Transcript Low-threshold Results from the Cryogenic Dark Matter

Low-threshold Results from the
Cryogenic Dark Matter Search Experiment
Ray Bunker—CDMS Collaboration
WIN`11
Cape Town, South Africa
Outline
• Dark Matter and WIMPs
• Direct Detection
• Evidence for a light WIMP
• Direct
• Indirect
• The CDMS experiment
• Detector technology
• Shallow-site low-threshold analysis
• Deep-site low-energy analysis
• Deep-site Neutrons
February 1st, 2011
Ray Bunker-UCSB HEP Group
2
The Dark Matter Problem
Milky Way Galactic Rotation Curve
• Use interstellar gas to probe galactic
galactic mass distribution
Vcircular
(km/s)
• Appears to contradict the R-1/2
falloff expected from luminous
matter
Y. Sofue, M. Honma and T. Omodaka
arXiv:0811.0859v2
Radius, R (kpc)
• Large uncertainties, but why should our
galaxy be any different than others?
The solar neighborhood
at ~8 kpc and ~220 km/s
Still the most compelling evidence for the existence
of dark matter in the solar neighborhood!
February 1st, 2011
Ray Bunker-UCSB HEP Group
3
The Dark Matter Problem
Komatsu et al. (WMAP), arXiv:1001.4538
• Concordance of observations of large-scale
structure, supernovae, and the cosmic
microwave background imply:
• Only Standard Model candidate is the neutrino,
however… if
0.01%
Metals (us)
Visible Baryons
0.5%
S.A. Thomas, F. B. Abdalla, and O. Lahav,
Phys. Rev. Lett. 105, 031301 (2010).
then,
Dark Baryons
4%
Physics beyond the Standard Model?
February 1st, 2011
Ray Bunker-UCSB HEP Group
Cold
Dark Matter
23%
Cosmological Constant
Dark Energy 
73%
4
WIMPsA Dark Matter Candidate
Weakly Interacting Massive Particles
• Massive ↔ Structure Formation
• Weakly Interacting ↔ Non-observance
WIMP
quarks,
Relic abundance obtained when annihilation too
slow to keep up with expansion
Being produced
and annihilating
(T ≥ MWIMP)
Production suppressed
(T < MWIMP)
leptons,
photons
WIMP
Freeze out
WIMP  1/annihilation
A Weak-scale Coincidence?
annihilation ~ weak scale
yields observed WIMP ~ ¼ !
February 1st, 2011
Ray Bunker-UCSB HEP Group
5
The Lightest Superpartner
• No stable WIMPs in the Standard Model
• SUSY extends physics beyond the SM
• Lots of new particles very popular among high energy physicists
• The LSP is often a WIMP
• Such as the neutralino 0:
• Non-appearance at LEP or Tevatron ↔ Massive (?)
• Neutral ↔ Dark
• Conserved R-parity ↔ Stable
• LEP 0 mass bound
• Chargino mass bound of ~103 GeV/c2  0 mass bound of 4060 GeV/c2
• Generally presumes gaugino mass unification
February 1st, 2011
Ray Bunker-UCSB HEP Group
6
Light SUSY WIMPs
• Relax gaugino mass unification:
• The chargino & neutralino masses are basically uncorrelated
• The 0 mass can evade the LEP chargino mass bound
• Must invoke cosmological constraints for 0 mass bound
Bottino et al., Phys. Rev. D69, 037302 (2004)
Scanning SUSY parameter space
Belanger et al. find 0 masses as
Similarly, Bottino,
Donato,
low as 6 GeV/c2
Fornengo and Scopel also find 0
masses as low as 6 GeV/c2
Lines indicate the sensitivities of
the ZEPLIN
I (solid),
ZEPLIN
II
Red points
for 
  CDMmin
(dashed),
CDMS
(dash-dotted)
and
Blue
points
for  < CDMmin
EDELWEISS (dotted) experiments
February 1st, 2011
0-nucleon cross section (nb)
0-nucleon cross section (pb)
Loose Interpretation of Belanger et al., J. High Energy Phys. 03 (2004) 012
DAMA Allowed Region
Ray Bunker-UCSB HEP Group
CDMS 2002 Limit
5 keV Threshold
EDELWEISS 2002
Upper Limit
2)
0 mass
(GeV/c
2)
mass
(GeV/c
0
7
Direct Detection
• Standard assumption 
Galactic WIMP Halo
• WIMP “wind” with ~220 km/s relative
velocity, or β = v/c ~ 7x10-4
• Direct detection attempts to measure:
Erecoil ~ ½ Mnucleus c2 β2
~ 10 to 20 keV
• Event rate  detector size,
 WIMP flux, &
 cross section
• More specifically, sensitivity depends
on detector composition, WIMP
mass, detection threshold, and
halo model
February 1st, 2011
Very roughly:
σ = 1x10-41 cm2, vescape = 544 km/s
Rate = N [atoms] x φ [cm-2day-1] x σ [cm2/atom]
Ge Target
Dark Matter Halo
Si Target
N = 8.3x1024 [atoms in a 1 kg Ge detector]
5 GeV/c2 WIMP
Thick Disk
φ = 6.1x109 [cm-2day-1]
Thin Disk
σ = 1x10-43Sun
[cm2/atom] (weak scale cross section)
