Bulletin of the American Physical Society
APS April Meeting 2014
Volume 59, Number 5
Saturday–Tuesday, April 5–8, 2014; Savannah, Georgia
Session H12: Dark Matter I |
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Sponsoring Units: DPF Chair: Robert Bernstein, Fermi National Accelerator Laboratory Room: 100 |
Sunday, April 6, 2014 8:30AM - 8:42AM |
H12.00001: Absolute ionization yield of 6.7 keV argon atoms stopping in liquid argon Peter Sorensen We have made a first measurement of the total electronic energy loss of low-energy argon atoms stopping in liquid argon. The importance of the measurement is underscored by the fact that state of the art theoretical treatment of this problem is only approximate. The results are of interest not only to condensed matter theory, but as a crucial calibration for experiments seeking to directly detect hypothetical dark matter particles, or coherent elastic neutrino nucleus scattering. Using quasi mono-energetic neutrons from a collimated and filtered 7Li(p,n) source, we measured the absolute ionization yield of nuclear recoils in liquid argon at 6.7 keV at applied electric fields ranging from 200-2130 V/cm. We will discuss the experimental setup used for these measurements, our findings and their implications, and finally our recent efforts to apply this technique to liquid xenon. [Preview Abstract] |
Sunday, April 6, 2014 8:42AM - 8:54AM |
H12.00002: The XENON1T Demonstrator: Impact of the recirculation of the gas above the liquid xenon on the purity Hugo Contreras XENON1T is the third detector in the XENON project for direct detection of Dark Matter. Its construction started at the end of 2013, and will achieve a sensitivity in the spin-independent cross section to ~2.e-47 cm2 for a 50GeV/c2 WIMP. XENON1T is a dual phase liquid xenon TPC detector with a fiducial mass of 1.1 ton and a length of 1m. To address the most important technical challenges involved in the scaling of this kind of detectors a facility was built in the Nevis Laboratories at Columbia University, the XENON1T Demonstrator, a dual-phase TPC with a 1-meter drift length. One of the main research topics in this facility has been the improvement of the purification process in XENON1T. The new approach includes the direct recirculation of the gas xenon layer on top of the active liquid xenon, to improve both the purification time and the maximum purity achievable in XENON1T. In this talk we will summarize the results obtained with this improved recirculation system in the XENON1T Demonstrator. [Preview Abstract] |
Sunday, April 6, 2014 8:54AM - 9:06AM |
H12.00003: A New Method for Electronic Recoil Calibration in Liquid Noble Dark Matter Detectors Sean MacMullin Calibration of next-generation liquid noble dark matter detectors present new challenges because radiation from external sources will not probe the entire target, owing to its large volume and high stopping power. For electronic recoil calibration in particular, a proposed solution is to dissolve a source of low-energy $\beta$-electrons directly into the liquid. A particularly promising candidate is $^{212}$Pb, a daughter of $^{220}$Rn. We have acquired a custom-made source of electrodeposited $^{228}$Th that efficiently emanates the desired $^{220}$Rn. Details of recent measurements of mixing $^{220}$Rn and its daughters in a liquid xenon detector and future prospects will be presented. [Preview Abstract] |
Sunday, April 6, 2014 9:06AM - 9:18AM |
H12.00004: Limits on GeV-scale WIMPs using charge signals in XENON100 Richard Wall Various theoretical models and recent experimental results have led to growing interest in the search for WIMP-like dark matter in the mass range of a few GeV. One important class of detector used in this study is based on the liquid-gas, dual-phase Xenon time projection chamber (as in XENON100 and LUX). These detectors nominally use both scintillation (S1) and ionization (S2) signals to localize collision events in their sensitive volumes and thus reject background events, but it is known that the efficiency for detecting small S1 signals (such as are expected from a GeV-scale WIMP interaction) is much smaller than the efficiency for detecting an S2 from the same recoil. By removing the requirement of an observed S1 signal, one can thus effectively lower the energy threshold of the detector, and study GeV-scale WIMPs with greater sensitivity. With this in mind, we measure the rate of WIMP candidates in 225 live days of XENON100 data in events with small S2 signals (with or without an accompanying S1) and which pass other simple selection cuts optimized for GeV-scale WIMPs. This rate is then used to set a limit on the WIMP-nucleon cross-section for the mass range 1-10 GeV. [Preview Abstract] |
Sunday, April 6, 2014 9:18AM - 9:30AM |
H12.00005: SuperCDMS SNOLAB Experiment and Active Neutron Veto Yu Chen The SuperCDMS SNOLAB experiment will attempt direct detection of the most promising candidate for dark matter, Weakly Interacting Massive Particles (WIMPs) using cryogenically cooled germanium and silicon semiconductors that provide sub-keV thresholds and excellent rejection of most radioactivity or cosmic-ray-induced backgrounds. An active neutron veto with high efficiency for tagging neutron-induced backgrounds will not only directly reduce the neutron background rate, but also provide an in-situ measurement of the neutron activity near the dark matter target. This active veto will consist of liquid scintillator doped with an isotope with high neutron-capture cross section. I will present a brief overview of the experiment, and report in detail on the current status of simulation and prototyping of this neutron veto. [Preview Abstract] |
