Henrique Araújo Imperial College London

HEP Seminar, Oxford University, 14 May 2013

OUTLINE

1. Why Dark Matters

2. The Noble Liquid Xenon

3. ZEPLIN-III at Boulby

4. LUX at SURF

5. LUX-ZEPLIN (LZ)

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WHY DARK MATTER(S)

COSMOLOGY

ASTROPHYSICS

PARTICLE PHYSICS To study these particles, we need to find them first: • Direct searches (scattering) • Indirect searches (annihilation, decay) • Accelerator searches (production)

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Via Lactea VL-1 simulation (2006)

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RECENT PALPITATIONS

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Cholis et al, JCAP12(2009)007

Phys. Rev. Lett. 110 (2013) 141102

AMS-02 CDMS-II

Agnese et al, arXiv:1304.4279

PLANCK

DARK MATTER PARTICLE ZOO

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There are many other candidates…

GALACTIC WIMP SIGNAL MODEL

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• WIMPs should scatter elastically off ordinary nuclei, producing measurable nuclear recoils

• Most backgrounds create electron recoils • Scalar (SI) and axial-vector (SD) χ-N interactions • Coherence-enhanced SI scattering rate

• Canonical halo model: isothermal sphere, no lumps with truncated Maxwellian velocity distribution

• Local density ρ0 ~ 0.3 GeV/cm3 (~1/pint at 100 GeV)

• v0=220 km/s, vesc=544 km/s, vE=232 km/s

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NUCLEAR RECOIL SPECTRUM

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• WIMP searches are hard: rare & low energy is bad combination

• Coherent neutrino-nucleus elastic scattering (CNS) also features in this story (SM-calculable process, yet to be observed)

Chepel & HA, JINST 8 R04401 (2013)

Heat & Ionisation Bolometers Targets: Ge,Si

CDMS, EDELWEISS, EURECA cryogenic (<50 mK)

Light & Heat Bolometers Targets: CaWO4, BGO, Al2O3

CRESST, ROSEBUD cryogenic (<50 mK)

Light & Ionisation Detectors

Targets: Xe, Ar ArDM, DarkSide, LUX, LZ, WARP, XENON, ZEPLIN

cold (LN2)

ionisation Q

WIMP SEARCH TECHNOLOGY ZOO

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Scintillators Targets: NaI, Xe, Ar

ANAIS, MiniCLEAN, DAMA, DEAP, KIMS, LIBRA,

NAIAD, XMASS, ZEPLIN-I

Ionisation Detectors Targets: Ge, Si, CS2, CdTe CoGeNT, DRIFT, DM-TPC

GENIUS, HDMS, IGEX, NEWAGE

Bolometers Targets: Ge, Si, Al2O3, TeO2

CRESST-I, CUORE, CUORICINO

Bubbles & Droplets CF3Br, CF3I, C3F8, C4F10

COUPP, PICASSO, SIMPLE

TWO-PHASE XENON DETECTORS

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S1: LXe is an excellent scintillator – Density: 3 g/cm3 – Light yield: >60 ph/keV (0 field) – Scintillation light: 178 nm (VUV) – Nuclear recoil threshold ∼5-10 keV

S2: Even better ionisation detector – Electroluminescence in vapour phase – S1+S2 allows mm vertex reconstruction – Sensitive to single ionisation electrons – Nuclear recoil threshold ∼1 keV

And a great WIMP target too – Scalar WIMP-nucleon scattering rate dR/dE∼A2

– Odd-neutron isotopes (129Xe, 131Xe) enable SD sensitivity; target exchange – Excellent ionisation threshold: ‘light WIMP’ searches using S2 only – No intrinsic backgrounds (85Kr, 222Rn removable, 136Xe 2νββ ok at low energy)

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• Direct and recombination luminescence via excimer state • Scintillation is quenched for Xe recoils relative to electronic recoils • LET picture fails for low energy electrons & recoils (tracks not cylindrical) • Hybrid model with Thomas-Imel box recombination (NEST sim model)

PROMPT SCINTILLATION (S1)

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Chepel & HA, JINST 8 (2013) R04401

*2Xe

electrons

IONISATION/E’LUM (S2)

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• E-field assisted track extraction, drift, emission, electroluminescence • Single electron sensitivity (∼30 phe/e) • NR ionisation yield improves with decreasing energy

