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1
Direct Detection of Dark Matter Status and
Perspectives
Bruno Serfass University of California,
Berkeley (CDMS experiment)
Rencontres de Moriond March 2006
2
Evidence for Dark Matter
Large Scale Structure
CMB
Fritz Zwicky observed in 1933 anomalous large
velocity dispersion in distant galaxy
clusters Since that time many
more observations have been made, at different
scale, supporting the presence of Dark Matter in
the universe.
SN Ia
Lensing
Galaxy clusters
Rotation curves of galaxies
3
Dark Matter Candidates

• Baryonic Dark Matter

• Dust, Gas, Stars
• MACHOs (lt 20 of halo)
• BBM, light element ratio observations
• ?B 0.05 0.005
• All evidence agrees that baryonic
• dark matter exists but does not
• constitute much of the total dark matter

• Hot Dark Matter

• Upper limit from CMB, tritium decay and
  neutrino oscillations ?? lt 0.0155
• Massive neutrinos exist, but not enough mass
  to explain dark matter
• Hot dark matter cannot produce observed
  large scale structure of universe

• Cold Dark Matter (rest of this talk)

• Two well-motivated candidates from particle
  theory
• WIMPs (Weakly Interacting Massive
  Particles), Axions
• Theory suggests that either could
  naturally explain measured
• matter density

4
Weakly Interacting Massive Particles (WIMPs)

• Cosmology provides a class of long lived or
  stable particles
• at the EW scale left over from Big Bang
• If such particles produced in thermal
  equilibrium in early universe,
• Freeze out when annihilation too slow to keep
  up with expansion
• Leaves a relic abundance
• ?c h2 ??10-27 cm3 s-1
  ????ann v??fr

• For W c0.3
• M 10-1000 GeV
• ?A electroweak

• Independently, supersymmetry theories predict a
  stable s-particle state
• whose properties are very similar to the
  hypothetical WIMPs

Note not only supersymmetry (extra
dimesions Kaluza-Kein excitation, etc.)
5
Searching for Dark Matter

• Indirect detection
• Direct detection

• Look for products of annihilations in the sun
  or earth
• (AMANDA, IceCube, Super-K, EGRET, etc.)
• Make dark matter in accelerators and detect
  products of interactions (LHC), however
• not sensitive to high mass
• still need to show that particle could make
• DM (stable particle)

• Measure them when they elastically scatter off
  nuclei in target
• - Suppress background (go deep
  underground, shielding..)
• - Low energy threshold
• - Large target mass needed

6
Direct Detection of WIMPs
Detect WIMPS via elastic scattering on nuclei in
targets (nuclear recoils)
Energy spectrum rate depend on target nucleus
masses and WIMP distribution in Dark Matter
halo Standard assumptions

• Isothermal and spherical
• Maxwell- Boltzmann velocity distribution
• ltVgt 270 km/s, ? 0.3 GeV / cm3

• Energy spectrum of recoils falling
• exponential with ltEgt 15 keV
• Rate (based on ?nc and ? ) is of the order of
  a
• fraction of 1 event /kg/day

Log(rate)
Direct detection experiments need low threshold,
low background, and high target mass very
challenging!
Erecoil
7
Experimental Signatures

• Nuclear Recoils WIMPs produce nuclear recoils,
  while most of the backgrounds produce electron
  recoils

• Electron produce more ionization
  (scintillation) than a nuclear recoil of equal
  energy
• Example ionization Yield Eionization
  /Ephonon 0.3 for Ge nuclear recoils
• However, neutrons also produce nuclear recoils

Ge, Si
Liquid Xe
(CDMS, Edelweiss)
(XENON, ZEPLIN II, III, IV)
Phonons
NaI, Xe
(DAMA, ZEPLIN I)
Light
CaWO4
(CRESST II)
Al2O3, LiF
8
Experimental Signatures

• Nuclear Recoils WIMPs produce nuclear recoils,
  while most of the backgrounds produce electron
  recoils
• Annual modulation variation of WIMPs flux with
  time of year (annual)

• Requires long exposure and large mass to
  measure small effect (5)
• Experiments DAMA, KIMS, etc.

