Low-threshold Results from the Cryogenic Dark Matter Search Experiment

Low-threshold Results from the Cryogenic Dark Matter Search Experiment

Ray Bunker—CDMS CollaborationWIN`11

Cape Town, South Africa

February 1st, 2011 2Ray Bunker-UCSB HEP Group

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 3

The Dark Matter ProblemMilky Way Galactic Rotation Curve

• Use interstellar gas to probe galactic galactic mass distribution

• Appears to contradict the R-1/2 falloff expected from luminous matter

Vcircular

(km/s)

Radius, R (kpc)

Still the most compelling evidence for the existence of dark matter in the solar neighborhood!

Y. Sofue, M. Honma and T. OmodakaarXiv:0811.0859v2

The solar neighborhoodat ~8 kpc and ~220 km/s

• Large uncertainties, but why should our galaxy be any different than others?

Komatsu et al. (WMAP), arXiv:1001.4538

February 1st, 2011 Ray Bunker-UCSB HEP Group4

The Dark Matter Problem• Concordance of observations of large-scale structure, supernovae, and the cosmic microwave background imply:

• Only Standard Model candidate is the neutrino, however… if

then,

Physics beyond the Standard Model?

S.A. Thomas, F. B. Abdalla, and O. Lahav, Phys. Rev. Lett. 105, 031301 (2010).

Metals (us) 0.01%

Visible Baryons 0.5%

Dark Baryons4%

Cold Dark Matter

23%

Cosmological ConstantDark Energy

73%

February 1st, 2011 5

WIMPsA Dark Matter Candidate

Weakly Interacting Massive Particles• Massive ↔ Structure Formation• Weakly Interacting ↔ Non-observance

Production suppressed(T < MWIMP)

Being producedand annihilating

(T ≥ MWIMP)

Freeze out

A Weak-scale Coincidence?

WIMP 1/annihilation

annihilation ~ weak scale

yields observed WIMP ~ ¼ !

Relic abundance obtained when annihilation too slow to keep up with expansion

WIMPquarks,

leptons,

photons WIMP

Ray Bunker-UCSB HEP Group


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 4060 GeV/c2

• Generally presumes gaugino mass unification


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

Scanning SUSY parameter spaceBelanger et al. find 0 masses as

low as 6 GeV/c2

Lines indicate the sensitivities of the ZEPLIN I (solid), ZEPLIN II

(dashed), CDMS (dash-dotted) and EDELWEISS (dotted) experiments

Belanger et al., J. High Energy Phys. 03 (2004) 012

0 mass (GeV/c2)

0-n

ucle

on c

ross

sec

tion

(pb)

0-n

ucle

on c

ross

sec

tion

(nb)

Bottino et al., Phys. Rev. D69, 037302 (2004)

0 mass (GeV/c2)

CDMS 2002 Limit5 keV Threshold

EDELWEISS 2002Upper Limit

Loose Interpretation ofDAMA Allowed Region

Similarly, Bottino, Donato, Fornengo and Scopel also find 0

masses as low as 6 GeV/c2

Red points for CDMmin

Blue points for < CDMmin


Direct Detection

Dark Matter Halo

Bulge

Thick Disk Thin Disk

Sun

Very roughly:

Rate = N [atoms] x φ [cm-2day-1] x σ [cm2/atom]

N = 8.3x1024 [atoms in a 1 kg Ge detector]

φ = 6.1x109 [cm-2day-1]

σ = 1x10-43 [cm2/atom] (weak scale cross section)

Rate = 5.1x10-9 [kg-1day-1]… totally hopeless rate per nucleon

But β << 1 Coherent scattering from entire nucleus ~A4 enhancement

Rate ~ (72.61)4 x 5.1x10-9 [kg-1day-1]

~ 0.1 events [kg-1day-1]… much more approachableA low-energy threshold is criticalfor detecting light WIMPs!

100 GeV/c2 WIMP

5 GeV/c2 WIMP

Ge TargetSi Target

σ = 1x10-41 cm2, vescape = 544 km/s• 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


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

10

Direct DetectionStrategies for overcoming backgrounds:

• Passive & active shielding All Experiments

• Minimum ionizing threshold suppression PICASSO & COUPP

• Large detector size, self shielding DAMA & XENON

• Measure 2 signals CDMS & LUX

•Event rate modulation DAMA & DRIFT

• Low threshold CoGeNT

• Pulse shape & timing CDMS

Hphonons

ionization

QL

scintillation

CDMSEDELWEISS

CoGeNTIGEX

DRIFT

XENONLUX

ZEPLIN II & IIIXMASS

DAMA/LIBRA ZEPLIN I

DEAP/CLEANNaIAD

ROSEBUD, CRESST II

CRESST I,PICASSO,COUPP

February 1st, 2011 Ray Bunker-UCSB HEP Group


Evidence for a Light WIMP

• 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 spin- independent WIMP-nucleon interactions… evidence for a light WIMP?

