Henrique AraújoImperial College London

IOP2010 – JOINT HEPP/APP GROUP MEETING29-31 March 2010, University College London

What are we looking for?• WIMPs attract most experimental effort,

but axion searches are a growth industry

• I assume here that the Lightest SUSY Particle

is the neutralino, , which is a great WIMP

• WIMPs should scatter off ordinary nuclei

producing measurable nuclear recoils

• But, essentially, WIMP searches are not really (PP-)model dependent…

type spin mass

Axion 0 eV – 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

• Scalar (SI) and axial-vector (SD) -N interactions(neutral current exchange):

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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+) + (40 keV), 73Ge(5/2+→9/2+)(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, )– Neutrons are irreducible background

NN

NN

NN *

3H. Araújo

Elastic scattering ratesCanonical model: not great, but we’re all in this together:• Isothermal sphere (no lumps), ∝ r−2

• Local density 0~0.3 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 4H. Araújo

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

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

• Spin-independent (scalar) interaction

– note A2 in enhancement factor

– cMSSM-favoured XS within reach of current detectors

• 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 targets

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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

• Ah: and there is no trigger…7H. Araújo

Building a WIMP detector• Consider 1 kg target

Sensitive to Edep>1 keV

• Expected WIMP rates – 0.1−0.000001 evt/day

• However…

• Cosmic rays, -rays– >1,000,000 evt/day

• Neutrons are THE background!– Several evt/day

1 kg

n

WIMP

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Building a WIMP detector

• Move underground

• Use radio-pure materials

• Shield external -rays

• Shield external neutrons

• Actively veto neutrons

• Discriminate e-recoils (, ) from n-recoils (WIMPs, n)

WIMP

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Nuclear recoils - backgrounds• Nuclear recoils – same signature

– Neutrons from (,n) and SFission from U/Th trace 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 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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Nuclear recoils - signal acceptance• Neutron elastic scattering populates WIMP acceptance region

– Calibration of detection efficiency with Am-Be (,n), Cf-252 (SF), D-D, D-T

• But there are complications:– Multi-element: in CaWO4 (CRESST), WIMPs couple mainly to heaviest material (W), but

neutrons scatter mainly off lightest (O). Signal acceptance must be calibrated indirectly– Quenching factor: in noble liquids (ZEPLIN,XENON,WARP,ARDM,…) conversion from

“electron-equivalent” to nuclear recoil energy is not straightforward (or favourable…)– Droplets: in C4F10 superheated droplets (SIMPLE,PICASSO) phase transition is independent

of energy. Calibration of signal acceptance threshold only

nR EA

AE 2

2cos

)1(

4

En

ER

En’

incomingneutron

nuclearrecoil

laboratory system

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

• 1 MeV neutron → ER,max = 30 keV

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Ge (CDMS-II)

Discrimination: single channels

Hphon

ons

ionisationQ

L

scintillation

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, SIMPLE12H. Araújo

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)

All 3 hybrid technologies> 99.9% discrimination @ >10 keV NR energy

Hphon

ons

ionisationQ

L

scintillation

Discrimination: hybrid detectors

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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 2Al~ 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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A few examples(not comprehensive and somewhat UK-centric)

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3 events observed in10-40 keVnr acceptance region 48 kg·days exposure (2007)Angloher et al, Astropart. Phys. 31 (2009) 270

CRESST: Scintillation & PhononsTarget: 0.6 kg CaWO4

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ZEPLIN-III: Scintillation & Ionisation

7 events observed in10-30 keVnr acceptance region 850 kg·days raw exposure (2008)(likely e-recoil background)Lebedenko et al, PRD 80 (2009) 052010

Target: 12 kg LXe

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CDMS-II: Ionisation & Phonons

J. Cooley, CDMS Collaboration

2 events observed in10-100 keVnr acceptance region612 kg·days exposure (2007-08)Background estimate 0.8±0.2!Ahmed et al, arXiv:0912.3592

Target: 4.4 kg Ge, 1.1 kg Si

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

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

(But what is modulated?and is it getting smaller?)

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DRIFT – NI Gas TPC

2

0

mincos)(exp

)(cos v

vvv

ddE

dR E

R

Target: 167 g/m3 CS2 (now CS2+CF4)

Unlikely that backgrounds mimic signal which appears as forward/backward asymmetry in galactic coordinates

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PICASSO: Superheated C4F10Target: 65+69 g C4F10

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CoGeNT - Ionisationp-type point contact (PPC) HPGe

Excess at low energies – a glimmer?Aalseth et al, arXiv:1002:4703v2)

No discrimination, too close to threshold…

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Target: 330 g Ge

World status & prospects (SI)

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World status & prospects (SD)

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World status & prospects (iDM)

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Schmidt-Hoberg & Winkler, JCAP09(2009)010

Akimov et al., arXiv:1003.5626 (ZEPLIN-III)

Next generation: a viewTWO-PHASE ARGON A=40, <ER>= 13 keV @50 GeV/c2, 35 keV @500 GeV/c2

very scalable (cheap, large LAr systems demonstrated) poor energy threshold, low atomic weight, Ar-39 background WARP, ArDM, (DEAP/CLEAN) working on 0.1—1 tonne targets 5-tonne system within 5 years is (optimistically) possible

CRYOGENIC GERMANIUM A=73, <ER>= 13 keV @50 GeV/c2, 57 keV @500 GeV/c2

excellent energy resolution, excellent discrimination difficult to scale (small detector modules, <50 mK cryostats) CDMS, EDELWEISS, (CRESST) working on 10—20 kg targets EURECA and SuperCDMS propose ~100 kg target in 5 years

TWO-PHASE XENON A=131, <ER>= 11 keV @50 GeV/c2, 85 keV @500 GeV/c2

scalable, low threshold control of xenon purity to <ppb is demanding ZEPLIN-III, XENON100, LUX350, (XMASS), working on 10-100 kg XENON1T and LUX-ZEPLIN propose 1 tonne two-phase xenon targets

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Proposals (>1 tonne)

CDEX, CLEAN, COUPP+, DAMA+, DARKSIDE, DARWIN, DEAP3600, DRIFT, EURECA, GEODM, KIMS+, LUX-ZEPLIN, MAX, SuperCDMS, XMASS, …

Next generation: a view

Araujo, S

trigari & T

rotta

Araujo, S

trigari & T

rotta

29H. Araújo

Ready to scale up!

• UK pioneered several search technologies – NaIAD, ZEPLIN-I, DRIFT-I, CRESST-I, ZEPLIN-II, DRIFT-II,

CRESST-II, ZEPLIN-III, ArDM, EDELWEISS, (EURECA, LZ)

• And pushed forward “underground science”– Dating back to Holborn Station Laboratory…– Creating the Boulby Underground Laboratory

(see Sean Paling’s talk tomorrow)

• But we’re running out of road…

30H. Araújo

Pack ?

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Thank you