Sunday, April 6, 2014 9:30AM - 9:42AM |
H12.00006: Optimizing SuperCDMS phonon energy sensitivity by studying quasiparticle transport in Al films Jeffrey Yen, Benjamin Shank, Blas Cabrera, Robert Moffatt, Peter Redl, Paul Brink, Astrid Tomada, Matt Cherry, Betty Young, Teddy Tortorici, John Mark Kreikebaum In order to further improve the phonon energy sensitivity of Cryogenic Dark Matter Search (CDMS) detectors, we studied quasiparticle transport at $\sim$ 40 mK in superconducting Al films similar in geometry to those used for CDMS detectors. Test structures of Al were deposited and photolithographically patterned on Si wafers using the same production-line equipment used to fabricate kg-scale CDMS detectors. Three Al film lengths and two film thicknesses were used in this study. In the test experiments described here, an 55Fe source was used to excite a NaCl reflector, producing 2.6 keV x-rays that hit our test devices after passing through a collimator. The impinging x-rays broke Cooper pairs in the Al films, producing quasiparticles that propagated into W transition edge sensors (TESs) coupled to the ends of the Al films. In this talk, we will give the motivation behind these studies, describe our experimental setup, and compare our data to results obtained using signal processing models constructed from basic physical parameters. We show that a non-linear, non-stationary optimal filter applied to the data allows us to precisely measure quasiparticle diffusion and other aspects of energy transport in our thin-film Al-W test devices. These results are being used to further optimize next-generation CDMS detectors. [Preview Abstract] |
Sunday, April 6, 2014 9:42AM - 9:54AM |
H12.00007: A Template-Matching Method For Measuring Energy Depositions In TES Films Benjamin Shank, Jeffrey Yen, Blas Cabrera, John Mark Kreikebaum, Robert Moffatt, Peter Redl, Betty Young, Paul Brink, Matthew Cherry, Astrid Tomada Transition edge sensors (TES) have a wide variety of applications in particle$\backslash$astrophysics for detecting incoming particles with high energy resolution. In TES design, the need for sufficient heat capacity to avoid saturation limits the ultimate energy resolution. Building on the TES model developed for SuperCDMS by Yen et. al. for tungsten TESs deposited next to aluminum collection fins, we outline a time-domain non-linear optimal filter method for reconstructing energy depositions in TES films. This allows us to operate devices into their saturation region while taking into account changing noise performance and loss of energy collection. We show how this method has improved our understanding of quasiparticle diffusion and energy collection in our superconducting sensors. [Preview Abstract] |
Sunday, April 6, 2014 9:54AM - 10:06AM |
H12.00008: Geant4 Simulations of SuperCDMS iZip Detector Charge Carrier Propagation Robert Agnese, Daniel Brandt, Peter Redl, Makoto Asai, Dana Faiez, Mike Kelsey, Enrico Bagli, Adam Anderson, Chandler Schlupf The SuperCDMS experiment uses germanium crystal detectors instrumented with ionization and phonon readout circuits to search for dark matter. In order to simulate the response of the detectors to particle interactions the SuperCDMS Detector Monte Carlo (DMC) group has been implementing the processes governing electrons and phonons at low temperatures in Geant4. The charge portion of the DMC simulates oblique propagation of the electrons through the L-valleys, propagation of holes through the $\Gamma$-valleys, inter-valley scattering, and emission of Neganov-Luke phonons in a complex applied electric field. The field is calculated by applying a directed walk search on a tetrahedral mesh of known potentials and then interpolating the value. This talk will present an overview of the DMC status and a comparison of the charge portion of the DMC to experimental data of electron-hole pair propagation in germanium. [Preview Abstract] |
Sunday, April 6, 2014 10:06AM - 10:18AM |
H12.00009: Spatial Imaging of Charge Transport in Germanium at Low Temperature Robert Moffatt, Blas Cabrera, Fedja Kadribasic, Peter Redl, Benjamin Shank, Betty Young, Daniel Brandt, Paul Brink, Matthew Cherry, Astrid Tomada Because germanium is an indirect-gap semiconductor, the energy minima of the conduction band occur at four locations on the edges of the Brillouin zone. These minima have differing anisotropic mass tensors, causing electrons to travel obliquely to an applied electric field and to separate into four distinct clusters. A better understanding this process may improve the reconstruction of particle interactions in the germanium detectors used by the Cryogenic Dark Matter Search (CDMS). In addition, the possibility exists that the distribution of electrons among the four minima may preserve some information about the initial direction of a dark matter recoil event. To observe this oblique propagation, we excited a point source of charge carriers with a focused laser pulse on one face of a 4mm thick germanium crystal. After the electrons were drifted through a uniform electric field, the pattern of charge density arriving on the opposite face was mapped and used to reconstruct the trajectories of the four clusters. This talk will present the latest results of the charge-transport experiment, including measurements of the electron and hole charge density patterns and the scattering rate between energy minima as a function of both temperature and electric field strength. [Preview Abstract] |
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