Dark matter direct detection with Edelweiss-II Eric Armengaud - CEA / IRFU Blois 2008 http://edelweiss2.in2p3.fr/ 1 Direct detection of dark matter : principles . A well-identified science goal: Detect the nuclear recoil of local WIMPs inside some material Target the electroweak interaction scale, m ~ GeV-TeV At least 3 strategies: Search for a global recoil spectrum V(Earth/Sun): Search for a - small - annual modulation V(Sun/Wimp gaz): Search for a - large - forward/backward asymetry Remove backgrounds!!! Go deep underground Use several passive shields Develop smart detectors to identify the remaining radioactivity interactions 2 Direct detection of dark matter . A well-identified science goal: Detect the nuclear recoil of local WIMPs inside some material Target the electroweak interaction scale, m ~ GeV-TeV At least 3 strategies: Search for a global recoil spectrum • Cryogenic bolometers: CDMS, CRESST, EDELWEISS… • Liquid noble elements: XENON, LUX, ZEPLIN, WARP, ArDM… • Superheated liquids: COUPP, PICASSO, SIMPLE… • Solid scintillator (DAMA, KIMS), low- threshold Ge (TEXONO), gaz detector (DRIFT)… A very active field with both R&D and large-scale setups 3 The XENON10 experiment • Quite easily scalable, « high » temperatures • First scintillation pulse (S1) + Second pulse due to e- extraction to the gaz (S2) ⇒ Nuclear recoil discrimination - especially @ low E • Position measurements ⇒ Fiducial cut (~5kg) • Low-energy threshold + Xe mass ⇒ sensitivity to low-mass Wimps Electronic recoils Nuclear recoil region But : not background-free (60 live days run) 4 The CDMS-II experiment • « Towers » of detectors @ 40mK •Heat and ionization pulses ⇒ remove bulk electronic recoils (γs) • Measurements of phonon delays and risetimes ⇒ remove surface events (βs : miscollected charges), but loose exposure • 15 cristals used, 1000 « raw » kg.days, 121 kg.days after cuts timing cut Still background free - only 1/2 of currently recorded data 5 The Edelweiss-II collaboration Based on the experience from Edelweiss-I : Conceptually close to CDMS Cryogenic bolometers with heat+ionization channels ⇒ identify bulk gamma-rays Was already limited, in the end, by the presence of surface (β) interactions with incomplete charge collection 62 kg.d with 3 detectors (final results 2005) Edelweiss-II : Completely new setup : host up to 100 detectors, of different kinds Development of new detectors EDW-II: • CEA Saclay • CSNSM Orsay • IPN Lyon • Institut Néel Grenoble • FZ / Universität Karlsruhe • JINR Dubna 6 The Edelweiss-II setup - an example of DM direct detection facility Operated at the Underground Laboratory of Modane (4μ/day/m2) Cryogenic installation (~ 20 mK) : Reversed geometry cryostat Use of pulse tubes Shieldings : Clean room + deradonized air Active muon veto (>98% coverage) PE shield Lead shield ⇒ γ background reduced by ~2 wrt EDW1 Facilities : Remotely controlled sources for calibrations + regenerations Detector storage & repair within the clean room 9 cool-downs already operated 7 The Edelweiss-II setup - an example of DM direct detection facility Operated at the Underground Laboratory of Modane (4μ/day/m2) Cryogenic installation (~ 20 mK) : Reversed geometry cryostat Use of pulse tubes Shieldings : Clean room + deradonized air Active muon veto (>98% coverage) PE shield Lead shield ⇒ γ background reduced by ~2 wrt EDW1 Facilities : Remotely controlled sources for calibrations + regenerations Detector storage & repair within the clean room 9 cool-downs already operated 8 The Edelweiss-II setup - an example of DM direct detection facility Operated at the Underground Laboratory of Modane (4μ/day/m2) Cryogenic installation (~ 20 mK) : Reversed geometry cryostat Use of pulse tubes Shieldings : Clean room + deradonized air Active muon veto (>98% coverage) PE shield Lead shield ⇒ γ background reduced by ~2 wrt EDW1 Facilities : Remotely controlled sources for calibrations + regenerations Detector storage & repair within the clean room 9 cool-downs already operated Can operate at the same time: - R&D on several detectors - Large exposure acquisitions 9 Edelweiss-II : the instrumented detectors The « standard » EDW-I NTD bolometers: 2 ionization channels (center+guard) NTD heat channel (edge of detector) New holders, covers, cabling May 2008 : 23 detectors (some with a β source on the cover for « calibration ») R&D bolometers for active β Run 9 setup (April-May) rejection « NbSi » detectors (May 2008: 4 detectors) « Interdigit » detectors (May 2008: 4 detectors) Some other detectors A Ge73 detector (SD search) A heat/scintillation Al2O3 bolometer 10 Standard NTD detector operation ~19 kg.d bckgd Q (spring 07) Erecoil • First background runs spring 07, « long » « Beta calibration » background run last winter • Already ~100 kg.d of fiducial exposure accumulated (after quality cuts) • Specific studies of detector’s response to surface events (« beta calibrations ») to have a quantitative understanding of the background and try to take it into account in the analysis 11 NbSi detectors • Detectors developed at CSNSM : identify surface events = athermal phonon measurement with Nb-Si films « replacing » the NTDs • Surface event rejection ok, some problems in 2007 with film contacts / leak currents ThermometerN bSii A ThermometerN bSi A 1.5 fiducial kg.d, 2007 ThermometterN bSi B ThermometterN bSiB Heat signal of thermometNerb SiA : 200 Surface event Bulk event 150 100 Transient Thermal 50 0 0 20 40 60 80 -3 Txi1m0 e (m s) 12 Interdigit detectors ‘a’ electrodes (+2V) collecting ‘b’ electrodes (-1V) field shapping • Keep the standard phonon detector • Modify the E field near the surfaces with interleaved electrodes (with guards: 6 ‘c’ (-2V) ‘d’ (+1V) ionization channels) • Use ‘b’ and ‘d’ signals as vetos agains surface events From preliminary see-level measurements: • Surface event rejection > 95% • Fiducial volume (for a 200g detector) ~ 50% 13 Interdigit detectors : background runs at LSM Few kg.d of background runs with a 200g detector: Performance as expected Now installing 3 more 400g detectors, and doing a β calibration, for further performance tests Q ⇒ A promissing, reasonably cheap and simple detector ~ 1.7 kg.d 14 Outlook Dark matter direct detection : A rapidly evolving field Several detection strategies available now: Indispensible to cross-check a possible discovery Each technology has its advantages and unknowns EDELWEISS-II is taking data : Quite large exposures in stable conditions already achieved, « 4-month cycles » with regular improvments and new detectors New, promissing « Interdigit » detector is operational ! The sensitivity of the experiment will - of course - depend on the ultimate detector performances (surface event rejection) and background encountered Aim ~10-8 pb, possibly even lower (Realistic) prospective example : background rate ~ 0.05 evts/kg.d ⊗ rejection factor 97% ⇒ with 1000 kg.d, expect 1 unrejected event, ie. reach ~ 2x10-8 pb sensitivity 15.