results from cryogenic dark matter search (cdms)...

Gaitskell

“Whither WIMPS?”

Results from Cryogenic Dark MatterSearch (CDMS) Experiment

Rick Gaitskell

Center for Particle AstrophysicsUC Berkeley

source at http://cdms.berkeley.edu/gaitskell/

CDMS Gaitskell IDM2000 September 2000 Rick Gaitskell

Summary

• Clean determination of nuclear recoil event (NR) rate in detectorsu 10.6 kg-day exposure of Ge

u Detectors specifically designed for WIMP direct detection search• Energy resolution better than 1 keV(recoil) FWHM• NR vs γ discrimination is very clean

• WIMP Detection Resultsu Summarise statistical analysis

• Outline assumptions made.

— Residual systematic errors smaller than Poisson fluctuations

• Can CDMS interpretation be wrong?

u Compare sensitivity to signal implied by DAMA annual modulation• DAMA and CDMS results are incompatible for scalar (SI) interactions

— (σ ~ A2)— DAMA ann. mod. signal predicts 40 events during Ge 10.6 kg-day

– (actually observe 13 NR single scatters)

• Conclusions are robust (i.e. even if CDMS does not rely on neutron hypothesis)


Nuclear Recoil Discrimination - Event by Event

• Nuclear recoils arise from

uu WIMPsWIMPsu Neutrons

• Electron Recoils arise fromu photons

u electrons

u alphas

(Typical Background)

• Ionization yieldu ionization/recoil energy strongly

dependent on type of recoil

• Recoil energyu Phonons give full recoil energy

•

Neutrons (external source)

Gammas (external source)

Phonon Trigger Threshold

NOT A SIMULATION!1334 gamma events, 616 neutron events


Run Overview

• CDMS-Iu This presentation will cover a full analysis of event data that

was taken as part of Cryogenic Dark Matter Searchexperiment based at the Stanford Underground Facility(*)

u 1998 (2 months)• 33 live days 100 g Si ZIP -> 1.6 kg-days after cuts

• (4 nuclear recoil events observed)

u 1999 (12 months)• 96 live days 4x165 g Ge BLIP -> 10.6 kg-days after cuts

• (17 nuclear recoil events observed)

(*) Used to be called Stanford Low Radioactivity Facility but the radiation safety paperwork became too onerous


Ge BLIP Ionization & Phonon Detectors

NTD Gethermistors

ionizationmeasurementcircuit

ionizationcollectionelectrodes

lockinamplifier

165 g p-type Ge

QinnerQouter

(electronics not shown)

Vpb

Rb

Vqb

• Four 165 g Ge detectors, for total massof 0.66 kg during 1999 Run

• Calorimetric measurement of total energy

• ENERGY Resolution• σ = Ionisation 220 eV, Phonons 250 eV

Inner Ionization Inner Ionization ElectrodeElectrode

Outer IonizationOuter IonizationElectrodeElectrode

Passive Ge shieldingPassive Ge shielding

(NTD Phonon Sensors on underside)

TowerTower•• WiringWiring•• heat sinkingheat sinking•• holds coldholds cold FETs FETs for for

amplifiersamplifiers

BLIP


Detector Environment

Stanford Underground Facility• 17 mwe of rock• hadronic component down by >1000• muon flux down by ~5

Low-Background Environment• 25 cm polyethylene reduces muon-

induced neutron flux from rock andlead by factor >100

• 15 cm Pb reduces photon flux byfactor >1000

• radiopure cold volume (10 kg)• additional internal (ancient) lead

shielding

Active Scintillator Muon Veto• muon veto >99.9% efficient• reject ~22 “internal” neutrons/ day

produced by muons within shield polyethyleneouter moderator

detectors inner Pbshield

dilutionrefrigerator

Iceboxouter Pb shieldscintillator

veto


Photograph of Shielding

Neutron moderatorPb shielding

Copper cold volume

Neutron moderator

Scintillator muon veto


Radiopure Cold Volume (“Icebox”)

Cold electronics

To internallead shield

anddetectors

Cutaways of lids ofconcentric copper cans

Electronics stem forsignal cables


4 x BLIP Tower Schematic (1999 Run)

• 1999 Run (net 10.6 kg-days)

• Low-radioactivity Cu and Ge housing

• Self-shielding through close packingu Spacing 3 mm vertically

Ge shielding

BLIP 4

BLIP 3

BLIP 5

BLIP 6

60 mm

60 mmOuter IonizationElectrodes• less shielded from electrons(surface events)

• imperfect ionization yield(electrode break)


