space of some supersymmetric models. The phase-II detectors with the target masses about 10 kg are currently starting to take data and the new limits down to 10-8 pb are expected within a year or two. The most favoured supesymmetric models predict, however, the cross-sections down to 10-10 pb indicating that further increase of the target masses up to one tonne with a significant reduction of background rate per unit mass of the target are necessary.
The EDELWEISS experiment
The EDELWEISS experiment is currently using about 4 kg of Ge crystals in a search for WIMPs at the Modane Underground Laboratory (France). The EDELWEISS Collaboration includes groups from universities and research institutions from France, Germany, Russia and the UK. The latest result included 384 kg×days of statistics and allowed us to set a limit on WIMP-proton spin-independent cross-section at a level of 4.4×10-8 pb for 85 GeV WIMP mass. More details about the experiment can be found at: http://edelweiss.in2p3.fr.
The EURECA project
The EURECA project is a merger of the two main European collaborations involved in the dark matter search with cryogenic bolometers - EDELWEISS and CRESST, with several other European groups from Spain, Switzerland and the UK joining the project. The final goal of EURECA is to discover dark matter WIMPs with the cross-section down to 10-10 pb using cryogenic bolometric technology - detecting phonons and either ionisation or scintillation from Ge and CaWO4/MoWO4 crystals with a target mass of up to one tonne. EURECA is planned to be constructed at the Modane Underground Laboratory.
Design and construction of such a detector represent a substantial challenge. Firstly, the scale-up from 10 kg to several hundred kg of target mass is not easy: cooling such a mass down to ~10-15 mK temperatures isolating all vibration noise is not straightforward, readout and data acquisition systems should be improved to be able to deal with thousands of channels. Secondly, simple increase in the target mass will provide only limited improvement in detector efficiency. Various background radiations impose severe constraints on the sensitivity.
One of the areas where the Sheffield team is working, is the Monte Carlo modelling of the background radiations (Vitaly Kudryavtsev, Matt Robinson and Vito Tomasello). Various types of radiation are key factors that limit sensitivity of many underground experiments in particle astrophysics. Detectors searching for WIMP dark matter, neutrinoless double-beta decay, proton decay and low-energy neutrinos are sensitive not only to the rare events in question, but also to any background radiation that should be suppressed or rejected using various techniques. Among the types of radiation, the most dangerous are gammas because of their high fluxes, and, especially, neutrons due to their high penetrating capability and ability to mimic the events that experiments are looking for. In dark matter detectors, for instance, neutrons are capable of mimicking WIMPs, producing nuclear recoils in a similar way to WIMP-nucleus interactions.
We are involved in the modelling of neutrons and gamma-rays for the EDELWEISS experiment. Our Monte Carlo of neutron background from radioactivity in rock and major detector components has shown that the expected number of nuclear recoils caused by this background in the region of interest, in 384 kg×days of statistics does not exceed 1.4.
Sheffield team is also working on simulations of neutron and gamma background originated from radioactivity in rock and detector components, and from cosmic-ray muons for the EURECA project. Some plots from initial work are shown below. The results of simulations will be used for designing passive shielding and active veto systems for EURECA and for choosing suitable materials for detector and shielding construction.
Sheffield team (Ed Daw) is also involved in the control of vibrations. Control of vibrations is a concern for the compressors and pumps of the 4He cryoplant and of the dilution refrigerator. Manufacturers will be requested to minimize noise and vibrations at the source, and vibration damping system is foreseen. Suitable vibration isolation scheme will be selected based on passive and/or active approaches.
Figure 1. Ionization yield vs recoil energy of fiducial events recorded by EDELWEISS-II with an exposure of 427 kg×d. The WIMP search region is defined by recoil energies between 20 and 200 keV, and an ionization yield inside the 90% acceptance band (solid red lines, corresponding to an effective exposure of 384 kg×d). WIMP candidates are highlighted in red. The average (worst) one-sided 99.99% rejection limits for electron recoils are represented by a solid (dashed) blue line. The average (worst) ionization thresholds are represented by a solid (dashed) green line.
Figure 2. Limits on the WIMP-proton spin-independent cross-section from the EDELWEISS-II experiment in comparison with other results.
Figure 3. Ge detectors in EDELWEISS-II
Figure 4. Simulated neutron spectrum in the Modane Underground Laboratory from radioactivity in rock around the lab
Figure 5. Simulated (black curve) and measured (red histogram) muon spectra in the Modane Underground Laboratory. Measurements are taken with the Frejus detector. Dips correspond to the directions where the detector acceptance was close to 0. The detector acceptance was also taken into account in simulations.
List of recent publications relevant to background radiation studies and EDELWEISS/EURECA work:
- E. Armengaud et al. (The EDELWEISS Collaboration). "Final results of the EDELWEISS-II WIMP search usinga 4-kg array of cryogenic germanium detectors with interleaved electrodes", arXiv:1103.4070v1 [astro-ph.CO] 21 Mar 2011.
- V. Tomasello, M. Robinson and V. A. Kudryavtsev. "Radioactive background in a cryogenic dark matter experiment", Astroparticle Phys., 34 (2010) 70-79.
- V.A.Kudryavtsev, M.Robinson and N.J.C.Spooner. "The expected background spectrum in NaI dark matter detectors and the DAMA result", Astroparticle Phys., 33 (2010) 91-96.
- A. Lindote, H. M. Araújo, V. A. Kudryavtsev, M. Robinson. “Simulation of neutrons produced by high-energy muons underground”, Astroparticle Phys. 31 (2009) 366-375.
- V. A. Kudryavtsev. “Muon simulation codes MUSIC and MUSUN for underground physics”, Computer Physics Communications 180 (2009) 339-346.
- V. Tomasello, V.A. Kudryavtsev, M. Robinson. “Calculation of neutron background for underground experiments”, Nucl. Instrum. and Meth. in Phys. Res. A 595 (2008) 431-438.
- H. M. Araújo et al. “Measurements of neutrons produced by high-energy muons at the Boulby Underground Laboratory”, Astroparticle Phys. 29 (2008) 471-481.
- V. A. Kudryavtsev, L. Pandola, V. Tomasello. “Neutron- and muon-induced background in underground physics experiments”, European Phys. J. A36 (2008) 171-180.
- V.A. Kudryavtsev. “Neutron background in the Boulby Underground Laboratory”, Journal of Physics: Conf. series 120 (2008) 042028.
- E. Tziaferi, M. J. Carson, V. A. Kudryavtsev, R. Lerner, P. K. Lightfoot, S. M. Paling, M. Robinson and N. J. C. Spooner. “First measurement of low-intensity fast neutron background from rock at the Boulby Underground Laboratory”, Astroparticle Physics, 27 (2007) 326-338.
- A. Tang, G. Horton-Smith, V. A. Kudryavtsev, A. Tonazzo. “Muon simulations for Super-Kamiokande, KamLAND and CHOOZ”, Phys. Rev. D, 74 (2006) 053007.