Dark Matter Detection with Liquid Xenon

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Dark Matter Detectionwith Liquid Xenon

• Masahiro Morii
• Harvard University
• Laboratory for Particle Physics and Cosmology

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Dark Matter

• Existence of Dark Matter is well establishedfrom
  its gravitational effects
• Coma cluster Zwicky, Galaxy rotation curve
  Rubin
• Weak gravitational lensing, Bullet cluster
• Amount of Dark Matter is inferred from
  cosmological data
• 22 of the energy of the Universe
• Local density 0.3 GeV/cm3
• Identity of Dark Matter is unknown
• Majority must be cold and non-baryonic
• i.e. made of particles that are not a part of the
  SM
• Dark Matter is a particle physics problemas much
  as a cosmology problem

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WIMP Dark Matter

• No shortage of candidates, but
• WIMPs are the front runners
• 100 GeV new particles with weak(and
  gravitational) interactions
• Such a particle would naturally havethe right
  thermal relic density
• Predicted in many BSM theories(e.g. the LSP)
• Since the annihilation cross section
  is constrained by therelic density, we can
  predict

Direct detection
Production at colliders
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Direct WIMP Detection

• Best limits on the WIMP-nucleoncross section are
  5x10-44 cm2
• CDMS Ge and Si crystals at 10 mK, 121 kg-day
  exposure
• XENON10 liquid Xe, 136 kg-day
• For LSPs, the interesting region is around 10-44
  cm2
• Smaller cross sections possible, but increasingly
  difficult to reconcile with the flavor problem
• Next generation of experiments aim for lt10-45 cm2

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Liquid Xenon

• WIMP-nucleus cross section ? A2
• Xe (A 131.3) gives high signal rate
• 100 kg-year exposure can probes(WIMP-p) lt 10-45
  cm2
• Key liquid Xe properties
• High density 3 g/cm3
• High boiling point 165K
• Good scintillator 42 photons/keV
• ? 175 nm easy to detect with PMTs
• High ionization yield W 15.6 eV
• High electron mobility, low diffusion
• No long-lived radioactive isotopes besides
  double-beta decays
• 85Kr must be removed by charcoal chromatography

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Two-Phase Xe Detector

• PMTs collect prompt (S1) and proportional (S2)
  light signals
• S1-S2 delay ? Drift length
• S2 light pattern ? Horizontal location
• S2/S1 ratio differs markedly between electron and
  nuclear recoil
• gt98.5 rejection of EM backgrounds
• Good scaling to larger masses
• 1 m3 holds 3 tonnes
• Instrumentation ? (mass)2/3
• Backgrounds improve with size due to self
  shielding

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LUX Experiment

• LUX is a 350 kg (100 kg fiducial) liquid Xe
  experiment
• Located in the Davis cavern, Sanford Underground
  Lab in Homestake, SD
• XENON10 technology has beenimproved to achieve
  lt1 bkgd. in100 kg-year
• Xe purification system has 300 kg/day throughput
  using a heat exchanger
• Ultra-low activity Ti vacuum vessel replaces SS
  Cu
• PMTs have low activity (9/3 mBq of U/Th per tube)
  and high QE (27)
• 183 m3 purified water tank shields the detector
  from neutrons
• Recoil energy threshold lt5 keV ? s(WIMP-p)
  5x10-46 cm2

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LUX Collaboration

• Brown, Case Western, LBNL, Harvard, LLNL,
  Maryland, Texas AM, Rochester, South Dakota,
  Yale
• Funded by DOE NSF

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Harvard Group

• Harvard joined LUX in June 2009
• Morii (50) is the PI
• Took up a critical-path item post-amplifier
• 120 channels of receiver-amplifier-shaper for the
  PMT signals
• Full system is needed in November
• Harvard took over production from UC Davis
• Oliver and Morii improved the LLNL design
• New LPPC engineer, Meghna Kundoor, working on
  testing
• Components in hand. PC boards in fabrication
• On track for November delivery
• Recruiting a postdoc and 12 graduate students
• Will take part in detector integration,
  commissioning
• Develop analysis software framework

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LUX Status and Schedule

• Prototype LUX0.1 is operating at Case
• 1 liter of liquid Xe viewed by 4 PMTs
• Test cryogenics and Xe purification system
• gt1 m electron drift achieved in 3 days
• Assembly of LUX in Sanford surfacebuilding will
  start in November
• All major components are in hand
• Building is being fitted out

• Fully-assembled LUX lowered to Davis cavern
  (4,850 ft) in Spring 2010
• Dark Matter search will start!

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LZ Proposal

• LZ LUX scaled up to 1500 kg (1200 kg fiducial)
• Joint collaboration of LUX and ZEPLIN-III
• LUX infrastructure designed to accommodate LZ
• s(WIMP-p) 2x10-47 cm2 in 2 years
• 2000-fold improvement over current limits
• Cost of liquid Xe 1000/kg
• Maximize the fiducial/total mass ratio by
  rejecting single-scatter ?-ray background with
  liquid scintillator
• Harvard will assume larger responsibilities
• Development of low radioactivity, high-QE PMT
• Complete analog electronics chain (pre postamp)
• MRI-R2 proposal submitted this month
• 3-year construction ? Data taking in 2013

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Summary and Prospect

• Exciting time for Dark Matter detection
• Cosmology points us to compelling particle
  physics
• Liquid Xe technology has the potential for first
  observation
• Harvard is entering DM hunting with strong
  commitment
• Producing critical component for the LUX
  experiment
• PMTs and analog electronics for the proposed LZ
  experiment
• Discovery potential of LUX is excellent
• s(WIMP-p) 5x10-46 cm2 covers the SUSY-favored
  region
• Dark Matter search run will start in 2010
• LZ will push the sensitivity to 2x10-47 cm2 by
  2015