TheFutureofCDMS

ScottFallows

UniversityofMinnesota

GLCWXChicago,14June2010

CDMSOverview(I)

• CDMS=CryogenicDarkMatterSearch

• HalfamileundergroundattheSoudanUndergroundLabinnorthernMinnesota

• AttemptsdirectdetectionofWIMPsthroughelasticscatteringonnucleiinGecrystals

• Detectorscontinuouslycooledindilutionrefrigeratortomaintainoperationaltemperatureforsuperconductingphononsensors

CDMSOverview(II)

• SolidGe“ZIP”detectorsmeasureionizationandphononsignalsfromparticleinteractions

• Basicstrategy:1. Discriminate

electronrecoils(γ,βbackground)fromnuclearrecoils(WIMPsignal,n0background)

2. Removeneutronbackground

DetectorBasics• Ionization:

– Electron‐holepairsfrominteractionseparatedbyappliedEfield,andcollectedatopposingelectrodes

• Phonons:– collectedbyAl,causing

diffusionintosuperconductingtungstenTES,drivingitnormal

– ChangeinRTESamplifiedbySQUIDsandreadout

Al

quasiparticletrapAlCollector

WTransition‐EdgeSensor

Ge

quasiparticlediffusion

phonons

T(mK)Tc~80mK

R TES(Ω

)

4

3

2

1

~10mK

IonizationYield

• Ratioofionizationtophononenergyisgreaterforelectronrecoilsthanfornuclearrecoils

• Allowsustorejectmoste‐,γbackgroundsto1:104levelforCDMS‐IIZIPs

SurfaceEvents• Incompletechargecollectionforeventsonouter

10‐μm“deadlayer”

• Suppressedionizationyield

• Phononpulsesshowdifferenttimingproperties

Surfaceevents

Surface‐EventRejectionCalibrationData

Keep

Reject

Phononpulse‐shapecontainsinformationoninteractiondepth

Rejectionpowerof1:350forsurfaceeventsonCDMS‐IIZIPs

(M.Fritts)

8

n0Background:ShieldingandTagging

Scintillatorvetocage:muon‐taggingefficiency>99.9%

Polyethyleneshieldsagainstexternalradiogenicneutrons

Cosmogenicbackgroundestimatedtobe0.04eventsforlastrun;similarforradiogenic

PlasticScintillator

Polyethylene(40cm,10cm)

Lead,AncientLead(23cm,4cm)

CopperCans(afewcm)

713mrock2090mwe

Rockreducesmuonfluxby5x104

≈1µ/minuteincidentonscintillatorcage

SoudanUnderground

Lab

ScalingUp

• Ge‐basedtechnologyisoneofseveralcandidatesforton‐scaledarkmattersearches– competingwithliquidXe,liquidAr,andothers

• Increaseexposurewhilelimitingbackground– increaseddetectorsize,number

– improveddetectordesign,analysistechniques– improvedhandling:lowercontamination(fewerβ)

– deepersitetolimitneutronbackground(SNOLAB)

mZIP• Relativelyminordesignchanges

• 1”thick,2.5xsuppressionofsurfaceevents

• “Stadium”phononsensordesign:

– Coversmoresurfacearea

– Improvesphononcollection

• “Mercedes”‐likephononsensorlayout

– Betterphononsignalattheouteredge

– Breaksdegeneraciesinpositionreconstruction

– Improvesphonontiminginformation

mZIPatSoudan

• SuperTower1installedatSoudan– Five1”thickmZIPs+twoendcapvetodetectors

– Tookdatafall/winter2009‐10– Dataanalysisongoing

iZIP• Majordesignchange

• Still3”x1”(650gGe)

• Phononsensors:onbothsidesforfull3Dreconstruction

• Chargeandphononchannelsinterleaved

iZIPChargeCollection

• Chargelinesalternatingat±2V/0Vcauseasymmetrice‐/h+collectionforsurfaceevents

• Chargecollectionissymmetricforbulkevents

• Vastlyreducesdominantbackground

e‐

h+

e‐h+h+ +g+g

e‐h+h+h+

e‐e‐e‐

ChargeAsymmetry

• Witha109Cdsourceontheelectron(+2V)side,holecollectionisreducedforsurfaceevents–theygotothee‐sideground

• 90%efficientq‐symmetrycutrejectssurfaceeventsto<1:1000

• Bonus:10xsmallerleakageoflowyieldsurfaceeventsintoNRbandevenbeforethiscut(strongertransverseEfields)

~100Hz109Cd

Holes

Electrons

Electrons

Holes

ReconstructioninPhonons

• Twoadditionalparameterstocombinewithq‐symm.1. Phononenergypartition(shown,center)

2. Timingdifferencebetweenrisingedgeofphononpulsesonupper/lowerdetectorfaces

• iZIPER/NRdiscriminationpowersatisfiesrequirementsofton‐scaleexperiment

θzpθzq

=

Phononenergy:sourceside

Phon

onene

rgy:otherside

θzp

θ zq

+

FromCDMSIItoSuperCDMStoGEODM

CDMSII3”x1cm~0.25kg/det16detectors=4kg~2yrsoperation(lastanalysis)

SuperCDMS3”x1”~0.64kg/detSoudan SNOLAB25detectors=15kg 150detectors=100kg2yrs~10,000kg‐d 3yrs~100,000kg‐d

SuperCDMSSNOLABandGe‐ObservatoryforDarkMatter(GEODM) ‏6”x2”~5.1kg/detSNOLAB DUSEL20detectors=100kg 300detectors=1.5ton3yrs~100,000kg‐d 4yrs~1.5Mkg‐d

(V.Mandic)

Summary

• NewiZIPdetectorsoffervastlyimprovedperformance– Chargesymmetry

– ReducedlowyieldsurfaceeventsinNRband– Phononenergypartition– Improvedphonontimingdiscrimination

• CombinedwithreducedneutronbackgroundatSNOLAB/DUSEL,Gedetectorsshouldbecompetitiveinton‐scaleWIMPsearch

Backup:HowDoWeGetThere?

• Currentsurface‐eventrejectionefficiency:3×10‐3.• SuperCDMS‐15kg(Soudan):

– x2(cuts)x2.5(1”thick)x1.6(lowerbkgd)⇒10x.• SuperCDMS‐100kg(SNOLAB):

– x2(lowercontamination)x2(electrodes)x2(phonontiming)x2(phononcollection)⇒150x.

• GEODM1.5ton(Homestake‐7400):– x2(2”thick)x2(lowercontamination)x3(detectorimprovements)⇒2000x.

• BulkEMbackground(gammas):similarfactorstoabove.• Lowercontamination(radonmitigation,materialscreening).• Neutronbackground:

– Deepersites(SNOLAB,Homestake‐7400).– Internalshielding(insidecryostat).– Externalmoderator(waterorpoly).