1. Direct Detection of WIMP Dark Matter with Liquid Argon The WARP Experiment
Frank Calaprice
Princeton University
International Workshop on Interconnection between Particle Physics and Cosmology
PPC 2007
Texas A&M May

2. Dark Matter Evidence for “dark matter” abounds:
Flattening of galactic rotation curves
Power spectrum of microwave background radiation (WMAP)
Gravitational lensing
Composition of Dark Matter: UNKNOWN

3. Leading Dark Matter Candidates
Axions
Motivated by strong CP problem
Extremely light: ~ 1 eV
Search by microwave cavity methods (ADMIX)
WIMPS (Weakly Interacting Massive Particles)
Measured abundance of cold dark matter compatible with a massive weakly interacting particle
Independent motivation from supersymmetry models of elementary particles

4. Direct Detection of Dark Matter WIMPS
Search for collisions of relic WIMPS with ordinary nuclei.
Low nuclear recoil energy expected
<100 keV
Low rate expected
Few events/ton/year if  ~ cm2

5. Detector Requirements
Low background from natural radioactivity
Beta and gamma radiation
Neutrons
Cosmic rays
Massive detector with a low (few keV) threshold.

6. Everything is radioactive What to do?
Build detector out of materials that have extremely low radioactivity (Big R&D).
Shield against external sources of radiation.
Underground sites
Develop detectors that have unique response to nuclear recoils compared to background.
Possible for radiation
Not possible for neutrons that scatter and produce nuclear recoil of same energy as WIMPS.
Both of the above

7. Nuclear Recoil Detector Strategies

8. Why argon and other noble gasses for WIMP detection?
Low threshold energy due to high scintillation light yield (~400 photons/keV)
Excellent ionization drift properties
Scintillation and ionization each distinguish nuclear recoil events from  background.
Readily scalable up to ton-size, or larger.

9. Noble Liquids as Ionization Detectors
Negligibly small attachment probability
Ar + e- -> Ar- in 1 in 1012 collisions
Thermal electron mobility relatively fast
Few mm/sec for E ~ 1 kV/cm
Many years of experience with LAr by Carlo Rubbia and group (ICARUS)
Multi-ton detectors with meter drift-lengths successfully developed.

10. Drift Properties

11. Argon as WIMP Target Form factor very different from Xe, Ge targets
Lower A results in lower rate per unit mass at 10 keV threshold
For Mχ>100 GeV, “Gold Plated” events (>60 keV) still abundant!
Can run with a significantly higher threshold than other experiments and be very competitive

12. Argon-39 Beta Background
39Ar -> 39K + e- + 
t1/2 = 269 yr
Emax = 565 keV
Produced in atmosphere by cosmic rays:
n+40Ar -> 39Ar + 2n
Abundance: 8 x 10-16
Rate ~ 1 Hz/kg.
Need 108 suppression to make good WIMP search.

13. Novel Properties of Argon for Suppression of  Background
Recoil atoms and  radiation have very different stopping powers (dE/dx)
Observed scintillation intensity and ionization charge depend on dE/dx
Scintillation pulse shape depends on dE/dx.
In argon, these two effects provide discrimination between recoil and  events of 108 or better.

14. Pulse shape Discrimination in Ar
Two decay components in scintillation of argon.
Triplet state is long lived (1.6 s)
Singlet state is short lived (7 ns)
Singlet/triple ratio depends on stopping power (dE/dx)
Betas mostly triplet (slow long pulse)
Recoils mostly singlet (fast short pulse)
Pulse shape discrimination is statistical- more photons detected, the better
Have achieved close to 108 discrimination

15. Ionization/Scintillation Discrimination
Charged particles produce ionization.
Recombination of electrons and ions is greater if density of ionization in track is high.
More recombination means more scintillation
Recoils produce dense track.
Betas produce diffuse track
For same energy deposited there will less ionization and more scintillation for recoils than betas.

