R. Mahapatra

Dark matter search results from the CDMS II experiment.

News from the Dark Side? Dark matter is thought to represent 85% of all matter in the universe and to have been responsible for the formation of structure in the early universe, but its nature is still a mystery. Ahmed et al. (p. 1619, published online 11 February; see the Perspective by Lang) describe the results from the completed Cryogenic Dark Matter Search (CDMS II) experiment, which searched for dark matter in the form of weakly interacting massive particles (WIMP). Two candidate signals were observed, whereas only one background event was expected. The probability of having two or more events from the background would have been 23%. The results of this analysis cannot be interpreted with confidence as evidence for WIMP interactions, but, at the same time, neither event can be ruled out as representing signal. Details of possible, but unlikely, detection events produced by dark matter are reported. Astrophysical observations indicate that dark matter constitutes most of the mass in our universe, but its nature remains unknown. Over the past decade, the Cryogenic Dark Matter Search (CDMS II) experiment has provided world-leading sensitivity for the direct detection of weakly interacting massive particle (WIMP) dark matter. The final exposure of our low-temperature germanium particle detectors at the Soudan Underground Laboratory yielded two candidate events, with an expected background of 0.9 ± 0.2 events. This is not statistically significant evidence for a WIMP signal. The combined CDMS II data place the strongest constraints on the WIMP-nucleon spin-independent scattering cross section for a wide range of WIMP masses and exclude new parameter space in inelastic dark matter models.Z. Ahmed, D.S. Akerib, S. Arrenberg, C.N. Bailey, D. Balakishiyeva, L. Baudis, D.A. Bauer, P.L. Brink, T. Bruch, R. Bunker, B. Cabrera, D.O. Caldwell, J. Cooley, P. Cushman, M. Daal, F. DeJongh, M.R. Dragowsky, L. Duong, S. Fallows, E. Figueroa-Feliciano, J. Filippini, M. Fritts, S.R. Golwala, D.R. Grant, J. Hall, R. Hennings-Yeomans, S.A. Hertel, D. Holmgren, L. Hsu, M.E. Huber, O. Kamaev, M. Kiveni, M. Kos, S.W. Leman, R. Mahapatra, V. Mandic, K.A. McCarthy, N. Mirabolfathi, D. Moore, H. Nelson, R.W. Ogburn, A. Phipps, M. Pyle, X. Qiu, E. Ramberg, W. Rau, A. Reisetter, 7 T. Saab, B. Sadoulet, 13 J. Sander, R.W. Schnee, D.N. Seitz, B. Serfass, K.M. Sundqvist, M. Tarka, P. Wikus, S. Yellin, 14 J. Yoo, B.A. Young, and J. Zhang (CDMS Collaboration) Division of Physics, Mathematics & Astronomy, California Institute of Technology, Pasadena, CA 91125, USA Department of Physics, Case Western Reserve University, Cleveland, OH 44106, USA Fermi National Accelerator Laboratory, Batavia, IL 60510, USA Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Department of Physics, Queen’s University, Kingston, ON, Canada, K7L 3N6 Department of Physics, St. Olaf College, Northfield, MN 55057 USA Department of Physics, Santa Clara University, Santa Clara, CA 95053, USA Department of Physics, Southern Methodist University, Dallas, TX 75275, USA Department of Physics, Stanford University, Stanford, CA 94305, USA Department of Physics, Syracuse University, Syracuse, NY 13244, USA Department of Physics, Texas A & M University, College Station, TX 77843, USA Department of Physics, University of California, Berkeley, CA 94720, USA Department of Physics, University of California, Santa Barbara, CA 93106, USA Departments of Phys. & Elec. Engr., University of Colorado Denver, Denver, CO 80217, USA Department of Physics, University of Florida, Gainesville, FL 32611, USA School of Physics & Astronomy, University of Minnesota, Minneapolis, MN 55455, USA Physics Institute, University of Zürich, Winterthurerstr. 190, CH-8057, Switzerland Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA

Search for low-mass weakly interacting massive particles using voltage-assisted calorimetric ionization detection in the SuperCDMS experiment

SuperCDMS is an experiment designed to directly detect weakly interacting massive particles (WIMPs), a favored candidate for dark matter ubiquitous in the Universe. In this Letter, we present WIMP-search results using a calorimetric technique we call CDMSlite, which relies on voltage-assisted Luke-Neganov amplification of the ionization energy deposited by particle interactions. The data were collected with a single 0.6xa0kg germanium detector running for ten live days at the Soudan Underground Laboratory. A low energy threshold of 170u2009u2009eVee (electron equivalent) was obtained, which allows us to constrain new WIMP-nucleon spin-independent parameter space for WIMP masses below 6u2009u2009GeV/c2.

