SEARCHING FOR WIMPS UNDERGROUND: - PowerPoint Presentation

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SEARCHING FOR WIMPS UNDERGROUND:THE EXPERIMENTAL QUEST

Henrique AraújoImperial College LondonIOP2011 NPPD CONFERENCE3-7 April 2011, University of Glasgow

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OutlineWIMP scattering signalThe experimental challengeRecent resultsGreat expectationsH. Araújo

2

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What are we looking for?

WIMPs attract most experimental effortA neutralino LSP would make a great WIMPWIMPs should scatter off ordinary nuclei producing measurable nuclear recoilsScalar (SI) and axial-vector (SD)

c

-N

interactions (neutral current exchange)

type

spin

mass

Axion

0

m

eV

– 10

meV

Axino

LSP1/2 eV - GeVInert Higgs Doublet050 GeVSterile Neutrino1/2 keVNeutralino LSP1/2 10 GeV - 10 TeVGravitino LSP3/2eV – TeVKaluza-Klein UED1TeV

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H. Araújo

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Low energy nuclear recoilsElastic 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 backgroundInelastic scatter off nucleus: Short-lived, low-lying excited states (easier signature?) 129Xe(3/2

+

→1/2

+

)

+

g

(40

keV

)

,

73

Ge

(5/2+→9/2+) + g (13 keV)Neutrons are irreducible backgroundInelastic 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 background4

H. Araújo

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Neutron elastic scattering populates WIMP acceptance regionCalibration of detection efficiency with Am-Be

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

Ge

(CDMS-II)

Signal

calibration

100

GeV

WIMP on

Xe

(A=131):

220 km/

s

WIMP →

E

R,max = 40 keV 1 MeV neutron → ER,max = 30 keV5H. AraújoXe (X100)

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

r−2Local density r0~0.3 GeV/c2/cm3 (~1/pint at 100

GeV

)

Maxwellian

(

gaussian

) velocity distribution

Characteristic velocity

v

0

=220 km/

s

,

Local escape velocity

vesc=600 km/sEarth velocity vE=232 km/s

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H. Araújo

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Elastic scattering ratesCoupling to protons and neutrons more useful than coupling to nucleusTo compare different target materials, indirect searches, LHC results1. Spin-independent (scalar) interaction

note A2 in enhancement factorcMSSM-favoured XS within reach of current detectors2. Spin-dependent (axial-vector) interactionnote

J

(nuclear spin) instead of

A

2

enhancement

cMSSM

-favoured XS out of reach for the time being…

7

H. Araújo

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SI scattering rates for 1 kg targets8H. Araújo

Probably just around the cornerby end 2011

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The experimental challengeLow-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…

9

H. Araújo

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BackgroundsNuclear recoils – same signatureRadioactivity neutrons: (a,n) and

SF from U/Th contaminationLaboratory 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 targetK-40, Cs-137, U/Th from walls, shields, vessels, componentsContamination of target bulk and surfacesU/Th betas and gammas (Pb-214, Bi-214, Pb-210,…)Cosmogenic (Ge-68, Ge-71,…), anthropogenic (Kr-85, Cs-137,…)10H. Araújo

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Heat & Ionisation BolometersTargets: Ge,SiCDMS, EDELWEISS

cryogenic (<50 mK)Light & Heat Bolometers

Targets

: CaWO

4,

BGO, Al

2

O

3

CRESST, ROSEBUD

cryogenic

(<50

mK

)

Light & Ionisation Detectors

Targets: Xe, ArArDM, LUX, WARP, XENON, ZEPLINcold (LN2)Hphonons

ionisationQ

L

scintillation

Discrimination

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H. Araújo

Scintillators

Targets:

NaI

,

Xe

,

Ar

ANAIS, CLEAN, DAMA,

DEAP, KIMS, LIBRA,

NAIAD, XMASS, ZEPLIN-I

Ionisation Detectors

Targets:

Ge

, Si,

CS

2

,

CdTe

CoGeNT

, DRIFT, GENIUS,

HDMS

,

IGEX, NEWAGE

Bolometers

Targets

:

Ge

,

Si, Al

2

O

3

, TeO

2

CRESST-I, CUORE, CUORICINOBubbles & DropletsCF3Br, CF3I, C3F8, C4F10COUPP, 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

Thermal phonon signal is lost with increasing mass:

must collect phonons

before

they thermalise in absorberJ. Cooley, CDMS CollaborationCryogenic: T0~50 mK12H. Araújo

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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

, CaWO

4

,

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 cryogenicDAMA/LIBRA Collaboration13H. Araújo

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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: T

0

~200 K

Edwards

et al.