Horn et al., PLB 705 (2011) 471

Santos et al, JHEP 12 (2011) 115

S1 S2

SE

S1+S2: DISCRIMINATION & POSITION

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electron recoils

nuclear recoils

• Discrimination drives down dominant backgrounds – Electron recoils from β & γ radioactivity

• 3D vertex reconstruction to exploit self-shielding – External γ-rays and neutrons from components/environment – Force (and resolve) multiple scattering in active volume

ZEPLIN-III

SELF-SHIELDING IN NOBLE LIQUIDS

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Liquid xenon

ρ=3 g/cm3

Sacrificial volume

Fiducial volume

LUX350 → LUX-ZEPLIN 1.5t

Neutrons (5-25 keV)

Gammas (5-25 keV)

S1 S2

S2 S2

S2

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

veto

make thin!

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Liquid Xenon

LUX350 → LUX-ZEPLIN 1.5t

Neutrons (5-25 keV)

Gammas (5-25 keV)

SELF-SHIELDING IN NOBLE LIQUIDS

ZEPLIN → LUX → LUX-ZEPLIN (LZ) • UK-led ZEPLIN programme at Boulby completed (2001-2011)

– Pioneered xenon technology, world class results from 3 experiments

• MOU between ZEPLIN-III and LUX groups signed in 2008

• LUX tested on the surface at Sanford Lab (Homestake), now including UK • Underground operation under way, first physics in 2013

• LZ project following ‘G2 down-selection’ process in the US (and similar in UK) – 1 of 4 WIMP experiments selected by DOE to progress to R&D/Design – Coordinated proposals to DOE/NSF/STFC in late 2013 – Construction planned to start mid 2014, commissioning from 2016/17

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Boulby Underground Laboratory, UK (1,100 m)

Henrique Araújo Imperial College London

On behalf of the ZEPLIN-III Collaboration:

Edinburgh University (UK) Imperial College London (UK)

ITEP-Moscow (Russia) LIP-Coimbra (Portugal)

Rutherford Appleton Laboratory (UK)

DARK MATTER SILVER JUBILEE 2012 Pacific Northwest National Laboratory

June 19-21, 2012

ZEPLIN-III XENON EMISSION DETECTOR • Time projection chamber with 12 kg of active liquid xenon • Readout of scintillation (S1) and ionisation (S2) with array of 31 PMTs • Strong electric field (∼4 kV/cm), planar design, no extraction grids • Construction from low background, xenon-friendly materials

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BOULBY UNDERGROUND LABORATORY

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BOULBY SCIENCE RUNS • First science run (FSR) at Boulby: 83 days in 2008

Strong constraints on WIMP-nucleon scattering XS

• Phase-II upgrades commissioned in 2009/10 - New photomultiplier array (ultra-low background) - New anti-coincidence veto (bk reduction, diagnostic) - New calibration hardware (reduction of systematics) - System automation (improved stability, underground effort)

• Second science run (SSR) 23 June 2010 to 7 May 2011 Longest run of any noble liquid WIMP detector (319 days) Effective fiducial exposure ~560 kg*days (4x FSR)

One month’s worth of daily LN2 refills

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UPGRADES: NEW PMT ARRAY AND VETO PMT γ-rays limited sensitivity of first run by a large factor New PMT model developed with manufacturers (ETEL) 20-fold reduction in γ-ray activity, but poor optical performance

52-module neutron veto installed around WIMP target Gd-loaded polypropylene surrounded by 1t of plastic scintillator 60% neutron efficiency, diagnostic tool

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Ghag et al. (2011), Astropart. Phys. 35: 76 Akimov et al. (2010), Astropart. Phys. 34: 151

Assembly of bespoke low-background PMTs ZEPLIN-III Veto

SSR BACKGROUNDS • Good (absolute) agreement with Monte Carlo

predictions from (sub-)component radio-assays • Gamma background reduced 18x (<1 dru) (expectation <1 evt assuming FSR discrimination) • Neutron background reduced 30x (expectation <1 evt after veto & efficiencies)

Electron and nuclear recoil background rates (5-50 keVnr in 6.5 kg fiducial, unity signal acceptance, before veto)

energy

radius

depth

Araujo et al., Astropart. Phys. 37: 495 (2012)

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MERCURY VERTEX RECONSTRUCTION • Reconstruction of (x, y, E) separately for the two responses (z from drift time)