9
Experimental Signatures

• Nuclear Recoils WIMPs produce nuclear recoils,
  while most of the backgrounds produce electron
  recoils
• Annual modulation variation of WIMPs flux with
  time of year (annual)
• Directionality diurnal modulation following
  the earth rotation on its axis

• Difficult measurement typical recoil range
• is of order of 20 nm in a crystal (for a 20 keV
  recoil in Ge) and 20 mm in gas (30 keV in Kr)
• Experiments at RD stage (DRIFT)

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Experimental Signatures

• Nuclear Recoils WIMPs produce nuclear recoils,
  while most of the backgrounds produce electron
  recoils
• Annual modulation variation of WIMPs flux with
  time of year (annual)
• Directionality WIMPs produce nuclear recoils,
  while most of the backgrounds produce electron
  recoils
• No multiple interactions mean free path of a
  WIMP order of a light-year neutrons order of
  cm
• ? can be used to identify neutrons
  which also produce nuclear recoils
• Recoil energy spectrum shape exponential,
  rather similar to background
• Uniform rate throughout the detector WIMPs
  interactions must be spread evenly throughout the
  detector
• ? if detector significantly larger than
  mean free path of high-energy photons or
  neutrons, the interactions will occur mostly near
  the surface
• Consistency between targets of different
  nuclei essential once first signal clearly
  identified

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WIMPs Direct Detection Strategies
2 main fundamental strategies (in practice
combined)

• Statistical method
• - Statistically estimate the
  background
• - Annual modulation, multiple
  scattering, pulse shape discr.
• - Large mass, simple detectors e.g.
  NaI (DAMA)

• Event by event discrimination
• - Active rejection with the best
  possible discrimination
• - For example, nuclear vs electron
  recoils
• - More sophisticated technology

12
Backgrounds for Direct Detection Experiments
?
?

• Put experiment underground so
• no cosmic-ray nuclei reach it
• very few muons (and hence fast neutrons) if deep
  enough
• Surround detectors with active muon veto

U/Th/K/Rn
Rock
n
?????,n
detector
U/Th/K/Rn
Rock
Lead, polyethylene
Veto (active)

• Use clean, low-radioactivity ( screened)
• materials
• Use passive shielding
• - Pb shielding to reduce EM backgrounds
  
• from radioactivity
• - Polyethylene contains hydrogen
  needed to
• moderate neutrons from radioactivity

13
WIMP-detection Experiments Worldwide
Funding scale 10M/experiment Collaborations
10-50 physicists/experiment
(Superheated droplets)
(Liq. Xenon)
(Cryogenic)
(HPGe)
(NaI)
(Liq. Xenon)
(Superheated superconducting granules)
(TPC)
(NaI)
(Cryogenic)
(HPGe)
(Cryogenic)
(HPGe)
(NaI)
(Ton of bare HPGe)
(Liq. Xenon)
(TeO2)
Next talk
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DAMA NaI Experiment
Glove box for calibration sources

• Very elegant experimental setup - in place gt1996
• Low Activity NaI scintillator9 ? 9.7 kg NaI
  crystals, each viewed by 2 PMTs
• - known technology
• - large mass
• - spin dependent interaction
• Located at Gran Sasso Underground Lab (3.8 kmwe)
  Photon and Neutron shielding
• Two modes of background discrimination
• - Pulse shape (not used)
• - Annual modulation 2 modulation
  amplitude

Copper
Lead
Polyethelene
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DAMA Search for annual modulation

• Not distinguish between WIMP signal and
  Background directly
• From the amplitude of the modulation, we can
  calculate the underlying
• WIMP interaction rate

WIMP Signal
Background
June
June
Dec
Dec
16
DAMA 7-year Annual Modulation Results
Source DAMA Riv. Nuovo Cim 26N1 (2003) 1-73

• 6.3s annual modulation is observed in
• the rate
• BUT, the modulation is only a 5 effect and
  
• is all in the lowest energy bin.

Whats next? DAMA?LIBRA, a 250-kg NaI experiment
has been operated since 2003
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KIMS - Similar to DAMA but with CsI

• Located at 700 mwe underground in Korea
• Challenge internal background from 137Cs
  contamination is most problematic.
• Can be reduce by using purified water in
  processing
• Recent result is based on 1 crystal with 6.6
  kg Currently running 4 crystals with
• mass 8.66 kg each
• Plan to start taking 100 kg data this summer
• Test DAMA data with similar
  crystal detector containing Iodine.
• ? should be helpful to confirm or deny
  claimed signal

DAMA
Phys. Lett. B 633 (2006) 201
18
Cryogenic detectors
Principle phonon mediated detectors Goals

• Sensitivity down to low energy. Phonons measure
  the full recoil energy
• Active rejection of background (event by event
  discrimination)
• recognition of nuclear recoil combine
• - with low field ionization measurement
• e.g. CDMS I and II, EDELWEISS (next talk)
• - or photon (CRESST II)
• More information on rare events

But, operation at very low temperature!
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CDMS II Overview
Measure simultaneously ionization and athermal
phonons