Search for low-mass weakly interacting massive particles with SuperCDMS.

We report a first search for weakly interacting massive particles (WIMPs) using the background rejection capabilities of SuperCDMS. An exposure of 577 kg-days was analyzed for WIMPs with mass < 30 GeV/c2, with the signal region blinded. Eleven events were observed after unblinding. We set an upper limit on the spin-independent WIMP-nucleon cross section of 1.2e-42 cm2 at 8 GeV/c2. This result is in tension with WIMP interpretations of recent experiments and probes new parameter space for WIMP-nucleon scattering for WIMP masses < 6 GeV/c2.

New Results from the Search for Low-Mass Weakly Interacting Massive Particles with the CDMS Low Ionization Threshold Experiment

The CDMS low ionization threshold experiment (CDMSlite) uses cryogenic germanium detectors operated at a relatively high bias voltage to amplify the phonon signal in the search for weakly interacting massive particles (WIMPs). Results are presented from the second CDMSlite run with an exposure of 70xa0kg day, which reached an energy threshold for electron recoils as low as 56xa0eV. A fiducialization cut reduces backgrounds below those previously reported by CDMSlite. New parameter space for the WIMP-nucleon spin-independent cross section is excluded for WIMP masses between 1.6 and 5.5u2009u2009GeV/c^{2}.R. Agnese, A.J. Anderson, T. Aramaki, M. Asai, W. Baker, D. Balakishiyeva, D. Barker, R. Basu Thakur, 23 D.A. Bauer, J. Billard, A. Borgland, M.A. Bowles, P.L. Brink, R. Bunker, B. Cabrera, D.O. Caldwell, R. Calkins, D.G. Cerdeno, H. Chagani, Y. Chen, J. Cooley, B. Cornell, P. Cushman, M. Daal, P.C.F. Di Stefano, T. Doughty, L. Esteban, S. Fallows, E. Figueroa-Feliciano, M. Ghaith, G.L. Godfrey, S.R. Golwala, J. Hall, H.R. Harris, T. Hofer, D. Holmgren, L. Hsu, M.E. Huber, D. Jardin, A. Jastram, O. Kamaev, B. Kara, M.H. Kelsey, A. Kennedy, A. Leder, B. Loer, E. Lopez Asamar, P. Lukens, R. Mahapatra, V. Mandic, N. Mast, N. Mirabolfathi, R.A. Moffatt, J.D. Morales Mendoza, S.M. Oser, K. Page, W.A. Page, R. Partridge, M. Pepin, ∗ A. Phipps, K. Prasad, M. Pyle, H. Qiu, W. Rau, P. Redl, A. Reisetter, Y. Ricci, A. Roberts, H.E. Rogers, T. Saab, B. Sadoulet, 4 J. Sander, K. Schneck, R.W. Schnee, S. Scorza, B. Serfass, B. Shank, D. Speller, D. Toback, R. Underwood, S. Upadhyayula, A.N. Villano, B. Welliver, J.S. Wilson, D.H. Wright, S. Yellin, J.J. Yen, B.A. Young, and J. Zhang

Search for Low-Mass WIMPs with SuperCDMS

We report a first search for weakly interacting massive particles (WIMPs) using the background rejection capabilities of SuperCDMS. An exposure of 577 kg-days was analyzed for WIMPs with mass < 30 GeV/c2, with the signal region blinded. Eleven events were observed after unblinding. We set an upper limit on the spin-independent WIMP-nucleon cross section of 1.2e-42 cm2 at 8 GeV/c2. This result is in tension with WIMP interpretations of recent experiments and probes new parameter space for WIMP-nucleon scattering for WIMP masses < 6 GeV/c2.

Silicon detector results from the first five-tower run of CDMS II

We report results of a search for weakly interacting massive particles (WIMPs) with the Si detectors of the CDMS II experiment. This report describes a blind analysis of the first data taken with CDMS II’s full complement of detectors in 2006–2007; results from this exposure using the Ge detectors have already been presented. We observed no candidate WIMP-scattering events in an exposure of 55.9 kg-days before analysis cuts, with an expected background of ∼1.1 events. The exposure of this analysis is equivalent to 10.3 kg-days over a recoil energy range of 7–100 keV for an ideal Si detector and a WIMP mass of 10u2009u2009GeV/c^2. These data set an upper limit of 1.7×10^(-41)u2009u2009cm^2 on the WIMP-nucleon spin-independent cross section of a 10u2009u2009GeV/c^2 WIMP. These data exclude parameter space for spin-independent WIMP-nucleon elastic scattering that is relevant to recent searches for low-mass WIMPs.