Anticoincident Ionisation Energy/Recoil Energy Plots

• Events over 10.6 kg-days,anticoincident with muonveto and other detectors

• Gamma Band

• Surface Electron Band

• Nuclear Recoil Bands(dashed lines indicate 90%acceptance region)

• B3 is clearly contaminated(discard in subsequentanalysis)

B6

B4B3

B5


60 mm

BLIP 4

BLIP 3

BLIP 5

BLIP 6

Contamination

Contamination

4 x BLIP Tower Schematic (1999 Run)

• Self shielding through close pack - spacing 3.5 mm vertically

Fiducial Region - contains ~0.25 kg of Ge

Outer IonizationElectrodes

•less shielded fromelectron-inducedsurface events•imperfect ionizationyield (particularly forevents in top-bottombreak)

60 mm

Ge shielding


1999 Run Ge BLIP Data Set Combined

all single-scattersNR candidates

Entire 96 live days operation GeBLIPs = 10.6 kg-days

u Gamma and electron bands wellseparated from NR band

• NR candidates are truly NR’s

u See a total of 13 events > 10 keVÜ ~ 1.2 events/kg/day

NR Band asymmetric (-3σ,+1.28σ)= 90% efficient

Expect 40 events for scalarWIMP σ given by the DAMAAnn Mod Signal


Nuclear Recoil Events

• 13 single NR ~1/3 that expected for the DAMA Ann Mod

• However, we have strong evidence that these events are causedby neutrons

u 4 multiple scatter nuclear recoils observed in Ge in same data set (**)

u 4 nuclear recoil events seen during 1.6 kg-days Si data

Observation of Ge multiple/Observation of Ge multiple/Si Si single events is consistentsingle events is consistentwith all single nuclear recoil events being due to neutronswith all single nuclear recoil events being due to neutrons


Neutron Multiple Scatters

• Observe 4 neutron multiple scattersin 10-100 keV multiple events

u 3 neighbors, 1 non-neighbor

• Calibration indicates negligiblecontamination by electron multiples

Ioni

zatio

n Yi

eld

B6

Ionization Yield B4

photons

neutronneutrons

Ioni

zatio

n Yi

eld

B5,

6

Ionization Yield B4,5

surfaceelectrons photons

Neighborinteraction

B4

B3

B5

B6

Non-Neighborinteraction

Neighbors Non-Neighbors


Muon-Coincident & Calibration Neutrons

• Agreement between MC and data is good - no free parametersu (i) Simulate neutrons generated in Pb/Cu shielding by muons

u (ii) Simulate neutrons from Am/Be source on top of Pb shield (poly out)

Singles/multiples ratios & shape in MC match data well ~15% syst much smaller than statistical fluctuation in final result


Consistency of Neutron Interpretation with MC

• Predicted ratios of numbers of events set by Monte Carlo simulations

• Ge multiples and Si singles imply large expected neutron background with largestatistical uncertainty

• Likelihood-ratio test indicates we should expect worse agreement 6% of the time

• Energy spectra consistent with expectations for neutrons (also with WIMPs)

Nuclear Recoil Events

4

13 16

1

3.4

Data w/ 68% confidence intervalPrediction based on Ge mult, SiPredictions based on most likely

+


• Follow ‘Unified Approach’ of Feldman and Cousins: ordering bylikelihood ratio R with parameters constrained to lie inside thephysical region:

u Large value of R(M, ) means the observed data is likely for the model (M, )

u Use Monte Carlo simulation to determine critical parameter R90% (which issmaller than 90% of simulated R’s). A model (M, ) is rejected by the data atthe 90% confidence level if R


mean over 2-6 keVee(22 – 66 keV recoil)

DAMA 2000 paper Figure 2

DAMA 15,000 kg-day

DAMA 215,000 kg-day

DAMA 3 + 438,000 kg-day

New CDMS Limit

CDMS 2000(90% CL)

• CDMS Limit (Published PRL17June2000) incompatiblewith DAMA (4σ) annualmodulation signal at 99.98%

MinimumDAMA NaI/1-4 (3σ)

Best fit to Ann Moddata alone

Best fit to Ann Moddata alone

Best FitDAMA NaI/1-4

DAMA 1996(90% CL)


Combined Likelihood CDMS limit & DAMA signal

DAMA (3 year)58,000 kg-days

NaI

Ann. Mod. Signal (fit to Fig 2)m = 50 GeV

σ = 14.4 10-42 cm2

+Background limit in2-3 keV binm = 52 GeV

σ = 7.2 10-42 cm2CDMS 1999

10.6 kg-daysGe

= 40 = 202.3σ away from best

fit to Ann. ModGe singles +

Ge multiples +Si singles

≤ 8(90% CL)

