Exploring the DEAP Dark Matter Experiment: Direct Detection with Liquid Argon

1. DEAP:Dark Matter Experiment with Argon PSD Mark Boulay Queen’s University arxiv.org:/astro-ph/0402007

2. Outline: • Dark Matter Problem • Current techniques for direct detection of dark matter • Direct detection with Liquid Argon (LAr) • Some advantages of LAr • Design of and Results from DEAP-0: • 1 kg LAr cryostat at LANL (preliminary results) • Plans for DEAP-1: • 10 kg LAr cryostat at Queen’s (SNOLab early space)

3. The Dark Matter Problem • Rotation curves • Mass density distributed more broadly than visible objects • Non-luminous halo required to describe rotation curves -First reported in 1933 by Zwicky

4. Precision WMAP measurements map.gsfc.nasa.gov • Host of precision measurements culminating in WMAP. • Interpret power spectrum data by fit to cosmology • Cold dark matter fraction accurately determined DARK MATTER PROBLEM is > 70 years old and experimentally sound : do not understand origin of large fraction of matter in universe.

5. Enter Supersymmetry… (…or a new type of particle makes up the dark matter…) • SUSY provides a “natural extension” to the standard model of • particle physics • “Attractive route towards unifying all four forces ” -theorists • Introduces a new symmetry (R-parity) and possible existence • of a new stable particle • New particle properties could well be consistent with those • needed to account for the missing dark matter • Generically, direct searches are looking for WIMPs, • Weakly Interacting Massive Particles, which would make up • the dark matter • So SUSY provides a particle physics solution to a • cosmological problem (SUSY not motivated by DM problem)

6. 40Ar c 40Ar c Direct WIMP detection in terrestrial experiment • WIMPs can elastically scatter in detectorproducingnuclear recoils • Rate in terrestrial detector depends on WIMP mass and WIMP- nucleon interaction cross-section • Energy spectrum of recoils is exponential with E ~ 50 keV • Experimental challenge is to detect small number of nuclear recoils • with low energy threshold (order event/1000 kg/year > 10 keV)

7. The problem with direct WIMP detection (…or why these experiments are so tough…) • Radioactive decays from materials, and cosmic rays and their by-products, are backgrounds to recoiling nucleus signal • Even clean materials can lead to billions decays/year for kg-scale detectors. • Background events can be further divided into two classes: • Events that will ‘look like’ nuclear recoils • Events that won’t ‘look like’ nuclear recoils

8. Backgrounds in WIMP searches • In general, this roughly approximates to: • Neutron related backgrounds (since n’s can elastically scatter the • target nuclei just like WIMPs can) • b/g radiation. This will deposit energy in a detector but not scatter • the target nuclei. The approach taken is to reduce: 1. by using very clean materials and running experiment underground 2. by using clean materials and distinguishing n.r. events from b/g s Principle difference between DM experiments is how the distinction of n.r. events from b/g events is accomplished

9. ASIDE: a-emitters plated out on detector surfaces as potentially dangerous background LAr Cryostat wall Decay in bulk LAr tagged by a-particle scintillation a 210Po on surface Decay from surface releases untagged recoiling nucleus a

10. CDMS (Cryogenic Dark Matter Search) Exploits difference in deposited charge versus phonon energy between b/g ‘s and nuclear recoils g rays Collection of small detectors simultaneously measure deposited energy in charge and phonon channels ~1 kg / “tower” Current best limit neutrons ZIP detector 250 g Ge Image from cdms.berkeley.edu

11. XENON (proposed experiment) Many (most) DM experiments are technically very complex in order to discriminate b/g ‘s from nuclear recoils Total Xe mass 1 tonne Exploits difference in ionization signal (electrons) versus scintillation signal (photons) between b/g‘s and nuclear recoils Figure from Elena Aprile Dark Matter 2004

12. DEAP (Dark Matter Experiment with Argon PSD) • Spin-independent WIMP-nucleon scattering on liquid 40Ar • Spherical volume of LAr instrumented with PMTs to detect • scintillation photons • Discrimination of g/b backgrounds using only scintillation time information from PMTs • Generic spherical design scaleable to large target mass • DEAP-n: n = log10(target mass [kg])

