Author(s): Agnese, R; Anderson, AJ; Aralis, T; Aramaki, T; Arnquist, IJ; Baker, W; Balakishiyeva, D; Barker, D; Basu Thakur, R; Bauer, DA; Binder, T; Bowles, MA; Brink, PL; Bunker, R; Cabrera, B; Caldwell, DO; Calkins, R; Cartaro, C; Cerdeno, DG; Chang, Y; Chagani, H; Chen, Y; Cooley, J; Cornell, B; Cushman, P; Daal, M; Di Stefano, PCF; Doughty, T; Esteban, L; Fascione, E; Figueroa-Feliciano, E; Fritts, M; Gerbier, G; Ghaith, M; Godfrey, GL; Golwala, SR; Hall, J; Harris, HR; Hong, Z; Hoppe, EW; Hsu, L; Huber, ME; Iyer, V; Jardin, D; Jastram, A; Jena, C; Kelsey, MH; Kennedy, A; Kubik, A; Kurinsky, NA; Leder, A; Loer, B; Lopez Asamar, E; Lukens, P; Macdonell, D; Mahapatra, R; Mandic, V; Mast, N; Miller, EH; Mirabolfathi, N; Moffatt, RA; Mohanty, B; Morales Mendoza, JD; Nelson, J; Orrell, JL; Oser, SM; Page, K; Page, WA; Partridge, R; Pepin, M; Penalver Martinez, M; Phipps, A; Poudel, S; Pyle, M; Qiu, H; Rau, W; Redl, P; Reisetter, A; Reynolds, T; Roberts, A; Robinson, AE; Rogers, HE; Saab, T; Sadoulet, B; Sander, J | Abstract: © 2018 American Physical Society. The SuperCDMS experiment is designed to directly detect weakly interacting massive particles (WIMPs) that may constitute the dark matter in our Galaxy. During its operation at the Soudan Underground Laboratory, germanium detectors were run in the CDMSlite mode to gather data sets with sensitivity specifically for WIMPs with masses l10 GeV/c2. In this mode, a higher detector-bias voltage is applied to amplify the phonon signals produced by drifting charges. This paper presents studies of the experimental noise and its effect on the achievable energy threshold, which is demonstrated to be as low as 56 eVee (electron equivalent energy). The detector-biasing configuration is described in detail, with analysis corrections for voltage variations to the level of a few percent. Detailed studies of the electric-field geometry, and the resulting successful development of a fiducial parameter, eliminate poorly measured events, yielding an energy resolution ranging from ∼9 eVee at 0 keV to 101 eVee at ∼10 keVee. New results are derived for astrophysical uncertainties relevant to the WIMP-search limits, specifically examining how they are affected by variations in the most probable WIMP velocity and the Galactic escape velocity. These variations become more important for WIMP masses below 10 GeV/c2. Finally, new limits on spin-dependent low-mass WIMP-nucleon interactions are derived, with new parameter space excluded for WIMP masses ≲3 GeV/c2.

Low-mass dark matter search with CDMSlite

We demonstrate that individual electron-hole pairs are resolved in a 1 cm$^2$ by 4 mm thick silicon crystal (0.93 g) operated at $\sim$35 mK. One side of the detector is patterned with two quasiparticle-trap-assisted electro-thermal-feedback transition edge sensor (QET) arrays held near ground potential. The other side contains a bias grid with 20\% coverage. Bias potentials up to $\pm$ 160 V were used in the work reported here. A fiber optic provides 650~nm (1.9 eV) photons that each produce an electron-hole ($e^{-} h^{+}$) pair in the crystal near the grid. The energy of the drifting charges is measured with a phonon sensor noise $\sigma$ $\sim$0.09 $e^{-} h^{+}$ pair. The observed charge quantization is nearly identical for $h^+$'s or $e^-$'s transported across the crystal.

