The campaign to observe dark matter using large Xenon detectors. Michael Witherell UC Santa Barbara INPA Dark Matter Workshop May 8, 2014
In 1988 UCSB-UCB-LBNL published the second non-observation of dark matter. “However, when it was decided to look for dark matter, we found that there was a rapidly rising background below about 400 keV. This was due to the presence of about half a gram of In, which undergoes a 486-keV β decay with a half-life of 4x10 14 yr! When the In was removed from one detector, the background for that detector became flat down to about 14 keV at a level of 0.5 counts/keV/kg/day, except for some x-ray peaks.” Present experiments achieve about 0.5 × 10 -4 in these units. The goal is 10 -6 . 2
What do we know about the local dark matter? • The energy density is about 0.3 GeV/cm 3 . • The DM particles have a broad velocity distribution relative to the earth with a characteristic velocity of about 220 km/s. • No particle in the Standard Model fits. – The mass of the particles is unknown. • The frequency of dark matter particle scatters in normal matter is no more than a few events per 100 kg per year. 3
How might we see dark matter? • A weakly interacting massive particle (WIMP) scatters from a xenon nucleus W W Xe • The xenon nucleus recoils with a small amount of energy (~5 keV). Then it escapes. 4
Strategy for direct detection WIMP scatters elastically from the entire nucleus . • Dominant γ and β create electron recoils (ER). • Neutrons produce nuclear recoils (NR), but scatter more than once. 5
Reducing background radiation • Minimize radioactive impurities – Uranium and Thorium chains; especially Radon. – Krypton-81 in Xenon, Argon-39 in Argon. • Reduce cosmogenic activity – Go deep to reduce rate of muons. – Keep xenon underground before operation. • Shield from external activity – Pure water • Neutrons are particularly dangerous, because they look more like WIMPs. 6
Why use liquid xenon to see dark matter? Liquid Xenon scintillates brightly at vacuum ultraviolet wavelengths, and is transparent to its own light. And it shields itself from radioactivity coming from the edges. S1 light is direct scintillation. S2 light counts ionization electrons, is delayed. Purified xenon can be kept very low in background. Krypton can be removed before operation. Because of self-shielding, the center of a large xenon detector is very, very quiet. => Internal calibrations! 7
Sensitivity per tonne 4 0.4 M χ =50 GeV/c 2 M χ =1000 GeV/c 2 integral rate, counts/tonne/year integral rate, counts/tonne/year σ χ ,SI = 10 -11 pb (10 -47 cm 2 ) σ χ ,SI = 10 -11 pb (10 -47 cm 2 ) 3 0.3 Xe Xe 2 0.2 Ge Ge Ar 1 0.1 Ar 0 0 0 10 20 30 40 50 0 10 20 30 40 50 threshold recoil energy, keV threshold recoil energy, keV The combination of low threshold and high mass makes xenon particularly sensitive per tonne over a wide range of WIMP masses 8
The LUX Collaboration Brown Collaboration Meeting, SD School of Mines Richard Gaitskell PI, Professor Sanford Lab, April 2013 Simon Fiorucci Research Associate Xinhua Bai PI, Professor Monica Pangilinan Postdoc Tyler Liebsch Graduate Student Doug Tiedt Graduate Student Jeremy Chapman Graduate Student Carlos Hernandez Faham Graduate Student David Malling Graduate Student SDSTA James Verbus Graduate Student Samuel Chung Chan Graduate Student David Taylor Project Engineer Dongqing Huang Graduate Student Mark Hanhardt Support Scientist Case Western Texas A&M Thomas Shutt PI, Professor James White PI, Professor Dan Akerib PI, Professor Robert Webb PI, Professor Carmen Carmona Postdoc Rachel Mannino Graduate Student Karen Gibson Postdoc Clement Sofka Graduate Student Adam Bradley Graduate Student Patrick Phelps Graduate Student Chang Lee Graduate Student UC Davis Kati Pech Graduate Student Mani Tripathi PI, Professor Bob Svoboda Professor Imperial College London Richard Lander Professor Henrique Araujo PI, Reader Britt Holbrook Senior Engineer Tim Sumner Professor John Thomson Senior Machinist Alastair Currie Postdoc Ray Gerhard Electronics Engineer Adam Bailey Graduate Student Aaron Manalaysay Postdoc Matthew Szydagis Postdoc Lawrence Berkeley + UC Berkeley Richard Ott Postdoc Jeremy Mock Graduate Student Bob Jacobsen PI, Professor University of South Dakota James Morad Graduate Student Murdock Gilchriese Senior Scientist University of Edinburgh Nick Walsh Graduate Student Kevin Lesko Senior Scientist Dongming Mei PI, Professor Michael Woods Graduate Student Victor Gehman Scientist Chao Zhang Postdoc Sergey Uvarov Graduate Student Alex Murphy PI, Reader