Bulge
-9 [kg
-1day-1]… totally hopeless rate per nucleon
2 WIMP
Rate = 100
5.1x10
GeV/c
But β << 1  Coherent scattering from entire nucleus
 ~A4 enhancement
Rate ~ (72.61)4 x 5.1x10-9 [kg-1day-1]
low-energy
threshold
is more
critical
-1]… much
~ 0.1Aevents
[kg-1day
approachable
for detecting light WIMPs!
Ray Bunker-UCSB HEP Group
8
Direct Detection
•Rate of interactions due to known backgrounds ~103 [kg-1day-1] !!!
• With low threshold (~1 keV), the expected rate for a light WIMP (< 10 GeV/c2) is
much larger… ~ 10 [kg-1day-1]
• Backgrounds rates increase rapidly at low energies (< 10 keV)… offsetting
higher expected rate for light WIMPs
February 1st, 2011
Ray Bunker-UCSB HEP Group
9
Direct Detection
Strategies for overcoming backgrounds:
• Passive & active shielding
All Experiments
• Minimum ionizing threshold suppression
PICASSO & COUPP
CoGeNT
IGEX
DRIFT
• Large detector size, self shielding
DAMA & XENON
• Measure 2 signals
CDMS & LUX
•Event rate modulation
XENON
LUX
ZEPLIN II & III
XMASS
ionization
Q
CDMS
EDELWEISS
DAMA & DRIFT
• Low threshold
CoGeNT
• Pulse shape & timing
CDMS
DAMA/LIBRA
ZEPLIN I
DEAP/CLEAN
NaIAD
CRESST I,
PICASSO,
COUPP
ROSEBUD, CRESST II
February 1st, 2011
Ray Bunker-UCSB HEP Group
10
Evidence for a Light WIMP
Installing the new DAMA/LIBRA detectors in HP Nitrogen atmosphere
IMAGE CREDIT: DAMA/LIBRA Collaboration
• The DAMA/LIBRA experiment located in the Gran Sasso
Laboratory (Italy): 200 kg of low-activity NaI operated
from September 2003 to September 2009
• Annual modulation in their residual event rate with
correct phase and period… significance of ~9σ
• Savage et al. have interpreted their data in terms of spinindependent WIMP-nucleon interactions… evidence for a
light WIMP?
C. Savage et al., JCAP, 0904, 010 (2009);
& JCAP, 0909, 036 (2009);
& arXiv:1006.0972v2 (2010)
February 1st, 2011
R. Bernabei et al., Eur. Phys. J C67, 39 (2010)
Ray Bunker-UCSB HEP Group
11
Evidence for a Light WIMP
• The CoGeNT experiment operates a ~½ kg Ge
diode detector... very low background &
very low threshold
• In a short exposure, they observe an excess in
their event rate that has the exponential shape
expected for a light WIMP
C.E. Aalseth et al., arXiv:1002.4703v2
D. Hooper et al. performed a combined analysis of
DAMA/LIBRA and CoGeNT data and find a region of
consistency that points to a WIMP with:
MWIMP ~ 7.0 GeV/c2
&
σWIMP-nucleon~ 2.0x10-40 cm-2
Hooper et al., Phys. Rev. D82, 123509 (2010)
February 1st, 2011
Ray Bunker-UCSB HEP Group
12
Indirect Evidence for a Light WIMP
D. Hooper and L. Goodenough, arXiv:1010.2752v2
• The FERMI Gamma Ray Space Telescope launched
in 2008
• The Large Area Telescope (LAT) has observed
gamma rays from the galactic center,
300 MeV to 100 GeV
D. Hooper and L. Goodenough, arXiv:1010.2752v2
• Dan Hooper & Lisa Goodenough have analyzed
the 1st two years worth of data for a WIMP
annihilation signal
• Emission spectrum from 1.25° to 10° is
consistent with π0 decay, inverse Compton
scattering and Bremsstrahlung
• Inner 0° to 1.25°, however, shows an excess
• Profile is consistent with a cusped halo of
7-10 GeV/c2 WIMPs, annihilating primarily
into tau pairs
February 1st, 2011
Ray Bunker-UCSB HEP Group
13
Direct Detection Low-mass WIMP Constraints
• Best constraints from the XENON100 experiment... however, low-energy scale controversial:
• Red dotted line = constant extrapolation
• Red solid line = decreasing extrapolation
E. Aprile et al., Phys. Rev. Lett., 105, 131302 (2010).
• The final CDMS II Ge limit is competitive
with 10 keV threshold:
Courtesy of M. Schumann
• Black solid line
Z. Ahmed et al., Science, 327, 1619 (2010).
• Very low-mass limit from the CRESST,
~½ keV threshold:
• Blue dashed line
G. Angloher et al., Astropart. Phys., 18, 43 (2002)
February 1st, 2011
Ray Bunker-UCSB HEP Group
14
CDMS Detector Technology
Standard Ionization Measurement
Drift Electrons & Holes with
-3 to -6 V/cm Electric Field
(Applied to Ionization Electrodes)
Inner Disk
Ionization Electrode
~85% Coverage
Holes
Outer Guard Ring
Ionization Electrode
eGe or Si Crystal
Phonon Sensors Held at Ground

