SEARCHING FOR WIMPS UNDERGROUND:THE EXPERIMENTAL QUEST

Henrique AraújoImperial College London

IOP2011 NPPD CONFERENCE3-7 April 2011, University of Glasgow

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Outline

• WIMP scattering signal

• The experimental challenge

• Recent results

• Great expectations

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What are we looking for?

• WIMPs attract most experimental effort

• A neutralino LSP would make a great WIMP

• WIMPs should scatter off ordinary nuclei

producing measurable nuclear recoils

• Scalar (SI) and axial-vector (SD) c-N

interactions (neutral current exchange)

type spin mass

Axion 0 meV – 10 meV

Axino LSP 1/2 eV - GeV

Inert Higgs Doublet 0 50 GeV

Sterile Neutrino 1/2 keV

Neutralino LSP 1/2 10 GeV - 10 TeV

Gravitino LSP 3/2 eV – TeV

Kaluza-Klein UED 1 TeV

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Low energy nuclear recoils• Elastic scatter off nucleus:

– Decreasing, featureless spectrum of low-energy recoils (<~50 keV)– Rate depends on target mass & spin, WIMP mass & spin, DM halo, …– Neutrons are irreducible background

• Inelastic scatter off nucleus: – Short-lived, low-lying excited states (easier signature?)– 129Xe(3/2+→1/2+) + g (40 keV), 73Ge(5/2+→9/2+) + g (13 keV)– Neutrons are irreducible background

• Inelastic dark matter (iDM):– “particles will scatter at DAMA but not at CDMS” (Smith & Weiner 2001)

– Recoil spectrum with threshold (mass splitting, d)– Neutrons are irreducible background

NN

NN

NN *

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• Neutron elastic scattering populates WIMP acceptance region– Calibration of detection efficiency with

Am-Be (a,n), Cf-252 (SF), D-D, D-T sources

Ge (CDMS-II)

nR EA

AE 2

2cos

)1(

4

Signal calibration

100 GeV WIMP on Xe (A=131):• 220 km/s WIMP → ER,max = 40 keV

• 1 MeV neutron → ER,max = 30 keV

En

ER

En’

incomingneutron

nuclearrecoil

laboratory system

Xe (X100)

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Elastic scattering ratesCanonical model (‘we’re all in it together’)• Isothermal sphere (no lumps), r ∝ r−2

• Local density r0~0.3 GeV/c2/cm3 (~1/pint at 100 GeV)

• Maxwellian (gaussian) velocity distribution• Characteristic velocity v0=220 km/s,

• Local escape velocity vesc=600 km/s

• Earth velocity vE=232 km/s

thE

RR

dEdE

dRR

max

min

322

0 )()(

2

v

vA

A

R

vdv

vfqF

mdE

dR

1)(

4,

2/

0

0 0

TW

TWrEE

R mm

mmre

rE

R

dE

dRR

RdE

dR

REthE

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Elastic scattering rates• Coupling to protons and neutrons more useful than coupling to nucleus

• To compare different target materials, indirect searches, LHC results

1. Spin-independent (scalar) interaction

– note A2 in enhancement factor

– cMSSM-favoured XS within reach of current detectors

2. Spin-dependent (axial-vector) interaction

– note J (nuclear spin) instead of A2 enhancement

– cMSSM-favoured XS out of reach for the time being…

22

2

)0( Aq pp

ASIA

2

,2

2 1

3

4)0( nnpp

SDnp

p

ASDA SaSa

J

Jq

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SI scattering rates for 1 kg targetsProbably

just around the corner

by end 2011

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The experimental challenge• Low-energy particle detection is easy ;)

E.g. Microcalorimetry with Superconducting TES

Detection of keV particles/photons with eV FWHM!

• Rare event searches are also easy ;)

E.g. Super-Kamiokande contains 50 kT water

Cut to ~20 kT fiducial mass (self-shielding)

• But doing both is hard!Small is better for collecting signal

Large is better for background

• And there is no trigger…

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Backgrounds• Nuclear recoils – same signature

– Radioactivity neutrons: (a,n) and SF from U/Th contamination• Laboratory walls, shields, vessels, components, target material

– Neutrons from atmospheric muon spallation• Difficult to shield completely even underground

– Recoils from alpha emitters (e.g. Rn-222 and progeny)• Contaminating active target bulk/surfaces, air, etc

– Eventually, even coherent neutrino-nucleus scattering!