S2: WLS fit

S1: ML fit

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Solovov et al., arXiv:1112.1481

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ITERATION 0 ITERATION 1 ITERATION 5

ηi(r) – Light Response Functions for each PMT (found iteratively)

σx,y =1.6 mm (FWHM)

SSR WIMP SEARCH RESULTS • Veto prompt tags (in green) – mostly gamma-rays (28% of electron recoils) • Lower γ-ray background than FSR, but poorer discrimination (1:280 v 1:7800) • Veto delayed tags (in red) consistent with predicted 0.7% accidentals rate • Negligible neutron background confirmed (e.g. no tags below NR median) • 8 candidate events in acceptance region (7-29 keVr, 2-45% acceptance)

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FSR SSR

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WIMP-NUCLEON ELASTIC XS LIMITS

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SPIN-INDEPENDENT SPIN-DEPENDENT WIMP-NEUTRON

Akimov et al., Phys Lett B 709: 14 (2012)

D.Akimov et al., WIMP-nucleon cross-section results from the second science run of ZEPLIN-III, Phys. Lett. B 709: 14 (2012) E.Santos et al., Single electron emission in two-phase xenon […] coherent neutrino-nucleus scattering, JHEP12 (2011) 115 V.Solovov et al., Position reconstruction in a dual phase xenon scintillation detector, Subm. IEEE Trans. Nuc. Sci. (2011); arXiv:1112.1481 P.Majewski et al., Performance data from the ZEPLIN-III second science run, Submitted to JINST (2011); arXiv.1112.0080 L. Reichhart et al., Quenching factor for low energy nuclear recoils in a plastic scintillator, Submitted to Phys. Rev. C (2011) M.Horn et al., Nuclear recoil scintillation and ionisation yields in liquid xenon from ZEPLIN-III data, Phys. Lett. B 705: 471 (2011) F.Neves et al., ZE3RA: the ZEPLIN-III reduction and analysis package, JINST 6, P11004 (2011) H.M.Araújo et al., Radioactivity Backgrounds in ZEPLIN-III, Astropart. Phys. 35(7): 495 (2012) C.Ghag et al., Performance of the veto detector incorporated into the ZEPLIN-III experiment, Astropart. Phys. 35(2): 76 (2011) D.Akimov et al., The ZEPLIN-III Anti-Coincidence Veto Detector, Astropart, Phys. 34: 151 (2010) D.Akimov et al., Limits on Inelastic Dark Matter with ZEPLIN-III, Phys. Lett. B 692: 180 (2010) F. Neves et al., Calibration of Photomultiplier Arrays. Astropart. Phys. 33: 13 (2009) V.N.Lebedenko et al., Limits on the spin-dependent WIMP-nucleon cross-section from […] ZEPLIN-III, Phys. Rev. Lett. 103: 151302 (2009) V.N.Lebedenko et al., Results from the First Science Run of the ZEPLIN-II Dark Matter Search Experiment, Phys. Rev. D 80: 052010 (2009) H.M.Araújo, Performance results from the first science run of ZEPLIN-III,. NIM A 604: 41 (2009) A. Lindote, et al., Simulation of neutrons produced by high energy muons underground, Astropart. Phys. 31: 366 (2009) H.M.Araújo, et al., Measurements of neutrons produced by high energy muons at the Boulby Underground Laboratory, Astropart. Phys. 29: 471 (2008) B.Edwards et al. Measurement of single electron emission in two-phase xenon, Astropart. Phys. 30: 54 (2008) D.Yu Akimov et al., The ZEPLIN III dark matter detector: instrument design, manufacture and commissioning, Astropart. Phys. 27: 46 (2007) G.J.Alner et al., First limits on WIMP nuclear recoil signals in ZEPLIN-II, Astropart.Phys. 28(3): 287(2007) G.J.Alner et al., Limits on spin-dependent WIMP-nucleon cross-section from the ZEPLIN-II data, Phys. Lett. B 653: 161(2007) A. Lindote et al., Preliminary results on position reconstruction for ZEPLIN-III, NIM A 573: 200 (2007) H.M.Araújo et al., The ZEPLIN III dark matter detector: performance study using end-to-end simulation tool, Astropart. Phys. 26: 140 (2006) G.J.Alner et al., First limits on nuclear recoil events from the ZEPLIN-I dark matter detector, Astroparticle Phys. 23: 444 (2005) G.J.Alner et al., Nuclear recoil limits from the ZEPLIN-I liquid xenon WIMP dark matter detector, New Astron. Rev. 49: 445 (2005) G.J.Alner et al., ZEPLIN-I: First limits on nuclear recoil rate, In “Proc. 5th Int. Workshop. Id. Dark Matter", World Scientific, p.218 (2005) J. V. Dawson et al., A study of the scintillation induced by alpha particles and gamma rays in liquid xenon, NIM A 545: 690 (2005) T.J.Sumner, The ZEPLIN-III dark matter project, New Astronomy Reviews 49: 277-281 (2005) H.M.Araújo, et al., Muon-induced neutron production and detection with GEANT4 and FLUKA,. NIM A 545: 398 (2005) M.Carson et al., Veto performance for large-scale xenon dark matter detectors, NIM A 548: 418 (2005) C.Bungau et al., Monte Carlo studies of shielding and veto techniques for neutron background reduction […],. Astroparticle Phys. 23(1): 97-115 (2005) P.F. Smith, et al., Simulation studies of neutron shielding, calibration and veto system for gaseous dark matter detectors, Astropart. Phys. 22: 409 (2005) H.M.Araújo et al., Low Temperature Study of 35 Photomultiplier Tubes for the ZEPLIN III Experiment, NIM A 521: 407 (2004) M.Robinson et al., Measurements of the muon flux at 1070 m vertical depth in the Boulby underground laboratory, NIM A 511: 347 (2001) T.J.Sumner, ZEPLIN-III: a two phase xenon dark matter detector, In “Proc. 3rd Int. Workshop. Id. Dark Matter”, World Scientific, p.452 (2001) D. Akimov et al., Scintillation two-phase xenon detector […] for dark matter searches, Proc 3rd Int . Symp. Sources and Detection of Dark Matter, p461 (1998) D.Cline, On a discriminating liquid-xenon detector for SUSY dark-matter observation, Nuclear Physics B (Proc. Suppl.) 51B:304 (1996) G.J.Davies et al., Liquid xenon as a dark matter detector: prospects for nuclear recoil discrimination by photon timing, Phys. Lett. B 320: 395 (1994) B.A.Dolgoshein, V.N.Lebedenko & B.U. Rodionov, New method of registration of ionising-particle tracks in condensed matter, JETP Lett. 11:351 (1970)