0
February 1st, 2011
Ray Bunker-UCSB HEP Group
15
CDMS Detector Technology
Z-sensitive Ionization & Phonon-mediated
ZIP Detector
SQUID array
Phonon A
R sh
Rfeedback
I bias
R
Superconducting
Quasiparticle-trap-assisted
Electrothermal-feedback
Transition-edge (QET)
phonon sensors
R0
A
D
B
C
T
T0
Q outer
Qinner
Vqbias
Aluminum Collector
Cooper Pair
Al
quasiparticle
trap
Tungsten
quasiparticle
Transition Edge
diffusion
Sensor (TES)
phonons
February 1st, 2011
Ray Bunker-UCSB HEP Group
Ge or Si Crystal
16
CDMS Detector Technology
• True recoil energy (Erecoil) measured on event-by-event basis by subtracting Luke phonons:
Lines due to decays of
internal radioisotopes tilted
• Ionization yield, Y ≡ Q / Erecoil
• Excellent separation between electron
recoils and nuclear recoils caused by
neutrons from 252Cf source
Electron Recoils
• Subtracting Luke phonons via average
ionization behavior more reliable for
low-energy nuclear recoils
February 1st, 2011
Nuclear Recoils
Ray Bunker-UCSB HEP Group
17
• Surface events can be misidentified as
nuclear recoils
• Phonon pulse shape and timing is
a powerful discriminator
• Allows for background-free analysis
Phonon pulse rise time (s)
Phonon pulse height (V)
CDMS Detector Technology
: reduced ionization collection
Time
since yield
trigger (s)
Ionization
Bulk  Recoil
February 1st, 2011
Ray Bunker-UCSB HEP Group
18
CDMS Shallow-site Run
• First tower of CDMS II ZIP detectors operated at shallow Stanford Underground Facility
FET Readout
(Ionization signals)
• Total Ge detector mass of ~0.9 kg and total Si mass of ~0.2 kg
17 mwe
SQUID Readout
(phonon signals)
• “Run 21” WIMP-search data taken between December 2001
and June 2002, yielding 118 live days of raw exposure 

