Cryogenic Dark Matter Search Research Articles

CUTE: A Cryogenic Underground TEst facility at SNOLAB

Low-temperature cryogenics open the door for a range of interesting technologies based on features like superconductivity and superfluidity, low-temperature phase transitions or the low heat capacity of non-metals in the milli-Kelvin range. Devices based on these technologies are often sensitive to small energy depositions as can be caused by environmental radiation. The Cryogenic Underground TEst facility (CUTE) at SNOLAB is a platform for testing and operating cryogenic devices in an environment with low levels of background. The large experimental chamber (O(10) L) reaches a base temperature of ∼ 12 mK; it can hold a payload of up to ∼ 20 kg and provides a radiogenic background event rate as low as a few events/kg/keV/day in the energy range below about 100 keV, as well as a negligible muon rate (O1)/month). CUTE was designed and built in the context of the Super Cryogenic Dark Matter Search experiment (SuperCDMS) that uses cryogenic detectors to search for interactions of dark matter particles with ordinary matter. The facility has been used to test SuperCDMS detectors since its commissioning in 2019. In 2021, it was handed over to SNOLAB to become a SNOLAB user facility after the completion of the testing of detectors for SuperCDMS. The facility will be available for projects that benefit from these special conditions, based on proposals assessed for their scientific and technological merits. This article describes the main design features and operating parameters of CUTE.

Radon Plate-out and the Effects of Airflow and Electric Charge for Dark Matter Experiments

The Cryogenic Dark Matter Search (SuperCDMS) is an international collaboration designed to search for and detect dark matter particles, which make up ~85% of the matter in the universe. The plate-out, or deposition of naturally occurring radioactive decay byproducts onto surfaces, can create backgrounds that interfere with dark matter detection experiments. In the first series of these experiments, we analyze the amount of radon progeny, 214Pb and 214Bi, that plate-out on polycarbonate samples while controlling factors such as electric charge and airflow. These samples are exposed to radon-spiked nitrogen gas in a polycarbonate wind tunnel to simulate plate-out conditions in a controlled environment. To determine radon progeny plate-out rates for each trial, the initial activity of radon progeny is calculated from the measurements of an Ortec alpha counter. In previous iterations of the experiment, we observed static charge buildup on surfaces, especially polycarbonate. This charge buildup was reduced by the implementation of an electric field source in the wind tunnel, yielding more consistent polycarbonate trials. After neutralizing the electric charge on polycarbonate, the second series of the experiment compares normalized radon daughter plate-out for polycarbonate and copper samples. Copper measurements demonstrated a positive correlation between air speed and radon daughter plate-out rate from speeds of 0 to 60 ft/min, stabilizing at speeds between 0 to 60 ft/min. Acrylic measurements demonstrated no observable relation between air speed and normalized plate-out rates. Results from both results deviate from the linear correlation of air speed and plate-out rate predicted by the Jacobi plate-out model [1]

Estimation of Interaction Locations in Super Cryogenic Dark Matter Search Detectors Using Genetic Programming-Symbolic Regression Method

The Super Cryogenic Dark Matter Search (SuperCDMS) experiment is used to search for Weakly Interacting Massive Particles (WIMPs)—candidates for dark matter particles. In this experiment, the WIMPs interact with nuclei in the detector; however, there are many other interactions (background interactions). To separate background interactions from the signal, it is necessary to measure the interaction energy and to reconstruct the location of the interaction between WIMPs and the nuclei. In recent years, some research papers have been investigating the reconstruction of interaction locations using artificial intelligence (AI) methods. In this paper, a genetic programming-symbolic regression (GPSR), with randomly tuned hyperparameters cross-validated via a five-fold procedure, was applied to the SuperCDMS experiment to estimate the interaction locations with high accuracy. To measure the estimation accuracy of obtaining the SEs, the mean and standard deviation (σ) values of R2, the root-mean-squared error (RMSE), and finally, the mean absolute error (MAE) were used. The investigation showed that using GPSR, SEs can be obtained that estimatethe interaction locations with high accuracy. To improve the solution, the five best SEs were combined from the three best cases. The results demonstrated that a very high estimation accuracy can be achieved with the proposed methodology.