R. Bernabei et al., Eur. Phys. J C67, 39 (2010)

Installing the new DAMA/LIBRA detectors in HP Nitrogen atmosphereIMAGE CREDIT: DAMA/LIBRA Collaboration

C. Savage et al., JCAP, 0904, 010 (2009);& JCAP, 0909, 036 (2009);

& arXiv:1006.0972v2 (2010)


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)



Indirect Evidence for a Light WIMPD. 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 • 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

D. Hooper and L. Goodenough, arXiv:1010.2752v2

D. Hooper and L. Goodenough, arXiv:1010.2752v2


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:

• 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)

Courtesy of M. Schumann


CDMS Detector Technology

Ge or Si Crystal

0

Holes

e-

Standard Ionization MeasurementDrift Electrons & Holes with -3 to -6 V/cm Electric Field

(Applied to Ionization Electrodes)

Inner DiskIonization Electrode

~85% Coverage

Outer Guard RingIonization Electrode

Phonon Sensors Held at Ground



Al

quasiparticle trapAluminum Collector Tungsten

Transition Edge Sensor (TES)

Ge or Si Crystal

quasiparticlediffusion

phonons

Cooper Pair

R0

R

TT0

SuperconductingQuasiparticle-trap-assisted Electrothermal-feedback

Transition-edge (QET) phonon sensors

Q inner

Q outer

D

C

A

B

Rsh

Ibias

SQUID array Phonon A

Rfeedback

Vqbias

Z-sensitive Ionization & Phonon-mediatedZIP Detector



• True recoil energy (Erecoil) measured on event-by-event basis by subtracting Luke phonons:

• Ionization yield, Y ≡ Q / Erecoil

• Excellent separation between electron recoils and nuclear recoils caused by neutrons from 252Cf source • Subtracting Luke phonons via average ionization behavior more reliable for low-energy nuclear recoils

Electron Recoils

Nuclear Recoils

Lines due to decays of internal radioisotopes tilted

Time since trigger (s)

Phon

on p

ulse

hei

ght (

V)


: reduced ionization collection

Bulk Recoil

Ionization yield

Phon

on p

ulse

rise

tim

e (

s)

• Surface events can be misidentified as nuclear recoils

• Phonon pulse shape and timing is a powerful discriminator

• Allows for background-free analysis


February 1st, 2011 Ray Bunker-UCSB HEP Group

19

CDMS Shallow-site Run

ZIP 1 (Ge)ZIP 2 (Ge)ZIP 3 (Ge)ZIP 4 (Si)ZIP 5 (Ge)ZIP 6 (Si)

SQUID Readout (phonon signals)

FET Readout(Ionization signals)

Cold Stages4 K to 20 mK

17 mwe

Detectors Inner Pb shieldPolyethylene

Pb Shield

Active Muon Veto

Fridge

Copper

nn

n

• First tower of CDMS II ZIP detectors operated at shallow Stanford Underground Facility

• Total Ge detector mass of ~0.9 kg and total Si mass of ~0.2 kg

• “Run 21” WIMP-search data taken between December 2001 and June 2002, yielding 118 live days of raw exposure

• 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)

• Analysis of 3V data with 5 keV recoil energy threshold published in 2002… Phys. Rev., D66, 122003 (2002)

• 6V data previously unpublished

Preliminary

Monte Carlo

Data

Beginningof Run 21

End ofRun 21

Cf-252 NeutronCalibration

1.3 keV from 68Ge & 71Ge DecaysElectron Capture from L-shell

10.4 keV from 68Ge & 71Ge DecaysElectron Capture from K-shell

66.7 keV from73mGe Decay

February 1st, 2011

CDMS Shallow-site Energy Calibration

20

• 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 • Si scale more difficult!