• Electron recoils – discrimination power is limited– Gamma-ray background external to target

• K-40, Cs-137, U/Th from walls, shields, vessels, components– Contamination of target bulk and surfaces

• U/Th betas and gammas (Pb-214, Bi-214, Pb-210,…)• Cosmogenic (Ge-68, Ge-71,…), anthropogenic (Kr-85, Cs-137,…)

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Heat & Ionisation BolometersTargets: Ge,Si

CDMS, EDELWEISScryogenic (<50 mK)

Light & Heat BolometersTargets: CaWO4, BGO, Al2O3

CRESST, ROSEBUDcryogenic (<50 mK)

Light & Ionisation Detectors

Targets: Xe, ArArDM, LUX, WARP,

XENON, ZEPLINcold (LN2)

Hphon

ons

ionisationQ

L

scintillation

Discrimination

ScintillatorsTargets: NaI, Xe, Ar

ANAIS, CLEAN, DAMA, DEAP, KIMS, LIBRA,

NAIAD, XMASS, ZEPLIN-I

Ionisation DetectorsTargets: Ge, Si, CS2, CdTe

CoGeNT, DRIFT, GENIUS,HDMS, IGEX, NEWAGE

BolometersTargets: Ge, Si, Al2O3, TeO2

CRESST-I, CUORE, CUORICINO

Bubbles & DropletsCF3Br, CF3I, C3F8, C4F10

COUPP, PICASSO, SIMPLE

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Phonons (microcalorimetry)

Superconducting Transition-Edge Sensor (as in CDMS)

• Collect high-frequency (athermal) phonons from particle interaction• Into superconducting Al contacts (threshold 2DAl~ meV)• Quasiparticles from broken Cooper pairs diffuse into a W TES• SQUID readout offers extremely high sensitivity• Channel threshold: 1 keV for Ge & Si nuclear recoils

C

ET max

Thermal phonon signal is lost with increasing mass: must collect phonons before they thermalise in absorber

J. Cooley, CDMS Collaboration

Cryogenic: T0~50 mK

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Scintillation (photomultipliers)

Scintillation detectors (as in DAMA)

• Best photomultipliers now approaching 50% quantum efficiency• Best NaI(Tl) crystals yield ~90 photons/keV for gamma rays• Typically require coincidence of two photomultipliers (2 phe)• Threshold: 0.3-3 keV for I nuclear recoils (depending on “channelling” effect)

NaI, CsI, CaWO4, LXe, LAr: many materials scintillate…Photomultipliers: ancient vacuum tube technology, but no-one has come up with a better alternative yet(and we’re trying…)

Room temperature, cold or cryogenic

DAMA/LIBRA Collaboration

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Ionisation (Electroluminescence, TES, HEMT, JFET)

Two-phase xenon detectors (as in ZEPLIN)

• Strong electric field across liquid-gas xenon target• Collect ionisation from particle track in liquid Xe• Drift up to surface, then emit into vapour phase• Electroluminescence photons detected with photomultipliers• Threshold: 0.2 keV for Xe nuclear recoils

Difficult to measure one electron, but not so hard to measure electroluminescence photons from one electron

Cold: T0~200 K

Edwards et al., Astroparticle Phys. 30 (2008) 54

(electroluminescence) S2

1e

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Self-shielding in noble liquids

Liquidxenon

r=3 g/cm3

neutrongamma

Sacrificial volume

Fiducial volume

LUX LUX-ZEPLIN 1.5t

Neutrons (5-25 keV)

Gammas (5-25 keV)

S1S2

S2S2

S2

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Water cherenkov, passive LXe, bare or loaded scintillator,…

veto

make thin!

Anticoincidence detectoraround WIMP target

LiquidXenonneutron

gamma

LUX LUX-ZEPLIN 1.5t

Neutrons (5-25 keV)

Gammas (5-25 keV)

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Anticoincidence detectoraround WIMP target

Akimov et al, arXiv:1103.0393

Effect of veto efficiency on the discovery power

of a rare event search with a single background

and no additional discrimination

NT is the number of tagged events observed

A veto buys you:1. Background reduction

• Up to order of magnitude for gammas and neutrons

2. Diagnostic power• Unexpected backgrounds• Radiation environment

3. Signal-free background sample• Calculation of background

expectations without compromising blind analysis

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Recent and future excitements

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DAMA/LIBRA: ScintillationTarget: 250 kg NaI(Tl)

8.9s CL modulationover 13 annual cyclesBarnabei et al, arXiv:1002.1028

(Something is modulated, but what?)