Constraints on Lightly Ionizing Particles from CDMSlite.

The Cryogenic Dark Matter Search low ionization threshold experiment (CDMSlite) achieved efficient detection of very small recoil energies in its germanium target, resulting in sensitivity to lightly ionizing particles (LIPs) in a previously unexplored region of charge, mass, and velocity parameter space. We report first direct-detection limits calculated using the optimum interval method on the vertical intensity of cosmogenically produced LIPs with an electric charge smaller than e/(3×10^{5}), as well as the strongest limits for charge ≤e/160, with a minimum vertical intensity of 1.36×10^{-7} cm^{-2} s^{-1} sr^{-1} at charge e/160. These results apply over a wide range of LIP masses (5 MeV/c^{2} to 100 TeV/c^{2}) and cover a wide range of βγ values (0.1-10^{6}), thus excluding nonrelativistic LIPs with βγ as small as 0.1 for the first time.

Optical properties of the Czochralski grown Cs2MoO4 crystal

Single crystal growth of novel dicesium molybdate (Cs2MoO4) crystal by Czochralski technique was carried out. The single crystal phase was confirmed by powders X-ray diffraction analysis. The optical band gap of 4.22 eV was estimated from the absorbance spectra, measured at room temperature. The luminescence properties were measured under the excitation of 4.42 eV UV light from an LED source in the temperature range of 10–300 K. The photoluminescence light yield increases upon cooling down the grown crystal to 10 K. The luminescence decay time was analyzed with the sum of three exponential decay-time functions in the entire temperature range. The comparison of luminescence light yield with Li2MoO4 (one of the promising scintillating bolometers for the neutrinoless double-beta decay search) at 10 K shows a similar light output. These preliminary investigations show that the grown crystal can be a good candidate crystal for the cryogenic dark matter and neutrinoless double-beta decay search.

Cryocooled cold trap system for the SuperCDMS dilution refrigerator

Operating 6,800 feet underground at the SNOLAB facility in Sudbury, Ontario, Canada, the dilution refrigerator-cooled SuperCDMS SNOLAB (Super Cryogenic Dark Matter Search at the Sudbury Neutrino Observatory Laboratory) experiment has been designed for maximum cryogenic up-time and remote operations. A key element in achieving these goals is a pair cold traps in the helium circulation stream of the dilution refrigerator; the first operating near liquid nitrogen temperatures and the second operating near liquid helium temperatures. Previous experience with the CDMS experiment, located underground at the Soudan Under-ground Laboratory, has given significant operational experience with dilution refrigerator cold traps and has solidified the demand of a system of dual cold traps. Unlike the CDMS-era system, the new SuperCDMS system will feature a cryocooler powered liquid nitrogen re-liquefying system (as opposed to regular under-ground re-filling of cold trap dewars using portable nitrogen dewars) and a cryogen-free 4 K cold trap, which eliminates the need for a bath of liquid helium.

SuperCDMS SNOLAB - Status and Plans

The Super Cryogenic Dark Matter Search (SuperCDMS) and its predecessor CDMS have been at the forefront of the search for Weakly Interacting Massive dark matter Particles (WIMPs) for close to two decades. Significant improvements in detector technology have opened up the low-mass parameter space (≲ 10 GeV/c2) where the experiment broke new ground with the CDMS low ionization threshold experiment (CDMSlite). Building on this success, SuperCDMS is preparing for the next phase of the experiment to be located at SNOLAB near Sudbury, Ontario. The new experimental setup will provide space for up to ∼200 kg of target mass in a considerably lower background environment. The initial payload of about 30 kg will be a mix of germanium and silicon targets in the form of both background discriminating iZIP and low-threshold HV detectors, pushing the sensitivity towards WIMPs with even lower masses and improving the cross-section reach of SuperCDMS by more than an order of magnitude. The long-term goal is to reach the neutrino-floor below 10 GeV/c2. We present the status of and plans for SuperCDMS at SNOLAB.

