LUX : A Liquid Xenon Dark Matter Detector Masahiro Morii Harvard University August 24, 2010 1

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Transcript of LUX : A Liquid Xenon Dark Matter Detector Masahiro Morii Harvard University August 24, 2010 1

  • LUX: A Liquid Xenon Dark Matter DetectorMasahiro MoriiHarvard University

    August 24, 2010


    LUX Dark Matter Detector

  • New Physics Where?*From the labs Unexplained experimental dataEx: nuclear decays violate energy conservation neutrinosMuch of the 20th-century particle physics was driven by data Today: all HEP data are consistent with the Standard ModelFrom imagination Quest for consistency/symmetry/beautyEx: equivalence principle general relativityFew of the many brilliant ideas succeedToday: too many ideas to fit on this slideFrom the skyEx: matter-dominant universe CP violationParticle physics and astronomy must agree on the observable universeToday: Dark Matter and Dark Energy

  • Evidence for Dark MatterOverwhelming evidences for Dark Matter from astronomyObserved motions of stars and galaxiesRotational curves in galaxiesVelocity dispersions in galaxy clustersMass distribution measured with gravitational lensingCluster mass-light ratios from strong lensingCluster mass maps in clusters from weak lensingSmoking gun: Separation of visible and dark matters in colliding clusters*Bullet cluster

  • *Abell 520 the train wreck cluster

  • Lambda-CDM UniverseObservation has improved dramatically in the last decadeCosmic Microwave Background anisotropy (WMAP)Accelerating expansion of the Universe (Type-1a SNe)Large-Scale Structure (2dFGRS, SDSS)Light-element abundances & Big-bang nucleosynthesisData support a universe with a positive cosmological constant and Cold Dark Matter CDM model*What (we think) we knowCold Dark MatterDark EnergyFlat UniverseWMAP, Astrophys. J. Suppl. 180 (2009) 225-245

  • What We Know About DMDark Matter must be there~23% of the energy of the UniverseDark Matter is not part of the Standard ModelNon-baryonicNo electromagnetic interactionsStableCold: non-relativistic in early Universe to create the LSSNear the Solar SystemLocal density: ~0.3 GeV/cm3 Give or take a factor of 2Velocity: assumed to be Maxwell-Boltzmann with v ~ 230 km/sSimplest model, but not necessarily true*

  • What is DM Made Of?NeutrinoSM neutrinos are too lightHeavy sterile neutrino?WIMP~100 GeV mass particles with Electroweak interactionsAxionCold even with small massesSuperWIMPWeaker-than-EW interactionsAxino, gravitino, KK gravitonsMany many others ...*Roszkowski, hep-ph/0404052

  • WIMP CandidatesMany theories beyond the SM predict new particles at O(100 GeV)A result of attempting to solve the hierarchy problemA conserved quantum number makes the WIMP stable

    This ensures that WIMPS are created/destroyed in pairsCanonical example is the lightest neutralino Admixture of gauginos and higgsinosIf they existed, how many WIMPs would be out there?*

    TheorySymmetryWIMPSupersymmetryR parityLightest Supersymmetric ParticleLittle HiggsT parityLightest T-odd ParticleUniversal Extra DimensionKK parityLightest Kaluza-Klein Particle

  • WIMP Relic DensityWIMPs were in thermal equilibrium with Standard Model particles after inflationAs the Universe cools, their density drops as

    This goes on until the WIMP annihilation rate becomes smaller than the expansion rate, i.e.,

    The WIMPs freeze out with the relic densitym = 100 GeV and g = gEW observed Dark Matter densityFormula above depends on gravity (mPl) and Hubble constant (H0) No relation to electroweak physics*

  • WIMP HuntingGoing beyond gravity, three ways to detect WIMPs*

  • WIMP HuntingHadron colliders produce WIMPs through decays of new particlese.g., gluino production followed by a long cascade ending in a neutralino Production rate depends on details of the particle spectrumIf a WIMP is found, its hard to identify it with the cosmic Dark MatterDoes it live cosmologically long? What is its coupling?Indirect searches rely on WIMP pair annihilation in the regions of high WIMP densitiese.g., Galactic center, Solar core Annihilation rate depends on the local density profileBackgrounds are poorly understoodAre there local sources of energetic radiations, e.g., pulsars?Direct searches depend on the local WIMP density and velocityAssumed to be 0.3 GeV/cm3, Maxwell distribution with *

  • mSUGRA ExampleCDM, g 2, and b s constraints allow 4 odd-shaped regionsBulk regionSmall m0 and m1/2 Light sparticlesWIMPs annihilate via sfermion exchange

    Critically EndangeredGood chance of discovery at the LHCStau co-annihilation regionA-funnel regionFocus point region*Baer & Balazs, JCAP 0305, 006 (2003)

  • mSUGRA ExampleCDM, g 2, and b s constraints allow 4 odd-shaped regionsBulk regionStau co-annihilation regionWIMP nearly degenerate with stauCo-annihilation with stau

    Difficult to see experimentallyA-funnel regionFocus point region

    *Baer & Balazs, JCAP 0305, 006 (2003)

  • mSUGRA ExampleCDM, g 2, and b s constraints allow 4 odd-shaped regionsBulk regionStau co-annihilation regionA-funnel regionWIMP mass 1/2 of pseudoscalar Higgs AResonant annihilation via A