Active Muon
Veto
Pb Shield
• Run 21 split into two periods distinguished by voltage bias
used:
• 1st half with Ge (Si) operated with 3V (4V) bias voltage (3V data)
• 2nd half with all detectors operated with 6V bias voltage (6V data)
Fridge
Cold Stages
4 K to 20 mK
• Analysis of 3V data n
with 5 keV recoil energy threshold
Copper
published in 2002… Phys. Rev., D66, 122003 (2002)
ZIP 1 (Ge)
ZIP 2 (Ge)
ZIP 3 (Ge)
ZIP 4 (Si)
ZIP 5 (Ge)
Detectors
Inner Pb shield
6 (Si)
Ray Bunker-UCSB HEPZIP
Group
n
• 6V data previously unpublished
Polyethylene
February 1st, 2011
19
CDMS Shallow-site Energy Calibration
• Electron-recoil energy scale calibrated with gamma-ray sources (137Cf & 60Co)
• Ge energy scale confirmed with lines
from decays of internal radioisotopes
• Confirmed 11.4 day half-life of 68Ge and
0.12 ratio of L- to K-shell captures
1.3 keV from 68Ge & 71Ge Decays
10.4 keV from 68Ge & 71Ge Decays
Electron Capture from L-shell
Electron Capture from K-shell
Beginning
Cf-252 Neutron
End of
of Run 21
Calibration
Run 21
• Si scale more difficult!
Monte Carlo
• Nuclear-recoil energy scale the most important
66.7 keV from
73mGe Decay
• Calibrated with neutrons from 252Cf source
• Ionization yield agrees well with expectation
from Lindhard theory
Data
• Ultimately, compare to GEANT simulation:
• Ge scale consistent (at low energy)
• Corrected Si for ~15% discrepancy
Preliminary
February 1st, 2011
20
CDMS Shallow-site Thresholds
FET Readout
(Ionization signals)
• ZIP 1 rejected as a low-threshold detector
•Hardware trigger efficiency:
SQUID Readout
(phonon signals)
• Average ionization yield used to estimate recoil energy
•Hardware thresholds vary from ~0.7 to 1.8 keV
• Software phonon energy threshold
• Based on Gaussian width of sub-threshold noise
Cold Stages
ZIP 2 (Ge)
4 K to 20 mK
• Software thresholds vary from ~0.6 to 1.6 keV
• Ultimate threshold efficiency
• Ge thresholds 0.7 to 1.1 keV
• Si thresholds 1.5 to 1.9 keV
(keV)
Energy(keV)
PhononEnergy
TotalPhonon
Total
• Events required to exceed 6σ noise width
ZIP 1 (Ge)
ZIP 2 (Ge)
ZIP 3 (Ge)
ZIP 4 (Si)
ZIP 5 (Ge)
ZIP 6 (Si)
ZIP 4 (Si)
February 1st, 2011
21
Run Number (6V
(3V data)
CDMS Shallow-site Event Selection
• WIMP candidates must pass several data cuts:
Ge
Si
1.3 keV Line
32%
0%
Zero-charge Events
30-40%
30-40%
• Data-quality cuts

99% efficient
Shallow-site Neutrons
6%
2%
• Fiducial-volume cut

~83% efficient
• Single-scatter criterion 
Compton γ Electron Recoils
10-20%
10-20%
100% efficient
• Muon-veto cut
 ~70-80% efficient
• Nuclear-recoil cut

• Combined data cuts
 ~50-60% efficient
Contamination β’s
0%
40%
Raw Spectrum in Blue
Corrected
for Efficiencies
Cut Efficiency
Black
Others
2-22%
0-18%
Average
Combined
in in
Orange
Further
Corrected
for Threshold
Efficiency
in Orange
90%
(statistical)
Lower-limit
Efficiencies
in Blue
• Are these really WIMPs?... probably not!
• While a low-mass WIMP could be hiding in
these data, we can claim no evidence of a
WIMP signal
Outer electrode ionization energy (keV)
~95% efficient
• 1080 candidate events in 72 kg-days of Ge exposure
970 candidate events in 25 kg-days of Si exposure
February 1st, 2011
14C
1080 Candidates
202 Candidates
970 Candidates
314 Candidates
130 Candidates
Ray Bunker-UCSB HEP GroupInner electrode ionization energy (keV)
22
CDMS Shallow-site Low-threshold Limits
• Large background uncertainties preclude background subtraction
• We use Steve Yellin’s Optimum Interval Method
(specially adapted for high statistics)
• Serialize detector intervals to make best
use of lowest-background detectors
Hooper et al. combined: Gray
CDMS Shallow-site Ge: Black —
CDMS Shallow-site Si:
Gray —
CoGeNT 2010: Orange --CRESST Saphire 2002:
Blue --XENON100 Decreasing:
Red —
XENON100 Constant:
Red ····
D. Akerib et al. (CDMS), Phys. Rev. D82, 122004 (2010)
• Include the effect of finite
energy resolution near threshold
• Standard WIMP halo model with
544 km/s galactic escape velocity
• Systematic studies indicate limits
are robust above ~3 GeV/c2
Exclude new parameter space for WIMP
masses between 3 and 4 GeV/c2!
February 1st, 2011
Ray Bunker-UCSB HEP Group
23
The CDMS Deep Site
17 mwe at SUF yielding
~500 Muons per second
in the CDMS shielding
5.2x104 m-2y-1
2100 mwe
2100 mwe at Soudan yielding
<1 Muon per minute
in the CDMS shielding
February 1st, 2011
Ray Bunker-UCSB HEP Group
24
The CDMS Deep Site
February 1st, 2011
Ray Bunker-UCSB HEP Group
25
CDMS Deep-site Low-energy Analysis
• Focused low-energy analysis of CDMS WIMP-search data taken at the Soudan Mine
• 5 Towers of ZIP detectors (30 total) operated from October 2006 to September 2008 (6 distinct runs)
• 8 lowest-threshold Ge detectors
analyzed with 2 keV threshold
26
CDMS Deep-site Low-energy WIMP Candidates
• Optimized nuclear-recoil selection to avoid zero-charge event background
• Band thickness due to variations in nuclear-recoil criterion from run to run
• Recoil energy estimated from phonon signal & average ionization yield behavior
Tower 1-ZIP 5
February 1st, 2011
Ray Bunker-UCSB HEP Group
27
CDMS Deep-site Low-energy Backgrounds
• A factor of ~10 reduction in background levels
• Improved estimates of individual background
sources
• Comparable detection efficiency for much larger
exposure (~3.5x)
• No evidence of a WIMP signal
Candidate Spectrum:
Black Error Bars
Zero-charge Events:
Blue Dashed
Surface Events:
Red +
Bulk Compton γ Events: Green Dash-dotted
1.3 keV Line:
Pink Dotted
Combined Background:
Black Solid
Average Efficiency
February 1st, 2011
Ray Bunker-UCSB HEP Group
28
CDMS Deep-site Low-energy Limit
Z. Ahmed et al. (CDMS), arXiv:1011.2482v1 (submitted to Phys. Rev. Letters)
Hooper et al. combined:
Gray
CDMS Shallow-site Ge:
Black —
CDMS Shallow-site Si:
Gray —
CRESST Saphire 2002:
Blue --XENON100 Decreasing: Orange —
XENON100 Constant: Orange ····
CDMS Deep-site Ge:
Red —
February 1st, 2011
Ray Bunker-UCSB HEP Group
29
CDMS Deep-site Low-energy Spin-dependent Limit
CDMS II Ge Deep-site
10 keV Threshold
CRESST
Saphire 2002
3σ DAMA Allowed Region
CDMS II Ge Deep-site
2 keV Threshold
XENON10
February 1st, 2011
Ray Bunker-UCSB HEP Group
30
Deep-site Neutron Background
• Less than one event expectec for CDMS II
• Limiting background for SuperCDMS… but how soon?
February 1st, 2011
Ray Bunker-UCSB HEP Group
31
Fast-neutron Detection
High Energy Neutron
No Veto, Small Prompt
Energy Deposit
Veto