Search for low-mass dark matter with CDMSlite using a profile likelihood fit

The Cryogenic Dark Matter Search low ionization threshold experiment (CDMSlite) searches for interactions between dark matter particles and germanium nuclei in cryogenic detectors. The experiment has achieved a low energy threshold with improved sensitivity to low-mass (<10 GeV/c$^2$) dark matter particles. We present an analysis of the final CDMSlite data set, taken with a different detector than was used for the two previous CDMSlite data sets. This analysis includes a data "salting" method to protect against bias, improved noise discrimination, background modeling, and the use of profile likelihood methods to search for a dark matter signal in the presence of backgrounds. We achieve an energy threshold of 70 eV and significantly improve the sensitivity for dark matter particles with masses between 2.5 and 10 GeV/c$^2$ compared to previous analyses. We set an upper limit on the dark matter-nucleon scattering cross section in germanium of 5.4$\times$10$^{-42}$ cm$^2$ at 5 GeV/c$^2$, a factor of $\sim$2.5 improvement over the previous CDMSlite result.

Energy loss due to defect formation from 206Pb recoils in SuperCDMS germanium detectors

The Super Cryogenic Dark Matter Search experiment at the Soudan Underground Laboratory studied energy loss associated with defect formation in germanium crystals at mK temperatures using in situ 210Pb sources. We examine the spectrum of 206Pb nuclear recoils near its expected 103 keV endpoint energy and determine an energy loss of (6:08 ± 0:18)%, which we attribute to defect formation. From this result and using TRIM simulations, we extract the first experimentally determined average displacement threshold energy of 19.7−0.5+0.6 eV for germanium. This has implications for the analysis thresholds of future germanium-based dark matter searches.

What is dark matter?

This study aims to review the early history of dark matter study, such as observational evidence from galactic rotational curves, gravitational lensing, and the cosmic microwave background, among others. The observation of the bullet clusters, which strongly supports the existence of dark matter rather than the theory of modified gravity, is discussed. The N body simulations also suggest the existence of cold non-relativistic dark matter and the existence of a universal form of the dark matter density distribution profile. We introduce a standard mechanism of thermal freeze-out for dark matter relic abundance, i.e., an explanation of how the dark matter particles that were kept in thermal equilibrium in the early stages of the universe are later unable to stay in equilibrium due to the expansion of the universe. The typical dark matter candidates are the so-called WIMPs (weakly interacting massive particles). Popular WIMP candidates include the lightest supersymmetric particles such as neutralinos. Other WIMP candidates such as the lightest T odd particles in the little Higgs models and the lightest KK (Kluza-Klein) modes in the universal extra dimension models are also discussed. Non-WIMP dark matter candidates such as axions are briefly discussed. The basic ideas and methods of dark matter detection, such as underground direct detection, which involves dark matter scattering off target nuclei, and indirect detection in space, which involves dark matter annihilation or decay in galaxies, are reviewed with a focus on recent experimental developments. For underground direct detection, we begin with the basic formulas for elastic dark matter nuclei scattering and the general features of nuclear number dependence. Three types of observables are discussed: (1) direct recoil events, (2) solar modulation of the recoil events, and (3) directional effects or the day-night differences of the events. Then, we focus on the experiments studying low-mass dark matter below 10 GeV, such as SuperCDMS (super cryogenic dark matter search) and CDEX (China dark matter experiment), among others, with a special focus on the CDEX experiment, which uses point contact germanium detectors. Experiments studying high-mass dark matter using liquid argon, such as Xenon, PandaX (particle and astrophysical xenon detector), and DarkSide are also discussed. Directional detection experiments, such as DRIFT (directional recoil identification from tracks) and MIMAC (MIcro-tpc MAtrix of Chambers), are briefly discussed. For indirect detection experiments in space, we first introduce the basic theory of dark matter signals in cosmic rays and then discuss the importance of the propagation effects of high-energy cosmic rays, such as electrons, positrons, protons, antiprotons, and heavier cosmic ray nuclei. The uncertainties originating from various sources are also discussed. We then review recent experimental results, such as that from Fermi LAT (Fermi large area telescope) and AMS-02(Alpha magnetic spectrometer 02), with a focus on the most recent dark matter particle explorer (DAMPE) experiments. As nearby sources may contribute to CRE (cosmic ray electron) structures at high energies, the recently released DAMPE results on the CRE flux, which hinted at a narrow excess at energy of 1.4 TeV, is discussed in some detail. In general, a spectral structure with a narrow width appears to reveal the space time distribution of the sources. Future perspectives of heavier cosmic-ray nuclei, such as anti-deuteron and anti-helium, are also reviewed.