• Nuclear-recoil energy scale the most important

• Calibrated with neutrons from 252Cf source

• Ionization yield agrees well with expectation from Lindhard theory

• Ultimately, compare to GEANT simulation:

• Ge scale consistent (at low energy)• Corrected Si for ~15% discrepancy


ZIP 1 (Ge)ZIP 2 (Ge)ZIP 3 (Ge)ZIP 4 (Si)ZIP 5 (Ge)ZIP 6 (Si)

SQUID Readout (phonon signals)

FET Readout(Ionization signals)

Cold Stages4 K to 20 mK

CDMS Shallow-site Thresholds

ZIP 2 (Ge)

To

tal

Ph

on

on

En

erg

y (

ke

V)

Run Number (3V data)

ZIP 4 (Si)

To

tal

Ph

on

on

En

erg

y (

ke

V)

Run Number (6V data)

• ZIP 1 rejected as a low-threshold detector •Hardware trigger efficiency:

• 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• Events required to exceed 6σ noise width

• 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

1080 Candidates

970 Candidates

Raw Spectrum in BlueCorrected for Cut Efficiency in Black

Further Corrected for Threshold Efficiency in OrangeAverage Combined Efficiencies in Orange

90% (statistical) Lower-limit Efficiencies in Blue

February 1st, 2011

CDMS Shallow-site Event SelectionGe Si

Ray Bunker-UCSB HEP Group 22

1.3 keV Line 32% 0%

Zero-charge Events 30-40% 30-40%

Shallow-site Neutrons 6% 2%

Compton γ Electron Recoils 10-20% 10-20%

14C Contamination β’s 0% 40%

Others 2-22% 0-18%

314 Candidates

202 Candidates

130 Candidates

• WIMP candidates must pass several data cuts:

• Data-quality cuts 99% efficient

• Fiducial-volume cut ~83% efficient

• Single-scatter criterion 100% efficient

• Muon-veto cut ~70-80% efficient

• Nuclear-recoil cut ~95% efficient

• Combined data cuts ~50-60% efficient

• 1080 candidate events in 72 kg-days of Ge exposure 970 candidate events in 25 kg-days of Si exposure

• 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

Inner electrode ionization energy (keV)

Out

er e

lect

rode

ioni

zatio

n en

ergy

(keV

)


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

• 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

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 ····

Exclude new parameter space for WIMPmasses between 3 and 4 GeV/c2!

D. Akerib et al. (CDMS), Phys. Rev. D82, 122004 (2010)


2100 mwe

5.2x104 m-2y-1

17 mwe at SUF yielding~500 Muons per secondin the CDMS shielding

2100 mwe at Soudan yielding<1 Muon per minute

in the CDMS shielding

The CDMS Deep Site


The CDMS Deep Site

26

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


CDMS Deep-site Low-energy WIMP Candidates

Tower 1-ZIP 5

• 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



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 BarsZero-charge Events: Blue Dashed

Surface Events: Red +Bulk Compton γ Events: Green Dash-dotted

1.3 keV Line: Pink DottedCombined Background: Black Solid

Average Efficiency


CDMS Deep-site Low-energy Limit

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 —

Z. Ahmed et al. (CDMS), arXiv:1011.2482v1 (submitted to Phys. Rev. Letters)


CDMS Deep-site Low-energy Spin-dependent Limit

CDMS II Ge Deep-site10 keV Threshold

CDMS II Ge Deep-site2 keV Threshold

XENON10

CRESST Saphire 2002

3σ DAMA Allowed Region


Deep-site Neutron Background

• Less than one event expectec for CDMS II

• Limiting background for SuperCDMS… but how soon?


Fast-neutron Detection

PMT PMT

Liquid ScintillatorGadolinium Loaded

Lead

VetoVeto

Veto

Hadronic Shower

Liberated Neutrons

High Energy NeutronNo Veto, Small Prompt

Energy Deposit

Capture on Gd, Gammas

(spread over 40 μs)


Fast-neutron Detection

Simulated 100 MeV NeutronsIncident on Lead Target

Detectable Neutron Multiplicity

Expected Number of sub-10 MeVSecondary Neutrons


A Fast-neutron Detector


Detector Installation

Lead Target

Electronics Rack

Source Tubes



Water Tanks

Cheap Labor



20” KamLANDPhototubes


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


Neutron Detection TechniqueTiming is Everything

• Neutron capture times microseconds

• A few 100 Hz of U/Th background milliseconds


Neutron Detection Technique

Pulse Height Likelihood

Puls

e tim

ing

Like

lihoo

d252Cf Fission Neutrons

Background U/ThGamma Rays

More Neutron Like

More Gamma Like


Pulse height (mV)

Even

t rat

e (a

rbitr

ary

units

)

Pulse height (mV)

Background U/ThGamma Rays

60Co ~1 MeVGamma Rays

252Cf FissionNeutrons

Actual data: shaded redSimulated data: black lines

Understanding Energy Scale


Understanding Energy Scale

Pulse height (V)

Even

t rat

e (a

rbitr

ary

units

)

~150 MeV

~50 MeV Endpoint

Low-threshold Results from the Cryogenic Dark Matter Search Experiment

Documents

Transcript of Low-threshold Results from the Cryogenic Dark Matter Search Experiment