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AN EXPERIMENTAL OVERVIEW OF Direct Dark Matter Searches. Henrique Araújo Imperial College London IOP2010 – JOINT HEPP/APP GROUP MEETING 29-31 March 2010, University College London. What are we looking for?. Scalar (SI) and axial-vector (SD) c -N interactions (neutral current exchange):.

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an experimental overview of direct dark matter searches

AN EXPERIMENTAL OVERVIEW OFDirect Dark Matter Searches

Henrique Araújo

Imperial College London

IOP2010 – JOINT HEPP/APP GROUP MEETING

29-31 March 2010, University College London

what are we looking for
What are we looking for?
  • Scalar (SI) and axial-vector (SD) c-N interactions
  • (neutral current exchange):

WIMPs attract most experimental effort,

but axion searches are a growth industry

I assume here that the Lightest SUSY Particle

is the neutralino, c, 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…

H. Araújo

low energy nuclear recoils
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

H. Araújo

elastic scattering rates
Elastic scattering rates

Canonical model: not great, but we’re all in this 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

H. Araújo

elastic scattering rates1
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…

H. Araújo

the experimental challenge
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…

H. 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, a, b, g-rays

>1,000,000 evt/day

Neutrons are THE background!

Several evt/day

m

b

g

n

a

Building a WIMP detector

WIMP

1 kg

H. Araújo

building a wimp detector1
Move underground

Use radio-pure materials

Shield external g-rays

Shield external neutrons

Actively veto neutrons

Discriminate e-recoils (g, b) from n-recoils (WIMPs, n)

Building a WIMP detector

WIMP

H. Araújo

nuclear recoils backgrounds
Nuclear recoils - backgrounds
  • Nuclear recoils – same signature
    • Neutrons from (a,n) and SFissionfrom 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,…)

H. Araújo

nuclear recoils signal acceptance

laboratory system

incoming

neutron

En

q

nuclear

recoil

ER

En

Nuclear recoils - signal acceptance

Ge (CDMS-II)

  • 100 GeV WIMP on Xe (A=131):
  • 220 km/s WIMP → ER,max = 40 keV
  • 1 MeV neutron → ER,max = 30 keV
  • Neutron elastic scattering populates WIMP acceptance region
    • Calibration of detection efficiency with Am-Be (a,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

H. Araújo

discrimination single channels

ionisation

Q

L

scintillation

H

phonons

Discrimination: single channels

Ionisation Detectors

Targets: Ge, Si, CS2, CdTe

CoGeNT, DRIFT, GENIUS,

HDMS, IGEX, NEWAGE

Scintillators

Targets: NaI, Xe, Ar

ANAIS, CLEAN, DAMA,

DEAP, KIMS, LIBRA,

NAIAD, XMASS, ZEPLIN-I

Bolometers

Targets: Ge, Si, Al2O3, TeO2

CRESST-I, CUORE, CUORICINO

Bubbles & Droplets

CF3Br, CF3I, C3F8, C4F10

COUPP, PICASSO, SIMPLE

H. Araújo

slide13

ionisation

Q

L

scintillation

H

phonons

Discrimination: hybrid detectors

Heat & Ionisation Bolometers

Targets: Ge,Si

CDMS, EDELWEISS

cryogenic (<50 mK)

Light & Ionisation Detectors

Targets: Xe, Ar

ArDM, LUX, WARP,

XENON, ZEPLIN

cold (LN2)

Light & Heat Bolometers

Targets: CaWO4, BGO, Al2O3

CRESST, ROSEBUD

cryogenic (<50 mK)

All 3 hybrid technologies

> 99.9% discrimination

@ >10 keV NR energy

H. Araújo

phonons microcalorimetry
Phonons (microcalorimetry)

Cryogenic: T0~50 mK

Thermal phonon signal is lost with increasing mass:

must collect phonons before they thermalise in absorber

  • 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

J. Cooley, CDMS Collaboration

H. Araújo

scintillation photomultipliers
Scintillation (photomultipliers)

DAMA/LIBRA Collaboration

Room temperature, cold or cryogenic

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…)

  • 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)

H. Araújo

ionisation electroluminescence tes hemt jfet

S2

(electroluminescence)

Ionisation(Electroluminescence, TES, HEMT, JFET)

Cold: T0~200 K

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

  • 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

1e

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

H. Araújo

cresst scintillation phonons
CRESST: Scintillation & Phonons

Target: 0.6 kg CaWO4

3 events observed in

10-40 keVnr acceptance region

48 kg·days exposure (2007)

Angloher et al, Astropart. Phys. 31 (2009) 270

H. Araújo

zeplin iii scintillation ionisation
ZEPLIN-III: Scintillation & Ionisation

Target: 12 kg LXe

7 events observed in

10-30 keVnr acceptance region

850 kg·days raw exposure (2008)

(likely e-recoil background)

Lebedenko et al, PRD 80 (2009) 052010

H. Araújo

cdms ii ionisation phonons
CDMS-II: Ionisation & Phonons

Target: 4.4 kg Ge, 1.1 kg Si

2 events observed in

10-100 keVnr acceptance region

612 kg·days exposure (2007-08)

Background estimate 0.8±0.2!

Ahmed et al, arXiv:0912.3592

H. Araújo

J. Cooley, CDMS Collaboration

dama libra scintillation
DAMA/LIBRA: Scintillation

Target: 250 kg NaI(Tl)

8.9s CL modulation

over 13 annual cycles

Barnabei et al, arXiv:1002.1028

(But what is modulated?

and is it getting smaller?)

H. Araújo

drift ni gas tpc
DRIFT – NI Gas TPC

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

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

H. Araújo

picasso superheated c 4 f 10
PICASSO: Superheated C4F10

Target: 65+69 g C4F10

H. Araújo

cogent ionisation p type point contact ppc hpge
CoGeNT - Ionisationp-type point contact (PPC) HPGe

Target: 330 g Ge

Excess at low energies – a glimmer?

Aalseth et al, arXiv:1002:4703v2)

No discrimination, too close to threshold…

H. Araújo

world status prospects idm
World status & prospects (iDM)

Schmidt-Hoberg & Winkler, JCAP09(2009)010

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

H. Araújo

next generation a view
Next generation: a view

TWO-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

Proposals (>1 tonne)

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

H. Araújo

next generation a view1
Next generation: a view

Araujo, Strigari & Trotta

Araujo, Strigari & Trotta

H. Araújo

ready to scale up
Ready to scale up!

Pack ?

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…

H. Araújo

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