1 CRESST Cryogenic Rare Event Search with Superconducting Thermometers Jens Schmaler for the CRESST group at MPI MPI Project Review December 14, 2009

Outline 1.The CRESST experiment 2.Status and progress of CRESST 3.New CRESST test facility 4.Outlook: Search for new scintillators 2

Outline 1.The CRESST experiment 2.Status and progress of CRESST 3.New CRESST test facility 4.Outlook: Search for new scintillators 3

 4 Dark Matter Detection goal: detection of WIMPs via elastic scattering off nuclei challenges: –low recoil energies: O(10 keV) –low event rate: < O(10 kg -1 yr -1 ) needs: –low energy threshold –excellent background rejection –large target mass –heavy target nucleus CRESST target: scintillating CaWO 4 crystals –up to 33 crystals (10 kg target mass) Ø 40 mm, h=40 mm, 300 g

5 Detector Principle detectors operated at ~ 10 mK evaporated tungsten film as thermometer: –stabilized in transition to superconducting state –particle interaction  temporarily warms up crystal  resistance change –SQUID-based readout electronics energy threshold: ~ 1 keV ∆R ∆T temperature [mK] film resistance [m  ] ΔRΔR ΔTΔT crystal W film thermal coupling

6 Passive Background Suppression dealing with very low signal event rates < O(10 kg -1 yr -1 ) : 1.Gran Sasso underground lab below 1400 m of rock 2.several layers of passive shielding against ambient radioactivity 3.only very radio-pure materials O(10 4 kg -1 yr -1 ) background events still remain copper (10 t) detectors lead (24 t) radon box PE (10 t) muon veto cryostat active background discrimination needed! 1 m

Active Background Discrimination event-by-event discrimination of particle type higher light output of crystal  better discrimination crystals + separate light detector → “detector module” simultaneous measurement of – energy in crystal: E – scintillation light: L define: 7 Light absorber W film CaWO 4 target W film reflective housing β+γ (LY=1 by def.) O-recoils (neutrons, LY=1/10) α (LY=1/5) W-recoils (WIMPs, LY=1/40) light yield := L / E

8 CRESST Detectors phonon detector light detector

Composite Detectors motivation: evaporation of W film on crystal degrades light output  reduces background discrimination power solution: –first evaporate film on small carrier crystal –glue carrier to large crystal light output can be increased by ~ 50% 9 crystal glue W film carrier crystal

Outline 1.The CRESST experiment 2.Status and progress of CRESST 3.New CRESST test facility 4.Outlook: Search for new scintillators 10

11 Reminder: 2008 Data Taking (1) 9 modules operated between August and December 2008 observation: varying number of no-light events in signal region properties of no-light events: –rate decreases with time –partly: different pulse shape in phonon detector → can be rejected by shape cut Rita/Steven, ~ 7 kgd 90% of W-recoils below this line no light with light phonon detector detector effect rather than particle interactions

Data Taking (2) most likely origin of no-light events: stress relaxation events (at contact surface between clamp and crystal) two possibilities: 1.in crystal („micro cracks“) 2.in clamps clamps were mostly covered with plastic layer  more no-light events than with pure metal clamps clamp

13 CRESST – Current Status (1) consequence of 2008 run: new clamps for all crystals 1.thinner material → softer (less pressure on crystals) 2.material: only metal, no plastic coverage new cooldown started mid of May 2009 –delayed by earthquake in L’Aquila region –fast cool-down (stronger thermal coupling of detectors installed) –10 detector modules operational and taking data since June two composite detectors one ZnWO 4 crystal (alternative target material)

CRESST - Current Status (2) number of no-light events strongly reduced no-light events with different pulse shape disappeared composite detectors work well –high light-output –together with good light detector: significantly lower discrimination threshold achievable 14 acceptance region standard composite 90% of all W-recoils below this line discrimination threshold prelim. preliminary 10 keV threshold 5 keV threshold

CRESST – Current Status (3) preliminary analysis: a few candidate events observed in the signal region origin of these events? –still detector effect (cracks,...)? –signal (e.g. inelastic Dark Matter)? more statistics needed to investigate properties 15 aim: continue this run at least until mid 2010

Inelastic Dark Matter assumes inelastic scattering can reconcile claimed DAMA signal with other experiments CRESST: –tightest limits of all experiments (due to heavy target nucleus) –already excludes most of parameter space 16 CRESST (2007 data only) CDMS δ=130 keV [Schmidt-Hoberg et al. arXiv: ] current data should bring decision on this scenario

Outline 1.The CRESST experiment 2.Status and progress of CRESST 3.New CRESST test facility 4.Outlook: Search for new scintillators 17

CRESST Test Facility (1) motivation: full-sized CRESST detector modules cannot be operated over ground (too high count rate) up to now: many tests of new detector developments only in CRESST cryostat itself –long cooling cycle (typically one run per year) –risk to reduce outcome of CRESST data taking aim: decouple R&D and science runs 18 small test cryostat installed in Gran Sasso underground lab by MPI group in 2009

CRESST Test Facility (2) small cryostat short cooling cycle (3 days) before: used at MPI (~300 cooldowns) moderate Pb shielding readout for one complete detector module (including new data taking system) ready to be cooled down for testing expected to be useable early

Outline 1.The CRESST experiment 2.Status and progress of CRESST 3.New CRESST test facility 4.Outlook: Search for new scintillators 20

Outlook: New Scintillators amount of scintillation light is limiting factor: 21 find new scintillating target materials higher light output and/or higher light yield for heavy elements find new scintillating target materials higher light output and/or higher light yield for heavy elements candidates? CdWO 4 CsI 1. discrimination threshold: 2. W-recoils: light yield 1/40 not safely distinguished from no-light events at low energy W-recoils CaWO 4 effect of 2x more light CaWO 4 effect of 2x more light β,γ no-light events W-recoils

Summary status of CRESST: –data taking with 10 detector modules ongoing –new holding clamps: strongly reduced no-light events –important role of CRESST: constrain inelastic Dark Matter model new CRESST test facility at Gran Sasso –allows for efficient detector R&D under low-background conditions –ready for operation plans for 2010: –continue data-taking to clarify nature of remaining events in the signal region –search for new scintillators as target 22

23 Backup Slides

24 CRESST Detectors The phonon detector: 300 g cylindrical CaWO 4 crystal. Evaporated tungsten thermometer with attached heater. The light detector: Ø=40 mm silicon on sapphire wafer. Tungsten thermometer with attached aluminum phonon collectors and thermal link. Part of thermal link used as heater.

25 Detector Readout SQUID-based readout of W-thermometer:

26 Background from Surface α-Decays in particular from decay chain after 222 Rn: 210 Po → 206 Pb (104 keV) + α (5.4 MeV) Po can be implanted in surface → Pb-recoil in the relevant energy range (below 40 keV) scintillating surrounding as veto when hit by α-particle crystal Pb  reflective housing Po

27 Commissioning Run – DM Limit [G.Angloher et al., Astropart. Phys. 31 (2009), ] 4.8x10 -7 pb for 50 GeV/c 2 WIMPs

28 Clamp Discrimination Detector modified phonon detector: –normal crystal + 1cm slice of CaWO 4 –parts glued together –thermometer and clamps: on small part signals from the small part should have different pulse shape  possibility to discriminate clamp-induced events thermometer clamps glue

Comp. Detectors – Pulse Shape 29 target crystal thermometer carrier