THE END OF ZEPLIN AT BOULBY

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Vadim Nikolaevitch Lebedenko 11/10/1939 – 11/05/2008

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NEAR-JUBILEE OF DM SEARCHES IN THE UK • UK Dark Matter Collaboration started by P. F. Smith (RAL) in 1987 • Exploring cryogenic bolometers, sodium iodide, gaseous TPC, liquid xenon • 3 decades in sensitivity achieved with only ∼10-fold increase in target mass

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SI UK results

LARGE UNDERGROUND XENON EXPERIMENT SEARCHING FOR DARK MATTER AT THE SANFORD UNDERGROUND LABORATORY

Brown University University of California, Berkeley University of California, Davis University of California, Santa Barbara Case Western Reserve University Edinburgh University Imperial College London Lawrence Berkeley National Laboratory Lawrence Livermore National Laboratory LIP-Coimbra, Portugal University of Maryland University of Rochester South Dakota School of Mines & Technology University of South Dakota Texas A&M University University College London Yale University

= LUX + University of Alabama Daresbury Laboratory Rutherford Appleton Laboratory SLAC National Accelerator Laboratory University of Wisconsin LZ

COLLABORATING INSTITUTES (US-UK-PT)

ZEPLIN-III Edinburgh University Imperial College London LIP-Coimbra Rutherford Appleton Laboratory ITEP-Moscow

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SANFORD UNDERGROUND LAB @HOMESTAKE

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THE LARGE UNDERGROUND XENON EXPERIMENT AT THE SANFORD UNDERGROUND LABORATORY

B. Edwards, Yale

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ENGINEERING RUN IN SURFACE LAB

• Operation in purpose-build water tank • Demonstrated performance of most sub-systems • Excellent light yield: 2.5x that of XENON100 • 3D reconstruction: a few mm resolution (x,y)

Akerib et al, NIM A 704: 111 (2013) Akerib et al, arXiv: 1210.4569 (2013)