13. Scintillation in liquid argon • ionizing radiation leads to formation of excited dimers in argon (Ar*2) • dimers are produced in either singlet or triplet excited states • decays to ground state have characteristic times, and can result • in photon emission • ~ 2 ns for singlet state (prompt) • 1.6 us for triplet state (delayed) • Fraction of dimers in singlet versus triplet state depends on • ionization density along track, and thus on incident particle • type • Net effect is a difference in the photon emission versus time • curve for g/b events and for nuclear recoils

14. http://arxiv.org/astro-ph/0411358 scintillation pulse-shape analysis for discrimination of e- vs nuclear recoils -> no electron-drift DEAP : Dark-matter Experiment with Argon PSD

15. Idea is to use scintillation photons only for discrimination in DEAP… …allows for simple detector design and possibly a more easily realizeable large-scale experiment

16. Some advantages of LAr • Inexpensive : 10 kg = 25$ or Lar • Good light yield, 40000 photons/MeV = good resolution • Used extensively, very large experiments underground • Easily accessible temperature (~85 K) • Same requirements as LN for cryogenic components • “Noble” noble gas • Liquid experiment can be continuously or periodically • purified (advantage over crystals) • Allows simple, inexpensive, scalable design

17. Simulation of discrimination in argon • 6 pe/keV for 75% coverage, with • 1500 Hz PMT noise • Backgrounds from Ham. R9288 (approx. 70 mBq/PMT) • 5 ns PMT resolution • 20% photon detection efficiency • 100 ns trigger window sets T0 • Fprompt = Prompt hits(100 ns)/Total hits(15 us) • ~2 kg Ar with 10 keVee threshold (60 pe) Dominant backgrounds assuming proper shielding, depth, and clean construction.

18. Background rejection with LAr (simulation) From simulation, rejection > 108 @ 10 keV (>>!) (Goal for SuperCDMS is 108) 108 simulated e-’s 100 simulated WIMPs

19. DM Sensitivity with LAr with 1-year exposure LAr with 10 keV (electron) threshold

20. Direct detection prediction from SUSY NMSSM (Next-to-MSSM) Prediction from talk by David Cerdeno at SUSY 2005 (JHEP 12 (2004) 048) 10-44 cm2 (10 kg LAr) 10-45 cm2 (100 kg LAr) Maybe within our reach!

21. DEAP-0 (1 kg) at LANL • PMT in air outside of large • vacuum chamber • ~1 kg LAr viewed by single • 2” PMT • calibration with g’s, n’s • (tagged 22Na and AmBe) • Demonstration of PSD • Test long term gain stability 40” DEAP-0 Timeline: Design: Jan 05 Order components: Feb 05 Rec’d all components: May 05 Assembly: June,Jul 05 Data run & analysis: Jul, Aug 05 (Analysis being completed) DEAP-0 M. Boulay, A. Hime, L. Rodriguez (LANL) Supported by LANL LDRD, with technical assistance and advice from: Steve Lamoreaux, Dan McKinsey, James Nikkel, Seppo Pentilla, …

22. Gas Handling System for DEAP-0 SAES purifier, < 0.1 ppb

23. DEAP-0 construction at LANL • Conflat construction, Cu gaskets, “standard” components • where possible to reduce cost • ~1 kg of liquid argon with 2” windows, viewed by 1 PMT • in air

24. DEAP-0 Construction at LANL Liquid nitrogen cooling, Ar gas in Cu coils

25. DEAP-0

26. DEAP-0 in vacuum chamber

27. DEAP-0 PMT setup at LANL PMT coupled to LAr through chamber window Source with CsI/PMT for gamma tag Vacuum chamber CsI tag LAr PMT source windows

28. DEAP-0 windows (post-warm-up) Window to argon chamber …room for improvement!

29. PMT pulses from LAr, in coincidence with g in CsI g-like neutron-like

30. Triplet lifetime check

31. Discrimination in liquid argon AmBe runs (neutron calibration) Na-22 runs 4 x 106 tagged g’s <pe/keV> = 0.1

32. Discrimination in liquid argon from DEAP-0 <pe> = 60 O(1in 105) consistent with random coincidence with room neutrons (preliminary) preliminary <pe> = 60 corresponds to 10 keV with 75% coverage • Final analysis and systematics evaluation being done (Kevin and Reuble)