GRA

N S

ASSO

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ANAIS: ScintillationTarget: aiming for 250 kg NaI(Tl)

ANAIS STATUSXXXIX IMFP CANFRANC 10-FEB-2011Carlos Pobes

- Various prototypes developed over last decade - Excessive K-40 contamination in existing crystals - Radio-pure detectors under development- Mass production from end 2011

Anais-0 being tested at old LSC Ready to be installed in new LSC facilities

With 500 kg.years data, the DAMA result could be reproduced if threshold ~2 keVee and

background <2 evt/kg/day/keV

CAN

FRAN

C

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EDELWEISS-IIIonisation & Phonons

384 kg·days from 14 months of operation 5 candidate events above 20 keVestimated background is <3.0 eventssSI<4.4×10−8 pb (90% CL) at 85 GeV

E. Armengaud et al, arXiv:1103:4070v2

Target: 4 kg Ge

MO

DAN

E S. Henry tomorrow

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CRESST: Scintillation & PhononsTarget: 3 kg CaWO4

GRA

N S

ASSO

Observed 57 events (yes, fifty seven!) in 730 kg*days in oxygen bandBackground prediction 35.6 events(of which 17.3 from neutrons, measured from only 3 multiple scatters)J. Schmaler (German Physical Society meeting, 30 Mar 2011)

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Light WIMPs: ‘excesses’ at low energies

Aalseth et al, arXiv:1002:4703v2)

A ‘glimmer’ or a ‘flicker’?

Phys. Rev. 26, 71–85 (1925)

DAMA

CoGeNT CRESST

CMSSM Buchmueller et alCMSSM Trotta et al

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What would ZEPLIN-III make of it?13 GeV WIMP AT sSI=3x10-5 pb

Z3 FIRST RUN OBSERVATION: 7 events near top of acceptance region in 2-16 keVee

You cannot be serious!

140 kg*days in FSR signal boxRecoil spectrum in xenon

30 events >2 keVee!

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COUPP: Bubble chamberTarget: 3.5 kg CF3I

E. Behnke et al, PRL 106, 021303 (2011)

Electron recoils do not nucleate bubblesBackground from neutrons and alphas

Ultrasound emission provides powerful discrimination between alphas and nuclear recoils (as demonstrated by PICASSO)

Run at shallow site (Fermilab): 3 candidate events were observed in 28.1 kg.days, consistent with neutron background.

SNO

LAB

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ZEPLIN-III: Scintillation & Ionisation

>280 days continuous operationResult from ~2,000 kg*days soonSensitivity 1-2x10-8 pb

Target: 12 kg LXe, 6.5 kg fiducial

BOU

LBY

CMSSM Buchmueller et alCMSSM Trotta et al

wed pm parallel session

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XENON100: PLR analysisTarget: 62 kg liquid xenon, 30 kg fiducial

E. Aprile,XIV Int. Workshop on Neutrino Telescopes,Venice, 16 Mar 2011

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XENON100: “result in weeks”

E. Aprile,XIV Int. Workshop on Neutrino Telescopes,Venice, 16 Mar 2011

Target: 62 kg liquid xenon, 30 kg fiducial

“ ‘unblinding procedure’ in final stage of internal review

‘Blind’ analysis of ~10x more data near completion

Results expected within weeks. Non negligible discovery potential”

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The futureGEN-1ANAISARDMCOGENTCOUPPCRESSTDAMADARKSIDEDM-TPCDRIFTEDELWEISSKIMSLUX350MiniCLEAN NEWAGEPICASSOSCDMSWARPXENON100XMASSZEPLIN-III…

GEN-3COUPPGEODMLZ20MAXCLEAN

GEN-2DARKSIDEDEAP-3600EURECALZSPICASSO-IISCDMSXENON1tXMASS-II

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Conclusions

• Dark matter is one of the hottest topics in science today

• The field of underground WIMP searches is very vibrant, attracting strong investment worldwide (mustn’t grumble…)

• Direct, indirect and accelerator searches are finally converging in sensitivity for neutralino-proton interactions

• An exciting 2011: new results expected from Gen-1 targets and significant design/construction activity at tonne scale