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 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 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

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

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

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.

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.

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 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

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 source Y = 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