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.

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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