Mia Ihm Graduate Student Angela Chiller Graduate Student Brian Lenardo Graduate Student James Dobson Postdoc Chris Chiller Graduate Student Dana Byram *Now at SDSTA Lawrence Livermore University of Maryland UC Santa Barbara Adam Bernstein PI, Leader of Adv. Detectors Group Carter Hall PI, Professor Yale Harry Nelson PI, Professor Dennis Carr Mechanical Technician Attila Dobi Graduate Student Mike Witherell Professor Kareem Kazkaz Staff Physicist Richard Knoche Graduate Student Daniel McKinsey PI, Professor Dean White Engineer Peter Sorensen Staff Physicist Jon Balajthy Graduate Student Peter Parker Professor Susanne Kyre Engineer John Bower Engineer Sidney Cahn Lecturer/Research Scientist Curt Nehrkorn Graduate Student Ethan Bernard Postdoc Scott Haselschwardt Graduate Student University of Rochester Markus Horn Postdoc LIP Coimbra Blair Edwards Postdoc Frank Wolfs PI, Professor Scott Hertel Postdoc Isabel Lopes PI, Professor University College London Wojtek Skutski Senior Scientist Kevin O’Sullivan Postdoc Jose Pinto da Cunha Assistant Professor Eryk Druszkiewicz Graduate Student Vladimir Solovov Senior Researcher Nicole Larsen Graduate Student Chamkaur Ghag PI, Lecturer Mongkol Moongweluwan Graduate Student Evan Pease Graduate Student Luiz de Viveiros Postdoc Lea Reichhart Postdoc Brian Tennyson Graduate Student Alexander Lindote Postdoc Ariana Hackenburg Graduate Student Francisco Neves Postdoc Claudio Silva Postdoc Elizabeth Boulton Graduate Student
Sanford Underground Research Facility 10
The LUX experiment 200 tons water 50 cm or 20" 250 kg of active xenon in a titanium vessel 122 photomultiplier tubes 11
The time projection chamber • The LUX TPC is a cylinder of liquid xenon (~50 cm h, ~48 cm d). • Thermosyphons passively cool xenon, operting from a liquid nitrogen reservoir. • A vertical electric field forces the freed electrons into the gas volume. • 122 photomultiplier tubes (above) detect the UV scintillation light 12
Sensitivity to low energy deposits • 1.5 keV electron recoil interaction – 5-fold coincidence for S1 – Larger S2 signal, delayed by 20 microseconds 13
Right at threshold 2 phe S1 event (near threshold) 14
Xenon shields itself. log10 evts/keVee/kg/day • The center of the detector is very quiet. – 118 kg fiducial mass • And it continues to get quieter as cosmogenic activity cools ( 127 Xe) • How can we calibrate the response in the center? Radius (cm) 15
Internal tritium calibration Tritium beta decay has an 25 endpoint energy of 18.6 keV, ideal 20 for calibrating the WIMP energy 15 region. 10 5 y (cm) LUX developed a method of 0 injecting CH 3 T into the xenon, − 5 taking calibration data, and − 10 removing the methane. − 15 − 20 − 25 LUX also injected 83 m Kr weekly − 20 − 10 0 10 20 x (cm) to determine the free electron XY distribution of tritium lifetime and the 3-d correction to photon detection efficiency. events. Circle at r=18cm. (9.4 and 32.1 keV deposits) 16
Electron and Nuclear Recoil Bands ER background rejected by 250x in region of interest 17
Efficiency for WIMP Detection • Universal S1, S2 efficiencies – AmBe NR calibration – Tritiated methane calibration – Mono-energetic neutron source relative detection efficiency 1 S2 0.8 S1 0.6 0.4 All cuts 50% - 4.3 keV nr 0.2 0 0 5 10 15 20 25 30 35 40 recoil energy (keV nr ) 18
Light and charge yields in LUX • Light and charge yields modeled fully (NEST) – NEST consistent with all experimental data • Includes effect of E-field, 77-82% of light yield w/ zero light • To be very conservative, for the initial analysis we assumed no charge or light below 3 keVnr, which we know is wrong. NEST: 19
Calibration with monoenergetic, collimated neutrons D-D neutron Water generator Tank e - 2.5 MeV e - e - e - e - θ neutrons e - e - LXe Drift time y distance into LXe Double scatter: angle gives E recoil Calibrate charge output S2. Then use single scatters to calibrate light output S1. 20
In-situ measurement of nuclear recoil events S2 – Ionization S1 – scintillation (double scatters) (single scatters) Blue Crosses - LUX DD Black line – NESTne - NEST 1 10 1 10 Energy from kinematics (keV nr ) Energy from S2 (keV nr ) Qualitative result: Current LUX 2014 PRL is indeed overconservative. Light & charge yields continuous below 3 keV. Close to NEST simulation. Updated result coming this fall with lower threshold 21
External backgrounds are understood. Full Simulation Model Data Background in 1-10 keV ee range can be predicted reliably because of this understanding. 22
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