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8.4 phe/keV (Cs-137, 0 field)

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The 8-metre diameter LUX water tank, Davis Campus, Sanford Underground Laboratory

Image: Carlos Faham, Brown University

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LUX UNDERGROUND AT SURF

• Surface run in 2012 • Successful move u/g • Water tank full • Xenon gas recirculation • Gas phase data • Cooldown/condensation • Final commissioning • Xenon purification • Calibration • Short run (∼60 days) • Long run (∼300 days)

H. Araujo Image: Matt Kapust, Sanford Underground Research Facility

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D. Malling, Brown

SIMULATED ER BACKGROUND • ∼1 mDRU expected at low energies • PMTs dominant contribution • Xe cosmogenic activation will die away • Krypton removal to ppt is essential

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KRYPTON REMOVAL C. Lee, CWRU

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STABILITY & XENON CIRCULATION

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INTERNAL CALIBRATION – 83mKr • How do you calibrate a detector which self-shields so effectively?

• Short-lived dispersed sources e.g. 83mKr, 37Ar, …

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J. Ch

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LOW ENERGY CALIBRATION EVENT

1.5 keV electron

interaction

LUX-ZEPLIN (LZ) Next-generation LXe experiment building on LUX and ZEPLIN programmes • Experimental approach: a progressive and low-risk programme

– UK-led ZEPLIN programme pioneered two-phase xenon for WIMP searches – LUX (now with UK) about to turn on – leading sensitivity in 2013 – LZ is another ∼20x mass scale-up factor to reach 10-12 pb sensitivity

• An aggressive science reach – Cover the parameter space open to direct searches – faster – LXe provides exciting physics for light & heavy WIMPs (GeV-TeV)

• Since we do not yet know what BSM physics looks like!

• UK contribution – Titanium cryostat; Xe detector systems; Screening capability; Management

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6 kg LXe (fid) >100 kg >5,000 kg

ZEPLIN-III LUX350 LZ

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SANFORD LAB, DAVIS COMPLEX (4850-ft) LZ INFRASTRUCTURE

49 cm 150 cm

LZ LUX350

• 7 tonne LXe TPC (∼9t total LXe) • Only 20-fold scale-up from LUX • But >100x increase in sensitivity • Layered veto strategy

- Instrumented LXe Skin detector - Liquid scintillator Veto detector - Instrumented water shield (same as LUX)

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IMPORTANT BACKGROUNDS

• Internal radioactivity neutrons and gamma-rays – Die out very quickly into xenon target, leaving ∼6-tonne fiducial – Layered detector strategy together with self-shielding and

accurate 3D position reconstruction are extremely effective 41 H. Araujo

PMTs + Cryostat

PMTs + Cryostat

INTRINSIC BACKGROUNDS

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• Intrinsic electron backgrounds • Controlled with modest discrimination (99.5%) • 85Kr: require <0.02 ppt Kr (best LUX production batch ∼0.2 ppt) • 214Pb: require <0.6 mBq radon in active volume (cf. ∼µBq in Borexino, SNO) • 2νββ from 136Xe dominates only >20 keVee (signal acceptance <10 keVee)

C. Lee, CWRU

DOMINANT BACKGROUNDS

• Solar pp ν-e elastic scattering is dominant e-recoil background – Requires 99.5% discrimination (as in X10; Z3 reached 99.99% at high field)

• CNS is dominant nuclear recoil background – From 8B solar neutrinos: significant number of events, but ∼0 above 5 keVr – Small background from Atmospheric and DSNB (<0.5 evts in ROI)

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OUTLOOK

• ZEPLIN-III, CDMS-II, EDELWEISS-II Technology development Competitive experiments at 10 kg scale

• XENON100 – present world’s best 62 kg active two-phase xenon

• LUX 300 kg fully active LXe, 370 kg total Operations underway, through to 2015

• LZ 7 tonnes fully active, 9 tonnes total Start 2017 To boldly go where no-one’s been… (and faster)

Here lie many other experiments (and theories) going back 25 years!

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ONE DAY, DARK MATTER WILL BE BORING… – Direct, indirect and accelerator searches are all closing in – LUX should be next world leading direct search (from 2013) – LZ will reach down to the neutrino window at 10-12 pb – It’s still very far from boring…

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Stephen Collins, The Guardian, Saturday 27 April 2013

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THANK YOU