• Most background sources (electrons, photons)
• scatter off electrons

Bulk Electron Recoils (133Ba)
Bulk Electron Recoils (133Ba)
Ionization Yield EQ/ER
Y 1 for electron recoils
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The CDMS ZIP Ionization Phonon Detectors

• 250 g Ge or 100 g Si crystal
• 1 cm thick x 7.5 cm diameter
• Photolithographic patterning
• Collect athermal phonons
• 4 quadrants provide information on xy position

• Informations on phonons pulse
• shape (ex. risetime), delay between
• charge and phonon pulses

Qouter
Qinner

• 6 detectors stacked together

? Allow detection of multiple
interactions (neutrons)

• 2 runs done
• Oct. 2003 - Jan 2004 1 tower (4 Ge and 2 Si)
• Mar Aug 2004 2 towers (6 Ge and 6 Si)

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Z-Position Sensitivity Rejects Surface events
Gammas
Surface event

• Energy deposited near the surface gives
• rise to slightly lower-frequency phonons
• ? undergo less scattering and hence
• travel ballistically
• ? Shorter risetime than bulk events

Neutrons
Bulk event Surface event
Signal region

• Overall rejection of surface events
• appears gt99
• We are only beginning to take full
• advantage of the information from the
• athermal phonon sensors!
• Improving modeling of phonon physics
• Extracting better discrimination parameters
• (timing and energy partition)

22
CDMSII second run results
Second Run with 2 towers 74.5 live days
96.8 (31.0) kg-days Ge (Si) before cuts
1 (0) event passed all cuts in Ge (Si)
consistent with background estimate also
this event occur during a time of known poor
detector performance
Si blind analysis
Ge blind analysis
Expected Background (7-100 keV recoil
energy) Surface events 0.40.20.2 for Ge
1.20.60.2 for Si Neutron 0.06 for Ge
0.05 for Si
Passed timing cuts outside nuclear recoil band
Passed timing cuts inside nuclear recoil band (
WIMP candidate)
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CDMSII spin-independent limit

• Upper limit on the WIMP-nucleon
• spin-independent cross-section
• is 1.6x10-43 cm2 for a WIMP with
• mass of 60 GeV/c2.
• Exclude large regions of SUSY
• parameter space

MSSM example from.Bottino et al., 2004 CMSSM
example from Ellis et al. (2005)
DAMA 3s and 90 CL. Scattering on NaGondolo
Gelmini (2005)
Phys. Rev. Lett. 96 (2006) 011302
24
CDMSII spin-dependent limit
CDMS has sensitivity to the spin dependent cross
sections as well!
Limiting case of pure neutron coupling 29Si and
73Ge include an unpaired neutron (4.68,)
(7.73)
Limiting case of pure proton coupling 73Ge has
nuclear excitations with non-zero ltSpgt
neutron
proton
Si
Ge (2 nuclear models)
Si
Ge (2 nuclear models)
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Whats next for CDMS?

• CDMSII

• 5 Towers now installed
• ? 30 detectors 19 Ge (4.75 kg) and 11 Si
  (1.1 kg)
• currently commissioning

ZEPLIN 1
EDELWEISS
CDMS II 2005
ZEPLIN 2
will run through 2007
CDMS II goal

• Toward 1 ton experiment
• SuperCDMS

SuperCDMS 25 kg
SuperCDMS 150 kg

• 3 phases 25 kg - 150 kg 1 ton of
• cold Ge det.
• move from Soudan to Snowlab
• (reduce muon flux by 500)
• increase detector thickness 1 inch
• (0.64 kg) instead of 1 cm (0.25 kg)

SuperCDMS 1000 kg
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CRESST II Phonons and Scintillation
Experiment located at Gran Sasso (3500 mwe)

• Nuclear recoils have much smaller light yield
  than electron recoils
• Photon and electron interactions can be
  distinguished from nuclear recoils (WIMPs,
  neutrons, ...)

• No problem of surface events
• Very small scintillation signal
• Scintillation threshold will
• determine minimum recoil energy
• Neutrons interact mainly with
• oxygen
• WIMPs interact mainly with
• tungsten

CaWO4
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CRESST II Phonons and Scintillation
Astropart. Phys. 23 (2005) 325 (astro-ph/0408006)

• Results from 20.5 kg-d exposure of two 300-g
  CaWO4 prototypes
• (Jan 31 Mar 23, 2004)
• No neutron shielding
• Observe 16 low-yield events consistent with
  neutron rates and oxygen cross section light
  yield
• Tungsten light yield not distinct from noise
  claim background-free low-yield region 12-40 keV
  for 10-kg-d exposure of better detector (Daisy)