99.98% 99.8%CDMS excludes all points in3σ (~99% CL) region at 75%

Ignore Gemultiples 99.93% 96.8%

Ge singles only(No Subtraction)

NWIMPs = 13≤ 19

(90% CL)99.5% 59%

•Expected sensitivity for CDMS with multiples subtraction (when exposure would see 1 multiplerather than 4, and 16 singles rather than 13) raises NWIMPs by 50%: ≤ 12 (90% CL)

•Combined likelihoods based on CDMS calculated fit to ann. mod. & an inferred likelihood for datawhen constraint of 2-3 keV bin included (DAMA actual likelihood function not yet made available)

NWIMPs is number of WIMPs in 10.6 kg-days Ge in CDMS run (Er>10 keV)

000919.3.rjg


Compatibility of CDMS and DAMA

• CDMS resultsincompatible withDAMA Figure 2 data(left) at > 99.98% CL

• Estimate full DAMAlikelihood function:

u Two experiments areincompatible at 99.8%CL

u Ignore multiplescatters: 96.8% CL

CDMS dataBest simultaneous fitto CDMS and DAMApredicts too littleannual modulation inDAMA, too manyevents in CDMS

predictedspectrum

DAMA 2000 paper Figure 2


Can CDMS be wrong? (1)

• The detector technology is too new to trust, inadequate technical write upu Yes this will be true of any new detector - we do have to convince you of

performance

u Within collaboration (25 active members) handling of systematics has beenextremely thorough

u Calibrations have been discussed

u PRD in preparation / Thesis (pp 500) also made available

• Completeness of Error Analysisu Because 13 Ge singles, 4 multiples, 4 Si Singles statistical fluctuations dominate

completely over systematic errors in final exclusion limit analysis

• Nuclear Recoil Efficiencyu Determined in situ using calibration-source neutrons; comparison to the

simulation of neutrons from this source indicates this efficiency is accurate to <20%. The stability of this efficiency is demonstrated by the veto-coincidentneutron rate, which is stable to 15%, consistent with statistical fluctuations, andagrees with the simulation of these neutrons to < 20%.


CDMS: Summary (I) and New Experiment (II)

• CDMS I probing supersymmetry regionu Results for scalar-interacting (σ~A2) WIMPs

(CDMS: 10.6 kg-day Ge) are incompatible with DAMA(58,000 kg-day NaI) signal at very high confidence

u PRL published / PRD to follow

u Will release additional analysis: 40% increase exposure (larger fiducial volume)

u Signal due to external neutron background from muons in rock

• 2000 Run underway at SUF shallow siteu 3 Ge and 3 Si detectors - better background subtraction

u Additional neutron moderator to cut background by ~2.3x

• CDMS II full funding (99-05) approved by NSF & DOE

R. Abusaidi et al., Phys. Rev. Lett. 84, 5699 (17June2000) astro-ph/0002471PRD in preparation + Thesis Golwala

000904.2.rjg

http://cdms.berkeley.eduhttp://dmtools.berkeley.edu


Inner Ionization Electrode

Increasing the 1999 Run BLIP Fiducial Volume

•Results I show today are based on the more restrictive cut --keep only events fully contained in inner ionization electrode

•Events near inner-outer gap haveionization energyshared between innerand outer electrodes

TopView

Region ofSharedEvents

Outer IonizationElectrode

•Internal multiple scattersalso appear as shared events

•Including "shared" regionwill increase exposure by~40% for WIMPs, by ~60% forneutrons.


Direct Detection: History & Future

Oroville (88)[m=20 GeV]

Homestake (87)

Heidelberg-Moscow (94)

Heidelberg-Moscow (98)

[m = ?? GeV - if significantlybetter limit obtained atdifferent mass]

DAMA (96)

UKDMC (96)[m=100 GeV]

DAMA (98/00)

90% CL Limit on Cross section for 60 GeV WIMP (scalar coupling)

CDMS SUF (T)

CDMS Soudan (T) 7 kg Ge+Si Cryodet

GENINO (T) 100 kg Ge Diode

GENIUS (T)100 kg Ge Diode

CryoArray (T)0.1-1 tonne Cryodet

~1 event kg-1 day-1

~1 event kg-1 yr-1

~1 event 100 kg-1 yr-1

LHC

Not meant to be a complete list - see http://dmtools.berkeley.edu

Different Different ColoursColoursIndicate DifferentIndicate Different

TechnologiesTechnologies

NOWNOWCDMS SUF (99)CDMS SUF (00)

Edelweiss (98)

000904.1.rjg

GeNaICryodet

(T) TargetSignal

Liq Xe


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