    Appears only for tan = 5458Good for indirect detectionFocus point region

    *Baer & Balazs, JCAP 0305, 006 (2003)

  • mSUGRA ExampleCDM, g 2, and b s constraints allow 4 odd-shaped regionsBulk regionStau co-annihilation regionA-funnel regionFocus point (hyperbolic branch) regionlarge m0 Heavy sfermionsSatisfies flavor/CP constraints without fine tuningWIMP higgsino Large coupling to W/ZWIMPs annihilate into W/Z

    Favored by data in wide range of tan*Baer & Balazs, JCAP 0305, 006 (2003)

  • Direct Detection Limits*Expt. limits on the WIMP-nucleon cross section are approaching 10-44 cm2XENON10 PRL100:021303 + PRC79:045807CDMS II arXiv:0912.3592 XENON100 arXiv:1005.0380Models in the focus point SUSY region predicts N ~ 10-44 cm2Upcoming experiments are aiming for N < 10-45 cm2= 1 zeptobarn

  • Experimental ChallengesSignal is elastic N N with small recoil energy EN ~10 keVSignatures: scintillation, ionization, heat (phonons)Background sources: rays from radioactivityRecoils electrons Different signatureNeutrons generated by cosmic raysRecoils nuclei Same signatureTo combat the backgroundsCombine multiple signatures to reject electron recoils rays and neutrons may scatter twice; WIMP will notScreen everything for radiopurityGo deep underground*DMnucleus~10 keV~230 km/selectron~10 keVfew MeV

  • Detector MaterialsWIMP-nucleus cross section A2WIMP scatters off A nucleons coherently because wavelength nuclear radiusXe beats Ge beats ArCaveat: recoil energy spectrum is softer Need lower energy thresholdDifferent strengths in background rejectionRejection of electron-recoil eventsGe (ionization vs. phonon)Ar (ionization vs. scintillation + scintillation pulse shape)Xe (ionization vs. scintillation)Liquid (Ar, Xe) can be continuously purified Solid (Ge) cantAr contains 1 Bq/kg of radioactive 39Ar*

  • Liquid XenonXe (A = 131.3) gives a high signal cross section A2100 kg-year exposure can probe 10-45 cm2Attractive liquid Xe propertiesHigh density: 3 g/cm3 Compact detectorBoiling point: 165 K is warmer than liquid N2 (77 K) Simpler cryogenicsLiquid Ar is 87 K. Ge (CDMS) is 10 mKGood scintillator: 42 photons/keV at 175 nmPMTs have good (~30%) quantum efficiency at this wavelengthAr scintillates at 128 nm Need wavelength shifterHigh ionization yield: 64 electron-ion pairs/keVShort radiation length: 2.77 cm Self shieldingBackground rays and neutrons cannot reach the fiducial volumeCost: $1000/kg*

  • Two-Phase Xe DetectorPMTs collect prompt (S1) and proportional (S2) lightS1-S2 delay Drift lengthS2 light pattern Horizontal locationS2/S1 ratio differs markedly between electron and nuclear recoilsNuclear recoils have higher ionization density higher recombination probability higher S1 yield>98.5% rejection of EM backgrounds*

  • S2/S1 Ratio in XENON10XENON10 calibration data show clean discrimination between electron and nuclear recoilslog(S2/S1) normalized to the electron recoil data is used to select the signal candidates*Phys. Rev. Lett. 100, 021303 (2008)

  • 5.4 kg 100 kg Liquid XenonXENON10 sets the benchmark for liquid-Xe detectors

    XENON100, LUX, and XMASS compete with different optimizationXENON100 is smaller Self shielding is less effectiveRely on good S2/S1 rejection against electron-recoil backgroundsPreliminary result used 40 kg x 11.2 daysXMASS is one-phase S1 light onlySpherical detector surrounded by PMTs in 4Rely on self shielding Large total massLUX is in between*

    XENON10XENON100LUXXMASSTotal Xe22 kg170 kg350 kg800 kgFiducial5.4 kg65 kg100 kg100 kgLive time58.6 days100 days100 days1 yearLocationGran SassoGran SassoSanfordKamioka

  • LUX Detector OptimizationLUX uses large (350 kg) total mass to take advantage of the self shielding of liquid XeElectron-recoil background in the 100 kg fiducial volume is 2030 events in 100 days before the S2/S1 cutModest (>98.5%) S2/S1 rejection will bring the background to
  • LUX Detector*

  • Internal Backgrounds-ray background must be moderately lowSuppressed by Xe self shielding and S1/S2 ratioPMTs are the dominant sourceScreened
  • Internal BackgroundsNeutron backgrounds come from fission and (,n) reactionsPMTs are the dominant sourceExpect 1.5 neutrons/yr/tubeSimulation (right) assumes 5 n/yr/tubeExpect
  • External BackgroundsDetector is suspended inside a 183 m3 purified water tankEfficient shield against fast neutrons generated in the rocks by cosmic raysAlso for radioactivity in the rocks

    Subdominant to internal back- grounds*

  • Energy ThresholdEnergy threshold is determined by the S1 light yieldDifferent for electron vs. nuclear recoilXENON10 performanceS1 light yield = 1 phe/keVAnalysis cut = 4.5 keVPMT was R8520 (1 sq.