Projected sensitivity of the SuperCDMS SNOLAB experiment

SuperCDMS SNOLAB will be a next-generation experiment aimed at directly detecting low-mass particles (with masses ≤ 10 GeV/c^2) that may constitute dark matter by using cryogenic detectors of two types (HV and iZIP) and two target materials (germanium and silicon). The experiment is being designed with an initial sensitivity to nuclear recoil cross sections ∼ 1×10^(−43) cm^2 for a dark matter particle mass of 1 GeV/c^2, and with capacity to continue exploration to both smaller masses and better sensitivities. The phonon sensitivity of the HV detectors will be sufficient to detect nuclear recoils from sub-GeV dark matter. A detailed calibration of the detector response to low-energy recoils will be needed to optimize running conditions of the HV detectors and to interpret their data for dark matter searches. Low-activity shielding, and the depth of SNOLAB, will reduce most backgrounds, but cosmogenically produced ^3H and naturally occurring ^(32)Si will be present in the detectors at some level. Even if these backgrounds are 10 times higher than expected, the science reach of the HV detectors would be over 3 orders of magnitude beyond current results for a dark matter mass of 1 GeV/c^2. The iZIP detectors are relatively insensitive to variations in detector response and backgrounds, and will provide better sensitivity for dark matter particles with masses ≳ 5 GeV/c^2. The mix of detector types (HV and iZIP), and targets (germanium and silicon), planned for the experiment, as well as flexibility in how the detectors are operated, will allow us to maximize the low-mass reach, and understand the backgrounds that the experiment will encounter. Upgrades to the experiment, perhaps with a variety of ultra-low-background cryogenic detectors, will extend dark matter sensitivity down to the “neutrino floor,” where coherent scatters of solar neutrinos become a limiting background.

Improved WIMP-search reach of the CDMS II germanium data

CDMS II data from the five-tower runs at the Soudan Underground Laboratory were reprocessed with an improved charge-pulse fitting algorithm. Two new analysis techniques to reject surface-event backgrounds were applied to the 612 kg days germanium-detector weakly interacting massive particle (WIMP)-search exposure. An extended analysis was also completed by decreasing the 10 keV analysis threshold to ∼5u2009u2009keV, to increase sensitivity near a WIMP mass of 8u2009u2009GeV/c^2. After unblinding, there were zero candidate events above a deposited energy of 10 keV and six events in the lower-threshold analysis. This yielded minimum WIMP-nucleon spin-independent scattering cross-section limits of 1.8×10^(−44) and 1.18×10^(−41) at 90% confidence for 60 and 8.6u2009u2009GeV/c^2 WIMPs, respectively. This improves the previous CDMS II result by a factor of 2.4 (2.7) for 60 (8.6)u2009u2009GeV/c^2 WIMPs.

Demonstration of surface electron rejection with interleaved germanium detectors for dark matter searches

The SuperCDMS experiment in the Soudan Underground Laboratory searches for dark matter with a 9-kg array of cryogenic germanium detectors. Symmetric sensors on opposite sides measure both charge and phonons from each particle interaction, providing excellent discrimination between electron and nuclear recoils, and between surface and interior events. Surface event rejection capabilities were tested with two ^(210)Pb sources producing ∼130 beta decays/hr. In ∼800 live hours, no events leaked into the 8–115u2009keV signal region, giving upper limit leakage fraction 1.7u2009×u200910^(−5) at 90% C.L., corresponding tou2009<u20090.6 surface event background in the future 200-kg SuperCDMS SNOLAB experiment.

The SuperCDMS Experiment

Modest improvements in the level and/or discrimination of backgrounds are needed to keep backgrounds negligible during the three phases of SuperCDMS. By developing production designs that require only modest testing, detector production rates may be improved sufficiently to allow an exposure of 500 ton d within a reasonable time and budget. Overall, the improvement estimates described above are conservative. Previous development efforts have shown that some areas prove easier and provide larger factors while others prove more difficult. The conservative estimates together with the broad approach reduce the risk and give us confidence that we will succeed, providing the surest way to probe to WIMP-nucleon cross sections of 10{sup -46} cm{sup 2}.