henrique araújo imperial college london iop2010 – joint hepp/app group meeting 29-31 march 2010,...
TRANSCRIPT
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 *
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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
13H. Araújo
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…
H. Araújo 24
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…
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Pack ?
Click to add funding
Thank you