Transcript The Borexino Solar Neutrino Experiment

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).



Develop detectors that have unique response
to nuclear recoils compared to background.



Shield against external sources of radiation.
Underground sites
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





t1/2 = 269 yr
Emax = 565 keV
Produced in atmosphere by
cosmic rays:


-> 39K + e- + 
n+40Ar
->
39Ar
+ 2n
Abundance: 8 x 10-16
Rate ~ 1 Hz/kg.
Need 108 suppression to
make good WIMP search.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
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.



Singlet/triple ratio depends on stopping power
(dE/dx)



Triplet state is long lived (1.6 s)
Singlet state is short lived (7 ns)
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

Isotopic Separation




39Ar
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