/pdf/thermal-detection-of-single-e-h-pairs-in-a-biased-silicon-2u6gd87pmd.pdf

Thermal detection of single e-h pairs in a biased silicon crystal detector

LUX-ZEPLIN (LZ) is a second-generation direct dark matter experiment with spin-independent WIMP-nucleon scattering sensitivity above $${1.4 \times 10^{-48}}\, {\hbox {cm}}^{2}$$
 for a WIMP mass of $${40}\, \hbox {GeV}/{\hbox {c}}^{2}$$
 and a $${1000}\, \hbox {days}$$
 exposure. LZ achieves this sensitivity through a combination of a large $${5.6}\, \hbox {t}$$
 fiducial volume, active inner and outer veto systems, and radio-pure construction using materials with inherently low radioactivity content. The LZ collaboration performed an extensive radioassay campaign over a period of six years to inform material selection for construction and provide an input to the experimental background model against which any possible signal excess may be evaluated. The campaign and its results are described in this paper. We present assays of dust and radon daughters depositing on the surface of components as well as cleanliness controls necessary to maintain background expectations through detector construction and assembly. Finally, examples from the campaign to highlight fixed contaminant radioassays for the LZ photomultiplier tubes, quality control and quality assurance procedures through fabrication, radon emanation measurements of major sub-systems, and bespoke detector systems to assay scintillator are presented.

/pdf/the-lux-zeplin-lz-radioactivity-and-cleanliness-control-3vmbny17x1.pdf

The LUX-ZEPLIN (LZ) radioactivity and cleanliness control programs

The search for particle-like dark matter with meV-to-GeV masses has developed rapidly in the past few years. We summarize the science case for these searches, the recent progress, and the exciting upcoming opportunities. Funding for Research and Development and a portfolio of small dark matter projects will allow the community to capitalize on the substantial recent advances in theory and experiment and probe vast regions of unexplored dark-matter parameter space in the coming decade.

Snowmass2021 Cosmic Frontier: The landscape of low-threshold dark matter direct detection in the next decade

LUX-ZEPLIN (LZ) is a second-generation direct dark matter experiment with spin-independent WIMP-nucleon scattering sensitivity above $1.4 \times 10^{-48}$ cm$^{2}$ for a WIMP mass of 40 GeV/c$^{2}$ and a 1000 d exposure. LZ achieves this sensitivity through a combination of a large 5.6 t fiducial volume, active inner and outer veto systems, and radio-pure construction using materials with inherently low radioactivity content. The LZ collaboration performed an extensive radioassay campaign over a period of six years to inform material selection for construction and provide an input to the experimental background model against which any possible signal excess may be evaluated. The campaign and its results are described in this paper. We present assays of dust and radon daughters depositing on the surface of components as well as cleanliness controls necessary to maintain background expectations through detector construction and assembly. Finally, examples from the campaign to highlight fixed contaminant radioassays for the LZ photomultiplier tubes, quality control and quality assurance procedures through fabrication, radon emanation measurements of major sub-systems, and bespoke detector systems to assay scintillator are presented.

/pdf/the-lux-zeplin-lz-radioactivity-and-cleanliness-control-557m7pir70.pdf

A potential source of dominant backgrounds for many rare-event searches or screening detectors is from radon daughters, specifically 210Pb, deposited on detector surfaces, often during detector fabrication. Performing a late-stage etch is challenging because it may damage the detector. This paper describes a late-stage etching technique that reduces surface 210 Pb and 210 Po by > 99 × at 90% C.L.

Removal of 210Pb by etch of crystalline detector sidewalls

Radiopure materials for detector components in rare event searches may be contaminated after manufacturing with long-lived $^{210}$Pb produced by the decay of atmospheric radon. Charged radon daughters deposited on the surface or implanted in the bulk of detector materials have the potential to cause noticeable backgrounds within dark matter regions of interest. Understanding the mechanics governing these background signals is therefore a paramount concern in dark matter experiments in order to distinguish a real signal from internal detector backgrounds. Teflon ($i.e.$ PTFE) is a specific material of interest because it makes up the walls of the inner detector of many liquid noble detectors such as the LUX-ZEPLIN experiment. The rate of radon daughter plate-out onto Teflon can be orders of magnitude larger than the plate-out rate onto other materials. Mitigation of plate-out onto Teflon and steel by proximity to other materials is demonstrated.

Radon Daughter Plate-out onto Teflon

Future dark matter direct detection experiments will reach unprecedented levels of sensitivity. Achieving this sensitivity will require more precise models of signal and background rates. Improving the precision of signal and background modeling goes hand-in-hand with novel calibration techniques that can probe rare processes and lower threshold detector response. The goal of this white paper is to outline community needs to meet the background and calibration requirements of next-generation dark matter direct detection experiments.

Snowmass2021 Cosmic Frontier White Paper: Calibrations and backgrounds for dark matter direct detection

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Removal of 210Pb by etch of crystalline detector sidewalls

Radon Daughter Plate-out onto Teflon

Snowmass2021 Cosmic Frontier White Paper: Calibrations and backgrounds for dark matter direct detection