Bulletin of the American Physical Society
APS April Meeting 2010
Volume 55, Number 1
Saturday–Tuesday, February 13–16, 2010; Washington, DC
Session A10: Dark Matter I |
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Sponsoring Units: DPF Chair: Angela Reisetter, University of Minnesota Room: Maryland B |
Saturday, February 13, 2010 8:30AM - 8:42AM |
A10.00001: DMTPC: a direction-sensitive dark matter search James Battat, Steve Ahlen, Thomas Caldwell, Denis Dujmic, Andrei Dushkin, Peter Fisher, Shawn Henderson, Andrew Inglis, Asher Kaboth, G. Kohse, Richard Lanza, Jeremy Lopez, Jocelyn Monroe, Gabriella Sciolla, B.N. Skvorodnev, Hidefumi Tomita, Roland Vanderspek, Hermann Wellenstein, Richard Yamamoto A WIMP detector with directional sensitivity could correlate signal events with astrophysical sources, thereby providing a definitive observable signature of dark matter. Our Dark Matter Time-Projection Chamber (DMTPC) collaboration uses a gas-based detector with optical and charge readout to achieve directional sensitivity. We have built a 10 liter prototype detector and operated it in a surface laboratory. The detector consists of two back-to-back time-projection chambers enclosed within a vacuum vessel which is filled with CF$_4$ gas at 75 Torr. I will report on the results from this run, including the first DMTPC limit on the spin-dependent cross section. In addition, I will describe our next-generation detector which we will deploy underground at the WIPP facility in New Mexico (1.6 km water equivalent depth). [Preview Abstract] |
Saturday, February 13, 2010 8:42AM - 8:54AM |
A10.00002: Estimation of Neutron Backgrounds in CDMS-II Matthew Fritts, Angela Reisetter, Priscilla Cushman In the Cryogenic Dark Matter Search (CDMS) neutrons can produce a signal indistinguishable from that of a WIMP. Potential sources of neutron background include cosmic-ray muons, radioactivity in the cavern rock, and radioactivity in the internal shielding and hardware. Methods to eliminate these backgrounds include the shielding from cosmic rays provided by the depth of the site, passive shielding from cavern radioactivity, an active scintillator cage to tag muons incident on the shielding, and event analysis to distinguish multiply-interacting neutrons from single nuclear recoils. I will describe the results of these efforts and estimates of the residual neutron background, based on Monte Carlo simulations of cosmogenic and radiogenic neutron production, screening of shielding and hardware materials, and analysis of data coincident with cosmic-ray muons tagged by the scintillator cage. [Preview Abstract] |
Saturday, February 13, 2010 8:54AM - 9:06AM |
A10.00003: Position and energy dependent calibration of CDMS-II detectors Kevin McCarthy, Zeeshan Ahmed, Scott Hertel, David Moore, Matthew Pyle, Bruno Serfass The Cryogenic Dark Matter Search (CDMS) detectors use measurements of the ionization and phonon energies deposited by interactions within the detector substrate to distinguish background electron recoils from WIMP candidate nuclear recoils. The primary discrimination quantities exhibit dependencies on the energy of an interaction and its position within a detector, which can significantly deteriorate the experiment's background rejection capabilities if not properly accounted for. I will describe the method used by the CDMS collaboration to estimate event position and calibrate the discrimination parameters to correct for position dependencies, and discuss how this position correction scheme affects the background rejection and nuclear recoil acceptance of the CDMS analysis. [Preview Abstract] |
Saturday, February 13, 2010 9:06AM - 9:18AM |
A10.00004: Estimation and Optimization of Surface Event Background Leakage in CDMS-II David C. Moore The Cryogenic Dark Matter Search (CDMS) measures both the ionization and phonon energy of interactions in the detectors to discriminate between electromagnetic backgrounds and nuclear recoils from possible WIMP interactions. For a given phonon energy, nuclear recoils produce a smaller amount of ionization than electromagnetic backgrounds. However, electromagnetic interactions near the surface of the detectors can also have suppressed ionization and constitute the dominant background for the CDMS-II experiment. Surface interactions can be distinguished from bulk nuclear recoils by measuring the promptness of the athermal phonon signal, allowing the rejection of $>99.5\%$ of these surface events while maintaining significant acceptance of nuclear recoils. I will describe the estimation of the surface event background from $^{133}$Ba calibration data, accounting for the differences in the energy spectrum and spatial distribution of surface events between calibration and WIMP search data. I will also discuss the optimization of the surface event rejection cut to maximize signal efficiency for a given expected leakage. [Preview Abstract] |
Saturday, February 13, 2010 9:18AM - 9:30AM |