Neutrons on oxygen

• CRESST II upgrades

March 2004 operation stopped to install -
neutron moderator, muon veto - 66 readout
channels (10 kg target)
90 W recoils Below this line
Currently working on detector holder system and
new analysis software, expect to be taking data
by end summer 2006
28
Liquid Xenon Detectors

• Potential to challenge cryogenic detectors
• High atomic number (A131) gives a high rate due
  to ?WIMP-Nucleon? A2 (if E is low)
• High density ( 3g/cm3 liquid)
• High light (175 nm) and ionization yield
• Easy to scale up to large volume
• Can be highly purified
• Challenges of Liquid Xenon
• Implementing dual-phase to improve
  scintillation signal near threshold
• Ionization signal/noise poor
• near threshold

Dual-phase LXe Time Projection Chamber (TPC)
gas
e-
t
B.A.Dolgoshein, V.N. Lebedenko, B.U. Rodionov,
JETP Lett. 11 (1970) 513.
Several programs XMASS, ZEPLIN, XENON, also
experiments using Argon, Neon, He
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UK/UCLA Collaboration Zeplin I

• Located in UK Boulby mine (2800 mwe)
• Three runs totaling 293 kg days exposure
• Discrimination factor is pulse shapes
• No in situ neutron calibration performed

Discrimination parameter
Gamma rays (Fast pulses)
Max sensitivity was 1.1x10-42 cm2 (Astropart 23
(2005) 44)
neutrons (Slow pulses)

• 5kg LXe target (3.1kg fid)
• 3 PMTs
• Cu construction
• Polycold cryogen cooling

• 1 tonne Compton veto
• PMT background tag

• Pulse shape

30
Zeplin II, III, and beyond
ZEPLIN II
ZEPLIN III
ZEPLIN IV/MAX

• Same 2 phases as ZEPLIN II but with 31, smaller
  2PMTs at the bottom of the detector
• Total mass 8 kg
• Calibrations and system checkouts done at the
  surface
• Expect to run 2007 to 2012

• Start planning 1 ton experiment
• To be installed in SNOlab 2008-2012
• Explore capabilities of nobel gases with new
  ideas and design (2013 and beyong)

• 2 phases liquid and gas Xe
• Calibrations and system checkouts underway in
  Boulby mine
• Expect to run 2007 to 2012

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XENON Experiment

• Dual Phase Liquid/Gas Xe
• The XENON design is modular
• Multiple 3D position sensitive LXeTPC
• modules, each with a 100 kg active Xe mass
• ? 1-ton scale experiment.
• The 100 kg fiducial LXe volume of
• each module is shielded by additional
• 50 kg LXe.

100 kg LXe

• A prototype (total mass 15 Kg LXe) is currently
  being shipped to
• Gran Sasso for assembly
• Plan to begin running with shield in may 2006

32
Bubble Chamber Revival
Example of CUOPP Experiment

• Principle Superheated liquid
• (e.g. CFbI)
• Idea arose from superheated droplet experiments
  (SIMPLE/PICASSO)
• Requires nucleation energy to overcome surface
  tension and form bubble
• Only high-ionization energy density tracks from
  nuclear recoils sufficient to cause nucleation
• Insensitive to gammas, betas,
• minimum ionizing particles
• Demonstrated bubble rates consistent with
  neutrons from cosmic rays at shallow site with 1
  liter prototype

CUOPP
Setting up now at 300 mwe site at
Fermilab Eventually will go to Soudan

• Challenges
• No energy information for given event
• Must do multiple exposures at different
  pressures to derive energy spectrum
• Possible alpha backgrounds Operational
  stability

33
DRIFT Look for diurnal modulation

• DRIFT I
• Cubic meter in Boulby
• since 2001
• Engineering runs completed
• DRIFT II extension to 10 kg
• module proposed

40 keV S recoil in 40 Torr CS2 (this is a
simulation)

• Sensitive to direction of recoiling nucleus
• Diurnal modulation signal galactic origin of
  signal
• Drift negative ions in TPC
• No magnet
• Reduced diffusion
• Electron recoils rejected via dE/dx

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Summary and Projections

• Dark matter
• Basic component of Universe
• Physics outside SM Wimps, axions, etc.
• Search broad range of approach
• (indirect, direct detection)
• Competitive reach for SUSY with LHC
• Several direct-search technologies on line and
  leading to steady progress
• CDMS, Edelweiss, Zeplin
• Dama/Libra systematics?
• Non-standard signal?
• Many other experiments
• Expansion to ton-scale
• Excellent prospects to see signal soon!
• ZepMAX, SuperCDMS, XENON
• EURECA (CRESSTII Edelweiss)
• Unfortunately, costs are also growing
• ? Field will likely contract to a few big