33. Conceptual design of DEAP-1 • ~10 kg Lar • Spherical geometry • PMTs coupled to inner • chamber through light guides • PMTs surrounded by polyethylene for n absorption • Inner chamber could be • (stainless steel,acrylic,copper) • Investigate using expanded • polystyrene for thermal insulation (vacuum chamber if needed)

34. Photon detection for DEAP-1 Acrylic light guide (UVA) Low background PMT window or acrylic vessel LAr 85K 300K 6” Acrylic guide backs off PMT to reduce (a,n) neutron backgrounds, and to reduce thermal load. Q = kA(Th-Tc)/L ~ 1 Watt

35. Photomultiplier tube (PMT) backgrounds in DEAP-1 For reference, 250 events/year for the ET9390 PMTs

36. Inner cryostat backgrounds in DEAP-1 • 4 neutrons/year/kg of SS (Peter Skensved) • ~3% leakage into signal region (Geant4 Monte-Carlo) • problematic background! • Investigating acrylic chamber for inner cryostat (Kevin Graham) • Could possibly use Copper cryostat • Internal backgrounds (impurities in LAr) • Will use gas purification and cold charcoal traps • In-situ assay of internal backgrounds with DEAP-1

37. Active neutron veto LAr Vacuum region n Neutron active veto (conceptual) Note: thermal neutron capture cross-section on Ar: 675 mbarn • Active veto can mitigate internal and external low-energy (a,n) neutrons • Relaxes internal (a,n) requirements • Possible overlap with SNO+ for liquid scintillator active veto

38. elg = 0.8 epmt = 0.25 a = 0 Y0 = 40 photons/keV Optimizing optics for DEAP-1 “Toy” optics model Model incorporating reflective losses and absorption: Y=R[1/S-1]elgepmt(1-a)Y0 Y = yield [photons/keV] R = surface reflectivity S = surface PMT coverage elg = light guiding efficiency epmt = PMT efficiency a = absorption Y0 = photon production yield Need real model to map inputs to yield, O(10%) (Kati N.)

39. Short-term activities for DEAP-1 at Queen’s • Develop optical model for use in chamber and light guide design, • test model in the lab: • GEANT4 simulation for optics • Most parts ordered for light guide/reflectivity tests using • a-scintillation in gaseous argon (~1 month to parts?) • Design cryostat for proper cooling and background requirements • for 10 kg detector and for optics/background tests with LAr • Inner chamber R&D (acrylic versus copper versus stainless) • Design and construction of clean room • Once these are in place, design and build clean 10 kg • experiment (DEAP-1)

40. DEAP-1 Timeline and SNOLab • Currently designing cryostat, selecting components • Clean room being constructed at Queen’s • Plan is to construct and commission DEAP-1 at Queens O(6 months) • Calibration & verification of PSD above ground, spring/summer 2006 • Seek “early” SNOLab space Fall 2006 for UG running • 10-44 cm2 with one year livetime Funding for DEAP-1 is in place with CFI/startup grant from Queen’s and LANL LDRD support O(750K total) • Philosophy is to design DEAP-1 so that scaling to DEAP-3 • is feasible, x 100 improvement in background required

41. Response of SNOLab Experiment Advisory Committee to DEAP: “Very interesting and recent technical developments using LAr provide the possibility for a conceptually simple and relatively inexpensive route to a large-scale detector. Given existing funds and plan to go forward, we strongly encourage submission of a technical proposal for DEAP-1.” -SNOLab EAC recommendations, August 2005 meeting

42. Conclusions • DEAP-0 (1 kg) succesfully executed at LANL • Demonstrated discrimination using PSD only in LAr • Currently designing DEAP-1 (10kg) for construction at Queen’s • Possibly ready for deployment in early SNOLab space (Sept 2006) • 10-44 cm2 sensitivity with 1 live-year, 10 keV threshold • Funding for DEAP-1 in place, CFI/startup + LANL LDRD • Opportunity for hardware design, analysis, Monte-Carlo simulation, underground experiment deployment and running experience, and • …potential Discovery of New Physics beyond the new Standard Model… on the timescale of a PhD