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 H. Araújo2

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)  -N interactions (neutral current exchange) typespinmass Axion0  eV – 10 meV Axino LSP1/2eV - GeV Inert Higgs Doublet050 GeV Sterile Neutrino1/2keV Neutralino LSP1/210 GeV - 10 TeV Gravitino LSP3/2eV – TeV Kaluza-Klein UED1TeV 3H. Araújo

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?) – 129 Xe (3/2 + →1/2 + ) +  (40 keV), 73 Ge (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 4H. Araújo

Neutron elastic scattering populates WIMP acceptance region – Calibration of detection efficiency with Am-Be ( ,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 → E R,max = 30 keV 5H. Araújo Xe (X100)

Elastic scattering rates Canonical model (‘we’re all in it together’) Isothermal sphere (no lumps),  ∝ r −2 Local density  0 ~ 0.3 GeV/c 2 /cm 3 ( ~ 1/pint at 100 GeV) Maxwellian (gaussian) velocity distribution Characteristic velocity v 0 =220 km/s, Local escape velocity v esc =600 km/s Earth velocity v E =232 km/s 6H. Araújo

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 A 2 in enhancement factor –cMSSM-favoured XS within reach of current detectors 2. Spin-dependent (axial-vector) interaction –note J (nuclear spin) instead of A 2 enhancement –cMSSM-favoured XS out of reach for the time being… 7H. Araújo

SI scattering rates for 1 kg targets 8H. Araújo Probably just around the corner by end 2011

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 And there is no trigger… 9H. Araújo

Backgrounds Nuclear recoils – same signature – Radioactivity neutrons: ( ,n) and SF from U/Th 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-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,…) 10H. Araújo

Heat & Ionisation Bolometers Targets: Ge,Si CDMS, 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, Ar ArDM, LUX, WARP, XENON, ZEPLIN cold (LN 2 ) H phonons ionisation Q L scintillation Discrimination 11H. 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, CUORICINO Bubbles & Droplets CF 3 Br, CF 3 I, C 3 F 8, C 4 F 10 COUPP, PICASSO, SIMPLE

Phonons ( microcalorimetry) Superconducting Transition-Edge Sensor (as in CDMS) Collect high-frequency (athermal) phonons from particle interaction Into superconducting Al contacts (threshold 2  Al ~ 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 absorber J. Cooley, CDMS Collaboration Cryogenic: T 0 ~50 mK 12H. Araújo

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: 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 cryogenic DAMA/LIBRA Collaboration 13H. Araújo

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 14H. Araújo

Self-shielding in noble liquids H. Araújo15 Liquid xenon  =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

Water cherenkov, passive LXe, bare or loaded scintillator,… veto make thin! Anticoincidence detector around WIMP target H. Araújo16 Liquid Xenon neutron gamma LUX  LUX-ZEPLIN 1.5t Neutrons (5-25 keV) Gammas (5-25 keV)

Anticoincidence detector around WIMP target H. Araújo17 Akimov et al, arXiv: Effect 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: 1.Background reduction Up to order of magnitude for gammas and neutrons 2.Diagnostic power Unexpected backgrounds Radiation environment 3.Signal-free background sample Calculation of background expectations without compromising blind analysis

Recent and future excitements 18H. Araújo

DAMA/LIBRA: Scintillation Target: 250 kg NaI(Tl) 8.9  CL modulation over 13 annual cycles Barnabei et al, arXiv: (Something is modulated, but what?) 19H. Araújo GRAN SASSO

ANAIS: Scintillation Target: aiming for 250 kg NaI(Tl) 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 20H. Araújo With 500 kg.years data, the DAMA result could be reproduced if threshold ~ 2 keVee and background <2 evt/kg/day/keV CANFRANC

EDELWEISS-II Ionisation & Phonons 384 kg·days from 14 months of operation 5 candidate events above 20 keV estimated background is <3.0 events  SI <4.4×10 −8 pb (90% CL) at 85 GeV E. Armengaud et al, arXiv:1103:4070v2 Target: 4 kg Ge 21H. Araújo MODANE  S. Henry tomorrow

CRESST: Scintillation & Phonons Target: 3 kg CaWO 4 22H. 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)

Light WIMPs: ‘excesses’ at low energies Aalseth 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

What would ZEPLIN-III make of it? GeV WIMP AT  SI =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

COUPP: Bubble chamber Target: 3.5 kg CF 3 I 25H. Araújo E. Behnke et al, PRL 106, (2011) 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) Run at shallow site (Fermilab): 3 candidate events were observed in 28.1 kg.days, consistent with neutron background. SNOLAB

ZEPLIN-III: Scintillation & Ionisation >280 days continuous operation Result from ~2,000 kg*days soon Sensitivity 1-2x10 -8 pb Target: 12 kg LXe, 6.5 kg fiducial 26H. Araújo BOULBY CMSSM Buchmueller et al CMSSM Trotta et al  wed pm parallel session

H. Araújo27 XENON100: PLR analysis Target: 62 kg liquid xenon, 30 kg fiducial E. Aprile, XIV Int. Workshop on Neutrino Telescopes, Venice, 16 Mar 2011

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 completion Results expected within weeks. Non negligible discovery potential”

The future H. Araújo29 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-3 COUPP GEODM LZ20 MAX CLEAN GEN-2 DARKSIDE DEAP-3600 EURECA LZS PICASSO-II SCDMS XENON1t XMASS-II

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. Araújo30