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SEARCHING FOR WIMPS UNDERGROUND: THE EXPERIMENTAL QUEST. Henrique Araújo Imperial College London IOP2011 NPPD CONFERENCE 3-7 April 2011, University of Glasgow. Outline. WIMP scattering signal The experimental challenge Recent results Great expectations. What are we looking for?.

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Searching for wimps underground the experimental quest

SEARCHING FOR WIMPS UNDERGROUND:THE EXPERIMENTAL QUEST

Henrique Araújo

Imperial College London

IOP2011 NPPD CONFERENCE

3-7 April 2011, University of Glasgow


Outline
Outline

  • WIMP scattering signal

  • The experimental challenge

  • Recent results

  • Great expectations

H. Araújo


What are we looking for
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)

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


Signal calibration
Signal calibration

Ge (CDMS-II)

Xe (X100)

  • 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 sources

H. Araújo


Elastic scattering rates
Elastic scattering rates

Canonical 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

  • 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

        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…

    H. Araújo


    Si scattering rates for 1 kg targets
    SI scattering rates for 1 kg targets

    Probably

    just around the corner

    by end 2011

    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 kTfiducial mass (self-shielding)

    • But doing both is hard!

      Small is better for collecting signal

      Large is better for background

    • And there is no trigger…

    H. Araújo


    Backgrounds
    Backgrounds

    • Nuclear recoils – same signature

      • Radioactivity neutrons: (a,n) and SF from U/Thcontamination

        • 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,…)

    H. Araújo


    ionisation

    Q

    L

    scintillation

    H

    phonons

    Discrimination

    Ionisation Detectors

    Targets: Ge, Si, CS2, CdTe

    CoGeNT, DRIFT, GENIUS,

    HDMS, IGEX, NEWAGE

    Light & Ionisation Detectors

    Targets: Xe, Ar

    ArDM, LUX, WARP,

    XENON, ZEPLIN

    cold (LN2)

    Heat & Ionisation Bolometers

    Targets: Ge,Si

    CDMS, EDELWEISS

    cryogenic (<50 mK)

    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

    Light & Heat Bolometers

    Targets: CaWO4, BGO, Al2O3

    CRESST, ROSEBUD

    cryogenic (<50 mK)

    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


    Self shielding in noble liquids

    LUX  LUX-ZEPLIN 1.5t

    Neutrons (5-25 keV)

    Gammas (5-25 keV)

    Self-shielding in noble liquids

    Liquid

    xenon

    r=3 g/cm3

    Sacrificial

    volume

    neutron

    gamma

    Fiducial

    volume

    S2

    S1

    S2

    S2

    H. Araújo

    S2


    Anticoincidence detector around wimp target

    LUX  LUX-ZEPLIN 1.5t

    Neutrons (5-25 keV)

    Gammas (5-25 keV)

    Anticoincidence detectoraround WIMP target

    veto

    Liquid

    Xenon

    make

    thin!

    neutron

    gamma

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

    H. Araújo


    Anticoincidence detector around wimp target1
    Anticoincidence detectoraround WIMP target

    Akimov et al, arXiv:1103.0393

    • 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

    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

    H. Araújo


    Recent and future excitements
    Recent and future excitements

    H. Araújo


    Dama libra scintillation
    DAMA/LIBRA: Scintillation

    GRAN SASSO

    Target: 250 kg NaI(Tl)

    8.9s CL modulation

    over 13 annual cycles

    Barnabeiet al, arXiv:1002.1028

    (Something is modulated, but what?)

    H. Araújo


    Anais scintillation
    ANAIS: Scintillation

    CANFRANC

    Target: aiming for 250 kg NaI(Tl)

    With 500 kg.years data, the DAMA result could be reproduced if threshold ~2 keVee and background <2 evt/kg/day/keV

    ANAIS STATUS

    XXXIX IMFP CANFRANC 10-FEB-2011

    Carlos 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

    H. Araújo


    Edelweiss ii ionisation phonons

    S. Henry tomorrow

    EDELWEISS-IIIonisation & Phonons

    MODANE

    Target: 4 kg Ge

    384 kg·days from 14 months of operation 5 candidate events above 20 keV

    estimated background is <3.0 events

    sSI<4.4×10−8pb (90% CL) at 85 GeV

    E. Armengaudet al, arXiv:1103:4070v2

    H. Araújo


    Cresst scintillation phonons
    CRESST: Scintillation & Phonons

    GRAN SASSO

    Target: 3 kg CaWO4

    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)

    H. Araújo


    Light wimps excesses at low energies
    Light WIMPs: ‘excesses’ at low energies

    DAMA

    CoGeNT

    CRESST

    A ‘glimmer’ or a ‘flicker’?

    Phys. Rev. 26, 71–85 (1925)

    CMSSM Buchmueller et al

    CMSSM Trotta et al

    Aalseth et al, arXiv:1002:4703v2)

    H. Araújo


    What would zeplin iii make of it
    What would ZEPLIN-III make of it?

    You cannot

    be serious!

    13 GeV WIMP AT sSI=3x10-5pb

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

    Recoil spectrum in xenon

    140 kg*days in FSR signal box

    30 events >2 keVee!

    H. Araújo


    Coupp bubble chamber
    COUPP: Bubble chamber

    SNOLAB

    Target: 3.5 kg CF3I

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

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

    Electron recoils do not nucleate bubbles

    Background from neutrons and alphas

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

    H. Araújo


    Zeplin iii scintillation ionisation

    wed pm parallel session

    ZEPLIN-III: Scintillation & Ionisation

    BOULBY

    Target: 12 kg LXe, 6.5 kg fiducial

    >280 days continuous operation

    Result from ~2,000 kg*days soon

    Sensitivity 1-2x10-8pb

    CMSSM Buchmueller et al

    CMSSM Trotta et al

    H. Araújo


    XENON100: PLR analysis

    Target: 62 kg liquid xenon, 30 kg fiducial

    E. Aprile,

    XIV Int. Workshop on Neutrino Telescopes,

    Venice, 16 Mar 2011

    H. Araújo


    XENON100: “result in weeks”

    Target: 62 kg liquid xenon, 30 kg fiducial

    E. Aprile,

    XIV Int. Workshop on Neutrino Telescopes,

    Venice, 16 Mar 2011

    “ ‘unblinding procedure’ in final stage of internal review

    ‘Blind’ analysis of ~10x

    more data near completion

    Results expected within weeks. Non negligible discovery potential”

    H. Araújo


    The future
    The future

    GEN-1

    ANAIS

    ARDM

    COGENT

    COUPP

    CRESST

    DAMA

    DARKSIDE

    DM-TPC

    DRIFT

    EDELWEISS

    KIMS

    LUX350

    MiniCLEAN

    NEWAGE

    PICASSO

    SCDMS

    WARP

    XENON100

    XMASS

    ZEPLIN-III

    GEN-2

    DARKSIDE

    DEAP-3600

    EURECA

    LZS

    PICASSO-II

    SCDMS

    XENON1t

    XMASS-II

    GEN-3

    COUPP

    GEODM

    LZ20

    MAX

    CLEAN

    H. Araújo


    Conclusions
    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

    H. Araújo


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