PMT






Liquid Scintillator
Gadolinium Loaded
Capture on Gd, Gammas
 (spread over 40 μs)
PMT


Lead
Veto
Hadronic Shower
Liberated Neutrons
February 1st, 2011
Ray Bunker-UCSB HEP Group
Veto
32
Fast-neutron Detection
Expected Number of sub-10 MeV
Secondary Neutrons
Simulated 100 MeV Neutrons
Incident on Lead Target
Detectable Neutron Multiplicity
February 1st, 2011
Ray Bunker-UCSB HEP Group
33
A Fast-neutron Detector
February 1st, 2011
Ray Bunker-UCSB HEP Group
34
Detector Installation
Electronics Rack
Lead Target
Source Tubes
February 1st, 2011
Ray Bunker-UCSB HEP Group
35
Detector Installation
Cheap Labor
Water Tanks
February 1st, 2011
Ray Bunker-UCSB HEP Group
36
Detector Installation
20” KamLAND
Phototubes
February 1st, 2011
Ray Bunker-UCSB HEP Group
37
Neutron Detection Technique
• Water-based neutron detector is challenging!
• Small fraction of energy visible as Cerenkov
radiation
• Poor energy resolution smears U/Th gammas
into signal region
February 1st, 2011
Ray Bunker-UCSB HEP Group
38
Neutron Detection Technique
Timing is Everything
• Neutron capture times  microseconds
• A few 100 Hz of U/Th background  milliseconds
February 1st, 2011
Ray Bunker-UCSB HEP Group
39
Neutron Detection Technique
More Gamma Like
Pulse timing Likelihood
Background U/Th
Gamma Rays
More Neutron Like
February 1st, 2011
252Cf
Fission Neutrons
Pulse Height Likelihood
Ray Bunker-UCSB HEP Group
40
Understanding Energy Scale
Background U/Th
Gamma Rays
252Cf
Fission
Neutrons
Event rate (arbitrary units)
Actual data: shaded red
Simulated data: black lines
Pulse height (mV)
60Co
~1 MeV
Gamma Rays
Pulse height (mV)
February 1st, 2011
Ray Bunker-UCSB HEP Group
41
Event rate (arbitrary units)
Understanding Energy Scale
~150 MeV
Pulse height (V)
~50 MeV Endpoint
February 1st, 2011
Ray Bunker-UCSB HEP Group
42