Dark Matter Detection with Liquid Xenon Masahiro Morii Harvard University Laboratory for Particle Physics and Cosmology 21 August

Dark Matter Existence of Dark Matter is well established from 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/cm 3 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 problem as much as a cosmology problem 21 August 2009Dark Matter2

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 have the right thermal relic density Predicted in many BSM theories (e.g. the LSP) Since the annihilation cross section is constrained by the relic density, we can predict: 21 August 2009Dark Matter3 Direct detection Production at colliders

Direct WIMP Detection Best limits on the WIMP-nucleon cross section are ~5x cm 2 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 cm 2 Smaller cross sections possible, but increasingly difficult to reconcile with the flavor problem Next generation of experiments aim for < cm 2 21 August 2009Dark Matter4

Liquid Xenon WIMP-nucleus cross section ∝ A 2 Xe (A = 131.3) gives high signal rate 100 kg-year exposure can probe σ(WIMP-p) < cm 2 Key liquid Xe properties High density: 3 g/cm 3 High boiling point: 165K Good scintillator: 42 photons/keV λ = 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 85 Kr must be removed by charcoal chromatography 21 August 2009Dark Matter5

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 >98.5% rejection of EM backgrounds Good scaling to larger masses 1 m 3 holds 3 tonnes Instrumentation ∝ (mass) 2/3 Backgrounds improve with size due to self shielding 21 August 2009Dark Matter6

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 been improved to achieve <1 bkgd. in 100 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 m 3 purified water tank shields the detector from neutrons Recoil energy threshold <5 keV  σ(WIMP-p) = 5x cm 2 21 August 2009Dark Matter7

LUX Collaboration Brown, Case Western, LBNL, Harvard, LLNL, Maryland, Texas A&M, Rochester, South Dakota, Yale Funded by DOE & NSF 21 August 2009Dark Matter8

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 1–2 graduate students Will take part in detector integration, commissioning Develop analysis software framework 21 August 2009Dark Matter9 PMT preamp postamp FADCDigital TriggerAnalog Trigger

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 >1 m electron drift achieved in 3 days Assembly of LUX in Sanford surface building 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! 21 August 2009Dark Matter10

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 σ(WIMP-p) = 2x cm 2 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 γ-ray background with liquid scintillator Harvard will assume larger responsibilities Development of low radioactivity, high-QE PMT Complete analog electronics chain (pre + postamp) MRI-R 2 proposal submitted this month 3-year construction  Data taking in August 2009Dark Matter11

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 σ(WIMP-p) = 5x cm 2 covers the SUSY-favored region Dark Matter search run will start in 2010 LZ will push the sensitivity to 2x cm 2 by August 2009Dark Matter12