Direct Detection of SUSY Cold Dark Matter in Liquid Xenon
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“ The XENON Project” Direct Detection of SUSY Cold Dark Matter in Liquid Xenon One Tonne - Have we got what it takes? Columbia University: E. Aprile (PI), E. Baltz, A. Curioni, K-L. Giboni, C. Hailey, L. Hui, M. Kobayashi, P. Majewski and K.Ni Brown University: Richard Gaitskell Princeton University: Tom Shutt Rice University: Uwe Oberlack LLNL: William Craig NESS02 – September 19, 2002 Elena Aprile The XENON Project: Overview • Liquid Xenon is an excellent target material for Cold Dark Matter WIMPs, and likely the only practical one, for a sensitive experiment of the scale required by most SUSY predictions. Driven by the compelling science case and with the confidence in the LXeTPC technology which Columbia has developed for g-ray astrophysics (with NASA support), we submitted a proposal to NSF on Oct 11, 2001, for an accelerated two year research program leading to a demonstration of the XENON design concept with a 10 kg prototype. • The 1- tonne XENON experiment would be realized with an array of ten position sensitive LXeTPCs, each with 100 kg Xe target and sourrounded by several cm of LXe as active scintillator shield. Using both light and charge (amplified in gas phase) and the intrinsic imaging capability of a TPC, the goals for XENON are a low energy threshold (~16 keV) and an excellent electron/nuclear recoil discrimination (>99.5%). With an estimated total background rate of 2 x 10-5 counts/kg/keV/day XENON should reach the sensitivity of 1 event /100 kg/year or s~10-10 pb, probing the lowest SUSY parameter space. • Following a SAGENAP review on March 12, 2002, NSF has funded the 2-year program, starting on September 1, 2002. The outcome of this R&D phase will define the design for the 100 kg unit module, taking into account the low activity materials requirement. For the next phase of construction and underground operation of the XENON array, both the development /support of US National Underground Laboratory, and a strong worldwide collaboration, will be vital for the success of the XENON dark matter experiment. NESS02 – September 19, 2002 Elena Aprile Current & Next Generation Experiments & SUSY Theory Range http://dmtools.berkeley.edu Edelweiss (June 2002) ~0.25 event/kg/d ~1 event/kg/yr ~ 1 event/100 kg/yr NESS02 – September 19, 2002 Elena Aprile Typical WIMP Signal Xe Eth=16 keVr gives 1 event/kg/day • dN Ú dE Er Example cross-section shown is at current (90%) exclusion limits of existing experiments † Experimental Requirements ‡ Energy Threshold : as low as possible ‡Target Mass: as high as possible ‡ Background: as low as possible NESS02 – September 19, 2002 Elena Aprile Liquid Xenon for WIMPs Direct Detection q High mass Xe nucleus (A ~131) good for WIMPs S.I. Interaction ( s ~A2 ) q Odd Isotopes with large spin-dependent enhancement factors q High atomic number (Z=54) and density (r=3g/cc) of liquid state good for compact and flexible detector geometry q Production and purification of Xe with << 1ppb O2 in large quantities for tonne scale experiment. “Easy” cryogenics at –100 oC q Excellent ionizer and scintillator with distinct charge/light ratio for electron/nuclear energy deposits for high background discrimination NESS02 – September 19, 2002 85 Elena Aprile q No long-lived radioactive isotopes. Kr contamination reducible to ppb level … and for Solar n and 0nbb Decay 124Xe 126Xe 128Xe 129Xe 130Xe 131Xe 132Xe 134Xe 136Xe (0.10%) (0.09%) (1.92%) (26.4%) (4.07%) (21.2%) (26.9%) (10.4%) (8.87%) Mostly Odd Mostly Even Separation here bb-nucleus 136 Odd enriched Even enriched:containing Xe • Solar neutrino • 2nbb/0nbb • Dark matter Spin dependent • Dark matter Spin independent XMASS EXO LXe prototype in Kamioka LXe prototype at Stanford NESS02 – September 19, 2002 Elena Aprile Xenon Phase Diagram NESS02 – September 19, 2002 Elena Aprile Properties of LXe vs Ge and Si NESS02 – September 19, 2002 Elena Aprile Ionization and Scintillation in Liquid Xenon I/S (electron) >> I/S (non relativistic particle) Ionization (Xe+, e) Excitation (Xe*) Alpha scintillation ) Electron charge electron scintillation Recombination L/L0 or Q/Q0 (% Alpha charge 1 3 Xe2* ( Su , Su ) fi 2Xe+hn (175 nm) Electric Field (kV/cm) Fast Slow NESS02 – September 19, 2002 Elena Aprile Recombination and Attachment reduce electron signal t=1/ks[S] l= t vd = tmE e- + S Æ S - ks • ‡ High drift field • ‡ High purity gas • ‡ low-outgassing materials choice of purifiers and materials must be compatible with the low cosmogenics requirements NESS02 – September 19, 2002 Elena Aprile Spatial Resolution Technical limits field line distortions, electronic noise, and effects specific to signal readout scheme Physical Limit Electron Diffusion electron cloud Ld r td = vd = mE m = mobility vd r spread in electron cloud: s = 2Dtd E vd transverse D = diffusion coefficient longitudinal diffusion depends on drift path Ld eD 2 eD = k T = < e > T =165K Æ ª 0.3eV m 3 LXe m r r electron energy depends on E Æ D(E) s a few mm ª Ld m NESS02 – September 19, 2002 Elena Aprile Energy Resolution • Statistical limit 1/ N N = E /W - value DE F FW • “Fano Factor” limit = 2.35 = 2.35 E N E WF (liquid argon) is 2.54 DE ª 4keV @ 1MeV WF (liquid xenon) is 0.64 DE ª 2keV @ 1MeV • If all charges are collected and if full energy is absorbed in the liquid, the contributions to the energy resolution of a liquid ionization chamber are: 2 2 2 2 1/ 2 DET = 2.35[DEi + DEn + DEs + DEr ] DEi •Ionization straggling DEn •Electronic noise DEs •Positive ion effect DEr •Rise time effect NESS02 – September 19, 2002 Elena Aprile Columbia Experience with LXe Detectors q A 30 kg Liquid Xenon Time Projection Chamber developed and successfully tested at balloon altitude for Compton Imaging and Spectroscopy of MeV Cosmic Gamma- Ray Sources q NASA supported R&D on LXe detector technology and development of balloon-borne LXeGRIT payload. q Road to LXeGRIT: extensive studies of LXe ionization and scintillation properties, purification techniques to achieve long electron drift for large volume application, energy resolution and 3D NESS02 – September 19, 2002 imaging, electron mobility etc. Elena Aprile The Columbia LXeTPC 30 kg • 30 kg active Xe mass • 20 x 20 cm2 active area • 8 cm drift with 4 kV/cm • Charge and Light readout • 128 wires/anodes digitizers • 4UV PMTs NESS02 – September 19, 2002 Elena Aprile Electron vs Nuclear Recoil Discrimination in XENON Measure both direct scintillation(S1) and charge (proportional scintillation) (S2) Nuclear recoils from •WIMPs •Neutrons Gas Electron recoils from •Gammas s •Electrons μ 1 ~ anode Proportional scintillation depends on type of recoil and grid applied electric field. e- Drift Time electron recoil → S2/ S1 >> 1 Liquid E nuclear recoil → S2/ S1 ~0 40ns but detectable if E large ~ g-ray cathode NESS02 – September 19, 2002 Elena Aprile The XENON Experiment : Design Overview • The XENON design is modular. An array of 10 independent 3D position sensitive LXeTPC modules, each with a 100 kg active Xe mass, is used to make the 1-tonne scale experiment. • The fiducial LXe volume of each module is self-shielded by additional LXe. Active shield very effective for charged and neutral background rejection. • One common vessel of ~ 60 cm diameter and 60 cm height is used to house the TPC teflon and copper rings structure filled with the 100 kg Xe target and the shield LXe (~50 kg ). NESS02 – September 19, 2002 Elena Aprile XENON TPC Signals Time Structure Both Direct and Proportional Scintillation Signals detected by the same PMTs Array t~45 ns 150 µs (300 mm) • Three distinct signals associated with typical event. Amplification of primary scintillation light with CsI photocathode important for low threshold and for triggering. • Event depth of interaction (Z) from timing and XY-location from center of gravity of secondary light signals on PMTs array. • Effective background rejection direct consequence of 3D event localization (TPC) NESS02 – September 19, 2002 Elena Aprile Detection of Xe Light with a CsI Photocathode • Stable performance of reflective CsI photocathodes with high QE of 31% in LXe has been demonstrated by the Columbia measurements • CsI photocathodes can be made in any size/shape with uniform response, and are inexpensive. • LXe negative electron affinity Vo(LXe)= - 0.67 eV and the applied electric field explain the favorable electron extraction at the CsI-liquid interface. Aprile et al. NIMA 338(1994) Aprile et al. NIMA 343(1994) NESS02 – September 19, 2002 Elena Aprile XENON Baseline Readout: PMTs • Hamamatsu Low Temperature Tube (R6041) u Developed for LXe detectors. Shown to work reliably at low T and at P< 5 atm u Metal construction, compact design, recent tests at Columbia with custom designed HV divider show simultaneous light/charge with good yield u Low Background version under study by Hamamatsu u Low Quantum Efficiency~10-15% • Hamamatsu Low Background Tube (R7281) u Being tested by Xmass Collaboration • Room temperature tests only so far u Metal construction, and giving lower backgrounds • ~500 cts/tube/day (XENON baseline goal:~ 100) u Higher Quantum Efficiency~27-30% • Uses longer optics which give better focusing (could be accommdated in XENON) NESS02 – September 19, 2002 Elena Aprile Light Collection Efficiency for XENON Hamamatsu R6041 Assumptions ÿ Wph : 13 eV ÿ lph: 1.7 m ÿ Quenching Factor: 25% ÿ Q.E. of PMTs: 26% ÿ Q.E. of CsI : 31% ÿ R.E of Teflon Wall: 90% ÿ Xe Mass: 100 kg ÿ 37 PMTs (2 inch) array NESS02 – September 19, 2002 Elena Aprile Baseline - Simulation Results 16 keV recoil threshold event • Assumes 25% QE for 37 phototubes, and 31% for CsI photocathode • With a Wph= 13 eV, a 16 keV (true) nuclear recoil gives ~ 24 photoelectrons.