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US LUX experiment reaches critical phase

The quest to find the most mysterious particles in the Universe is entering a critical phase, scientists say.
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Title: A Detailed Look at the First Results from the Large Underground Xenon (LUX) Dark Matter Experiment
Author: M. Szydagis, D.S. Akerib, H.M. Araujo, X. Bai, A.J. Bailey, J. Balajthy, E. Bernard, A. Bernstein, A. Bradley, D. Byram, S.B. Cahn, M.C. Carmona-Benitez, C. Chan, J.J. Chapman, A.A. Chiller, C. Chiller, T. Coffey, A. Currie, L. de Viveiros, A. Dobi, J. Dobson, E. Druszkiewicz, B. Edwards, C.H. Faham, S. Fiorucci, C. Flores, R.J. Gaitskell, V.M. Gehman, C. Ghag, K.R. Gibson, M.G.D. Gilchriese, C. Hall, S.A. Hertel, M. Horn, D.Q. Huang, M. Ihm, R.G. Jacobsen, K. Kazkaz, R. Knoche, N.A. Larsen, C. Lee, A. Lindote, M.I. Lopes, D.C. Malling, R. Mannino, D.N. McKinsey, D.-M. Mei, J. Mock, M. Moongweluwan, J. Morad, A.St.J. Murphy, C. Nehrkorn, H. Nelson, F. Neves, R.A. Ott, M. Pangilinan, P.D. Parker, E.K. Pease, K. Pech, P. Phelps, L. Reichhart, T. Shutt, C. Silva, V.N. Solovov, P. Sorensen, K. O'Sullivan, et al. (13 additional authors not shown)

LUX, the world's largest dual-phase xenon time-projection chamber, with a fiducial target mass of 118 kg and 10,091 kg-days of exposure thus far, is currently the most sensitive direct dark matter search experiment. The initial null-result limit on the spin-independent WIMP-nucleon scattering cross-section was released in October 2013, with a primary scintillation threshold of 2 phe, roughly 3 keVnr for LUX. The detector has been deployed at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, and is the first experiment to achieve a limit on the WIMP cross-section lower than 10^-45 cm^2. Here we present a more in-depth discussion of the novel energy scale employed to better understand the nuclear recoil light and charge yields, and of the calibration sources, including the new internal tritium source. We found the LUX data to be in conflict with low-mass WIMP signal interpretations of other results.

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Xenon (LUX) experiment
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Title: First results from the LUX dark matter experiment at the Sanford Underground Research Facility
Author: LUX Collaboration: D.S. Akerib, H.M. Araujo, X. Bai, A.J. Bailey, J. Balajthy, S. Bedikian, E. Bernard, A. Bernstein, A. Bolozdynya, A. Bradley, D. Byram, S.B. Cahn, M.C. Carmona-Benitez, C. Chan, J.J. Chapman, A.A. Chiller, C. Chiller, K. Clark, T. Coffey, A. Currie, A. Curioni, S. Dazeley, L. de Viveiros, A. Dobi, J. Dobson, E.M. Dragowsky, E. Druszkiewicz, B. Edwards, C.H. Faham, S. Fiorucci, C. Flores, R.J. Gaitskell, V.M. Gehman, C. Ghag, K.R. Gibson, M.G.D. Gilchriese, C. Hall, M. Hanhardt, S.A. Hertel, M. Horn, D.Q. Huang, M. Ihm, R.G. Jacobsen, L. Kastens, K. Kazkaz, R. Knoche, S. Kyre, R. Lander, N.A. Larsen, C. Lee, D.S. Leonard, K.T. Lesko, A. Lindote, M.I. Lopes, A. Lyashenko, D.C. Malling, R. Mannino, D.N. McKinsey, D.-M. Mei, J. Mock, M. Moongweluwan, J. Morad, M. Morii, A.St.J. Murphy, et al. (38 additional authors not shown)

The Large Underground Xenon (LUX) experiment, a dual-phase xenon time-projection chamber operating at the Sanford Underground Research Facility (Lead, South Dakota), was cooled and filled in February 2013. We report results of the first WIMP search dataset, taken during the period April to August 2013, presenting the analysis of 85.3 live-days of data with a fiducial volume of 118 kg. A profile-likelihood analysis technique shows our data to be consistent with the background-only hypothesis, allowing 90% confidence limits to be set on spin-independent WIMP-nucleon elastic scattering with a minimum upper limit on the cross section of 7.6 x 10-46 cm2 at a WIMP mass of 33 GeV/c2. We find that the LUX data are in strong disagreement with low-mass WIMP signal interpretations of the results from several recent direct detection experiments.

