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Direct Detection of WIMP Dark Matter with Liquid ArgonThe WARP Experiment
Frank Calaprice
Princeton University
International Workshop on Interconnection between Particle Physics and Cosmology
PPC 2007
Texas A&M May 14-18 2007
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
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
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
~ 10-46 cm2
Detector Requirements
Low background from natural radioactivity Beta and gamma radiation Neutrons Cosmic rays
Massive detector with a low (few keV) threshold.
Everything is radioactiveWhat 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
Nuclear Recoil Detector Strategies
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.
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.
Drift Properties
• 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
Argon as WIMP Target
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.
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Novel Properties of Argon forSuppression 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.
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
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.
Wimp ARgon Program(WARP) Collaboration
INFN and Università degli Studi di PaviaP. 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 NapoliF. Carbonara, A. Cocco, G. Fiorillo, G. Mangano
INFN Laboratori Nazionali del Gran SassoR. Acciarri, F. Cavanna, F. Di Pompeo, N. Ferrari,
A. Ianni,O. Palamara, L. Pandola
Princeton UniversityF. Calaprice, D. Krohn, C. Galbiati, B. Loer, R. Saldanha
IFJ PAN KrakowA.M. Szelc
INFN and Università degli Studi di PadovaB. Baibussinov, S. Centro, M.B. Ceolin,
G. Meng, F. Pietropaolo, S. Ventura
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
“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
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
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
The 3.4 kg Detector Chamber
First Dark Matter Results
Selected events in the n-induced single
recoils window during the WIMP search run:
None
Recoil Energy Calibration AmBe neutron source
AmBe sourceY = 1.26±0.15
ph.el/keV
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.
Dark Matter Limits Currently ~ 10-42 cm2
New run underway
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
WARP 140-kg Detector(under construction)
Background Sources
Neutron Sources
Delivery of External Cryostat
Projected Sensitivity
One year 140 kg null measurement with 30 keV threshold
~ 10-45 cm2
One year 1400 kg null measurement with 30 keV threshold
~ 10-46 cm2
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!
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
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 10-45 cm2