,

Astroparticle

Phys. 30 (2008) 54

(

electroluminescence)

S2

1e

14

H. Araújo

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Self-shielding

in noble liquidsH. Araújo15

Liquid

xenon

r

=3 g/cm

3

neutron

gamma

Sacrificial

volume

Fiducial

volume

LUX



LUX-ZEPLIN 1.5t

Neutrons (5-25 keV)

Gammas (5-25 keV)

S1

S2

S2

S2

S2

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

veto

make

thin!

Anticoincidence detector

around WIMP target

H. Araújo

16

Liquid

Xenon

neutron

gamma

LUX



LUX-ZEPLIN 1.5t

Neutrons (5-25 keV)

Gammas (5-25 keV)

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Anticoincidence detectoraround WIMP targetH. Araújo17

Akimov et al, arXiv:1103.0393Effect of veto efficiency on the discovery power of a rare event search with a single background and no additional discrimination

N

T

is the number of tagged events observed

A veto buys you:

Background reduction

Up to order of magnitude for gammas and neutrons

Diagnostic power

Unexpected backgrounds

Radiation environment

Signal-free background sample

Calculation of background expectations without compromising blind analysis

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Recent and future excitements18H. Araújo

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DAMA/LIBRA: Scintillation

Target: 250 kg NaI(Tl)

8.9

s

CL modulation

over 13 annual cycles

Barnabei

et al

, arXiv:1002.1028

(Something is modulated, but what?)

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H. Araújo

GRAN SASSO

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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

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H. Araújo

With 500

kg.years

data, the DAMA result could be reproduced if threshold

~

2

keVee

and background <2

evt

/kg/day/keV

CANFRANC

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EDELWEISS-IIIonisation & Phonons384 kg·days from 14 months of operation 5 candidate events above 20 keVestimated background is <3.0 eventss

SI<4.4×10−8 pb (90% CL) at 85 GeVE. Armengaud et al, arXiv:1103:4070v2Target: 4 kg

Ge

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H. Araújo

MODANE



S. Henry tomorrow

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CRESST: Scintillation & Phonons

Target: 3 kg CaWO422H. Araújo

GRAN SASSO

Observed 57 events (yes, fifty seven!)

in 730 kg*days in oxygen band

Background 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 energiesAalseth et al, arXiv:1002:4703v2)

H. Araújo23

A ‘glimmer’ or a ‘flicker’?

Phys. Rev. 26, 71–85 (1925)

DAMA

CoGeNT

CRESST

CMSSM Buchmueller et al

CMSSM Trotta et al

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What would ZEPLIN-III make of it?24

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 box

Recoil spectrum in xenon

30 events >2

keVee

!

H. Araújo

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COUPP: Bubble chamberTarget: 3.5 kg CF3I

25H. AraújoE. 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.

SNOLAB

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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

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H. Araújo

BOULBY

CMSSM Buchmueller et al

CMSSM Trotta et al



wed pm parallel session

Slide27

H. Araújo27

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

E.

Aprile

,

XIV Int. Workshop on Neutrino Telescopes,

Venice, 16 Mar 2011

Slide28

H. Araújo28

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 completionResults expected within weeks. Non negligible discovery potential”

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The futureH. Araújo29

GEN-1ANAISARDMCOGENTCOUPPCRESSTDAMADARKSIDEDM-TPCDRIFTEDELWEISSKIMSLUX350MiniCLEAN NEWAGEPICASSO

SCDMS

WARP

XENON100

XMASS

ZEPLIN-III

…

GEN-3

COUPP

GEODM

LZ20

MAX

CLEAN

GEN-2

DARKSIDEDEAP-3600EURECALZSPICASSO-IISCDMSXENON1tXMASS-II

Slide30

ConclusionsDark matter is one of the hottest topics in science todayThe 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 interactionsAn exciting 2011: new results expected from Gen-1 targets and significant design/construction activity at tonne scaleH. Araújo30