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LUX detector
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Dark matter detector installed and submerged in South Dakota

An experiment to look for one of nature's most elusive subatomic particles is finally under water, in a stainless steel tank nearly a mile underground beneath the Black Hills of South Dakota.
The Large Underground Xenon experiment, nicknamed LUX, will be the most sensitive device yet to look for dark matter. Thought to comprise more than 80 percent of the mass of the universe, dark matter has so far eluded direct detection. The LUX detector, under construction for more than three years in South Dakota, was installed underground in a protective tank in July. The tank was filled with water last week, and all systems are functioning well.
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Title: Technical Results from the Surface Run of the LUX Dark Matter Experiment
Authors: LUX Collaboration: D. S. Akerib, X. Bai, E. Bernard, A. Bernstein, A. Bradley, D. Byram, S. B. Cahn, M. C. Carmona-Benitez, J. J. Chapman, T. Coffey, A. Dobi, E. Dragowsky, E. Druszkiewicz, B. Edwards, C.H. Faham, S. Fiorucci, R. J. Gaitskell, K. R. Gibson, M. Gilchriese, C. Hall, M. Hanhardt, M. Ihm, R. G. Jacobsen, L. Kastens, K. Kazkaz, R. Knoche, N. Larsen, C. Lee, K. T. Lesko, A. Lindote, M. I. Lopes, A. Lyashenko, D. C. Malling, R. Mannino, D. N. McKinsey, D. Mei, J.Mock, M. Moongweluwan, M. Morii, H. Nelson, F. Neves, J. A. Nikkel, M. Pangilinan, K. Pech, P. Phelps, A. Rodionov, T. Shutt, C. Silva, W. Skulski, V. N. Solovov, P. Sorensen, T. Stiegler, M. Sweany, M. Szydagis, D. Taylor, M. Tripathi, S. Uvarov, J. R. Verbus, L. de Viveiros, N. Walsh, R.Webb, J. T. White, M. Wlasenko, et al. (3 additional authors not shown)

We present the results of the three-month above-ground commissioning run of the Large Underground Xenon (LUX) experiment at the Sanford Underground Research Facility located in Lead, South Dakota, USA. LUX is a 370 kg liquid xenon detector that will search for cold dark matter in the form of Weakly Interacting Massive Particles (WIMPs). The commissioning run, conducted with the detector immersed in a water tank, validated the integration of the various sub-systems in preparation of the underground deployment. Using the data collected, we report excellent light collection properties, achieving 8 photoelectrons per keV for 662 keV electron recoils without an applied electric field, measured in the center of the WIMP target. We also find good energy and position resolution in relatively high-energy interactions from a variety of internal and external sources. Finally, we have used the commissioning data to tune the optical properties of our simulation and report updated sensitivity projections for spin-independent WIMP-nucleon scattering.

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  Lying in Wait for WIMPs

Although it's invisible, dark matter accounts for at least 80 percent of the matter in the universe. No one knows what it is, but most scientists would bet on weakly interacting massive particles, or WIMPs.
LUX, the Large Underground Xenon detector at the Sanford Underground Research Facility in the Black Hills of South Dakota, is calling that bet with a titanium bottle holding 350 kilograms of liquid xenon, placed in a cavern 4,850 feet down in the former Homestake gold mine. LUX is a trap set for dark-matter WIMPs.
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Large Underground Xenon experiment
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Title: An Ultra-Low Background PMT for Liquid Xenon Detectors
Authors: D.S. Akerib, X. Bai, E. Bernard, A. Bernstein, A. Bradley, D. Byram, S.B. Cahn, M.C. Carmona-Benitez, D. Carr, J.J. Chapman, Y-D. Chan, K. Clark, T. Coffey, L. deViveiros, M. Dragowsky, E. Druszkiewicz, B. Edwards, C. H. Faham, S. Fiorucci, R. J. Gaitskell, K. R. Gibson, C. Hall, M. Hanhardt, B. Holbrook, M. Ihm, R.G. Jacobsen, L. Kastens, K. Kazkaz, N. Larsen, C. Lee, K. Lesko, A. Lindote, M. I. Lopes, A. Lyashenko, D. C. Malling, R. Mannino, D. McKinsey, D. Mei, J. Mock, M. Morii, H. Nelson, F. Neves, J.A. Nikkel, M. Pangilinan, K. Pech, P. Phelps, T. Shutt, C. Silva, W. Skulski, V.N. Solovov, P. Sorensen, J. Spaans, T. Stiegler, M. Sweany, M. Szydagis, D. Taylor, J. Thomson, M. Tripathi, S. Uvarov, J.R. Verbus, N. Walsh, R. Webb, J. T. White, M. Wlasenko, F. L. H. Wolfs, M. Woods, C. Zhang

Results are presented from radioactivity screening of two models of photomultiplier tubes designed for use in current and future liquid xenon experiments. The Hamamatsu 5.6 cm diameter R8778 PMT, used in the LUX dark matter experiment, has yielded a positive detection of four common radioactive isotopes: 238U, 232Th, 40K, and 60Co. Screening of LUX materials has rendered backgrounds from other detector materials subdominant to the R8778 contribution. A prototype Hamamatsu 7.6 cm diameter R11410 MOD PMT has also been screened, with benchmark isotope counts measured at <0.4 238 U / <0.3 232 Th / <8.3 40 K / 2.0±0.2 60 Co mBq/PMT. This represents a large reduction, equal to a change of x 1/24 238U / x 1/9 232Th / x  1/8 40K per PMT, between R8778 and R11410 MOD, concurrent with a doubling of the photocathode surface area (4.5 cm to 6.4 cm diameter). 60Co measurements are comparable between the PMTs, but can be significantly reduced in future R11410 MOD units through further material selection. Assuming PMT activity equal to the measured 90% upper limits, Monte Carlo estimates indicate that replacement of R8778 PMTs with R11410 MOD PMTs will change LUX PMT electron recoil background contributions by a factor of x 1/25 after further material selection for 60Co reduction, and nuclear recoil backgrounds by a factor of x 1/36. The strong reduction in backgrounds below the measured R8778 levels makes the R11410 MOD a very competitive technology for use in large-scale liquid xenon detectors.

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