A10.00005: SuperTower 1 - Status/Backgrounds/Leakage Manungu Kiveni The Cryogenic Dark Matter Search (CDMS) collaboration employs depth-sensitive detectors to search for the dark matter that pervades the universe in the form of Weakly Interacting Massive Particles (WIMPs) by measuring simultaneously the ionization and the athermal phonons produced when WIMPs interact with the detectors. With this technique, CDMS discriminates the nuclear recoils (WIMP-like interactions) from electron recoil backgrounds with a discrimination power of $>$10$^{4}$:1. The SuperCDMS Soudan experiment has started, and the first of five SuperTowers of detectors has been installed and taking data since 06/05/2009. Each SuperTower contains 5 improved detectors, each 2.5 times thicker than CDMS-II detectors. Together, the five SuperTowers will have a total detector mass of 15 kg. In this presentation, I will report on the status of data-taking with the first SuperTower, focusing on measurements of surface event backgrounds and background rejection. Backgrounds are estimated from measurements of the photon rate and by the rate of tagged alpha-particle interactions, which determine the dominant background of surface electron events. The leakage of background events past discrimination cuts is estimated with data taken with a 133-Ba photon calibration source. [Preview Abstract] |
Saturday, February 13, 2010 9:30AM - 9:42AM |
A10.00006: Measurements of Charge Transport Properties for the Cryogenic Dark Matter Search Kyle Sundqvist The Cryogenic Dark Matter Search (CDMS) utilizes high-purity germanium detectors to seek for weakly interacting massive particles (WIMPs) via their interactions with nuclei. Operating at a temperature of $40 ~mK$, the ionization and phonons generated by particle interactions enable CDMS to discriminate putative WIMPs from electromagnetic background. The discrimination potential of these signals has fundamental dependencies on the transport properties of drifting electrons and holes. We have performed simulations of charge dynamics for these conditions, where the dominant drift-limiting mechanism is the spontaneous emission of phonons. To corroborate our theoretical understanding of charge scattering processes, we have recently performed high-bandwidth ionization measurements on CDMS detectors. Electrons and holes are always ``hot'' under our typical operating conditions. Structure in their drift velocity response to applied field we identify as evidence of the inelastic and anisotropic emission of phonons that is unique to this non-equilibrium regime. We will present how this understanding is beneficial to future detector development. [Preview Abstract] |
Saturday, February 13, 2010 9:42AM - 9:54AM |
A10.00007: Monte Carlo of Cryogenic Dark Matter Search large germanium detectors Steven Leman, Kevin McCarty, Blas Cabrera, Matthew Pyle, Kyle Sundqvist, Bernard Sadoulet A description of the Cryogenic Dark Matter Search (CDMS) detector Monte Carlo (MC) is given along with a comparison to calibration data obtained in 3" diameter, 1" thick [100] germanium crystals. Prompt phonons are generated from electron-recoil interactions along with Luke phonons created by charges as they drift through the crystal via our ionization channels' electric field. The MC phonon transport is described by quasidiffusion, which includes anisotropic propagation, isotope scattering and anharmonic decay, until the phonons are absorbed in either the Transition Edge Sensor based phonon channels or lost in surface interactions. Charge creation is a powerful discriminator for electron-recoil and nuclear-recoil events and also surface interaction rejection. Unlike holes, electrons transports obliquely to the electric field in our detectors due to the germanium [100] crystal orientation and the indirect semiconductor band structure. We are improving the agreement between MC and calibration data in different detector designs, which provides a powerful consistency test of our phonon and charge models. [Preview Abstract] |
Saturday, February 13, 2010 9:54AM - 10:06AM |
A10.00008: Projections for the LUX Dark Matter Experiment from LUX\_0.1 Prototype Results David Malling The LUX experiment will facilitate direct detection of Weakly Interacting Massive Particles (WIMPs) with a 350~kg liquid xenon TPC. LUX will be able to detect 100~GeV WIMPs with scalar cross-section as low as $7\times10^{-46}$~cm$^2$, equivalent to $\sim$0.5~events/100~kg/month in a 100~kg inner fiducial volume. Background event rates $< 8\times10^{-4}$~events/keVee/kg/day are ensured with up to 99.9\% electron-recoil rejection and 50\% nuclear-recoil acceptance. Light collection from primary scintillation events is expected to reach $\sim$10~phe/keVee (ZF). Results from the LUX\_0.1 prototype detector, including greatly enhanced light collection and high xenon purity, are very promising for LUX. All impurities resulting in the degradation of either electron drift lengths or photon mean free paths in liquid xenon are expected to be reduced to negligible levels. A further testing phase above ground at Sanford Lab in early 2010 will further investigate these results prior to underground deployment in mid-2010. [Preview Abstract] |
Saturday, February 13, 2010 10:06AM - 10:18AM |
A10.00009: LUX\_0.1 Prototype Results: Cryogenics and Circulation Adam Bradley LUX is a new dark matter direct detection experiment to be carried out at the Sanford Lab, the renewed underground facility at the Homestake mine in Lead, SD. The detector's large size supports effective internal shielding from natural radioactivity of the surrounding materials and environment. The LUX detector consists of a cylindrical vessel containing 350 kg of liquid xenon (LXe) cooled down using a novel cryogenic system. We tested a small-scale four PMT prototype utilizing over 300 gm of active xenon, installed in the full-sized cryostat. We report the efficiency of a unique internal heat exchanger with standard gas phase purification using a heated getter, which allows for very high flow purification without requiring large cooling power, as well as the efficiency of a thermosyphon-based cooling system. Such systems are required for multi-ton scale up. [Preview Abstract] |
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