16. Wimp ARgon Program (WARP) Collaboration
INFN and Università degli Studi di Pavia
P. Benetti, E. Calligarich, M. Cambiaghi, L. Grandi,
C. Montanari, A. Rappoldi, G.L. Raselli, M. Roncadelli,
M. Rossella, C. Rubbia, C. Vignoli
INFN and Università degli Studi di Napoli
F. Carbonara, A. Cocco, G. Fiorillo, G. Mangano
INFN Laboratori Nazionali del Gran Sasso
R. Acciarri, F. Cavanna, F. Di Pompeo, N. Ferrari,
A. Ianni,O. Palamara, L. Pandola
Princeton University
F. Calaprice, D. Krohn, C. Galbiati, B. Loer, R. Saldanha
IFJ PAN Krakow
A.M. Szelc
INFN and Università degli Studi di Padova
B. Baibussinov, S. Centro, M.B. Ceolin,
G. Meng, F. Pietropaolo, S. Ventura

17. The Underground Halls of the Gran Sasso Laboratory
Halls in tunnel off A24 autostrada with horizontal drive-in access
Under 1400 m rock shielding (~3800 mwe)
Muon flux reduced by factor of ~106 to ~1 muon/m2/hr
WARP in Hall B ~20mx20mx100m
To Rome ~ 100 km

18. “Two Phase” Liquid-Gas Detector
WIMP hits nucleus, causing ionization due to recoil.
Partial recombination of electron-ion pairs produces scintillation S1 in liquid.
Remaining electrons from ionization drifted by E1-field to gas-liquid interface.
Electrons extracted from liquid by E2 and accelerated in gas to produce scintillation S2

19. Scintillator Pulse Shapes (S1)
The  scintillatio is very slow (1.6 s)
The recoil signal is very fast (7 ns)
Pulse shape provides discrimination
Use prompt/total ratio

20. Ionization/Scintillation Ratio (S2/S1)
More ionization (S2) relative to S1 scintillation for electrons
Less ionization (S2) to scintillation (S1) for recoils
Ratio S2/S1 is bigger for electrons than recoils

21. The 3.4 kg Detector Chamber

22. First Dark Matter Results
Selected events in the n-induced single
recoils window during the WIMP search run:
None

23. Recoil Energy Calibration AmBe neutron source
AmBe source
Y = 1.26±0.15 ph.el/keV

24. Recoil-  Discrimination
After recent electronics upgrade, pulse shape discrimination between m.i.p. and nuclear recoils better than 3x10-7
Shape of distribution does not change by applying S2/S1 cut. Two discriminations seemingly independent.

25. Dark Matter Limits Currently ~ 10-42 cm2 New run underway

26. The 140-kg WARP Detector Goal: achieve 10-45 cm2 sensitivity (SUSY)
Excellent Neutron Suppression:
Efficient External 4 neutron detector with 9 tons of active LAr viewed by 300 PMTs
Veto events with signals in both detectors (e.g., neutrons)
3D Event Localization with drift chamber
Veto multi-hit events (e.g., neutrons)
Define fiducial volume

27. WARP 140-kg Detector (under construction)

28. Background Sources

29. Neutron Sources

30. Delivery of External Cryostat

31. Projected Sensitivity
One year 140 kg null measurement with 30 keV threshold
~ cm2
One year 1400 kg null measurement with 30 keV threshold
~ cm2

32. WIMP Signatures Induces nuclear recoils, instead of electron recoils
WIMP signals do not have multiple interactions sites (as neutrons)
Recoil energy spectrum shape
Diurnal detection modulation
Consistency between different targets!

33. Sources of Argon with low 39Ar
Isotopic Separation
Russian Centrifuge production
5- kg sample delivered March ‘07 to LNGS
Expensive
Underground Argon
Abundant sources available
Measurements of 39Ar in underground samples underway by Princeton -Notre Dame -Harvard Argonne National Lab collaboration
First Measurements to be made with Accelerator Mass Spectrometry Spring ‘07

34. Conclusions
Beta/gamma backgrounds are under control with pulse shape and ionization/scintillation ratio
Neutron backgrounds are under control with the external neutron veto
WARP is poised to go the 100 kg level and reach the sensitivity of cm2