Nuclear-recoil energy scale in CDMS II silicon dark-matter detectors

The Cryogenic Dark Matter Search (CDMS II) experiment aims to detect dark matter particles that elastically scatter from nuclei in semiconductor detectors. The resulting nuclear-recoil energy depositions are detected by ionization and phonon sensors. Neutrons produce a similar spectrum of low-energy nuclear recoils in such detectors, while most other backgrounds produce electron recoils. The absolute energy scale for nuclear recoils is necessary to interpret results correctly. The energy scale can be determined in CDMS II silicon detectors using neutrons incident from a broad-spectrum 252Cf source, taking advantage of a prominent resonance in the neutron elastic scattering cross section of silicon at a recoil (neutron) energy near 20 (182)keV. Results indicate that the phonon collection efficiency for nuclear recoils is 4.8−0.9+0.7% lower than for electron recoils of the same energy. Comparisons of the ionization signals for nuclear recoils to those measured previously by other groups at higher electric fields indicate that the ionization collection efficiency for CDMS II silicon detectors operated at ∼4V/cm is consistent with 100% for nuclear recoils below 20keV and gradually decreases for larger energies to ∼75% at 100keV. The impact of these measurements on previously published CDMS II silicon results is small.

Performance of the first 150 mm diameter Cryogenic silicon ionization detectors with contact-free electrodes

Cryogenic semiconductor detectors operated at temperatures below 100 mK are commonly used in particle physics experiments searching for dark matter. The largest such germanium and silicon detectors, with diameters of 100 mm and thickness of 33 mm, are planned for use by the Super Cryogenic Dark Matter Search (SuperCDMS) experiment at SNOLAB, Canada. Still larger individual detectors are being investigated to scale up the sensitive mass of future experiments. We present here the first results of testing two prototype 150 mm diameter silicon ionization detectors. The detectors are 25 mm and 33 mm thick with masses 1.7 and 2.2 times larger than those currently planned for SuperCDMS. These devices were operated with contact-free bias electrodes to minimize leakage currents which currently limit operation at high bias voltages. The results show promise for the use of such technologies in solid state cryogenic detectors.

Feasibility Study for an IR-LED-Based Calibration System for SuperCDMS Detectors

The Super Cryogenic Dark Matter Search is one of the leading experiments in the direct search for weakly interacting massive particles in the mass range below 10 GeV/c2. Particles are detected in cryogenic semiconductor detectors; their energy deposition produces phonons and liberates charges which are measured in TES-based phonon sensors and charge-collecting electrodes. The next generation of the experiment will be deployed at SNOLAB and aims to further reduce the detection threshold to a few tens of eV by reducing the noise in the readout circuit and improving the design of the phonon sensors. Traditionally, radioactive sources are used to calibrate the energy scale and to monitor detector stability. However, in most cases, it takes a long time to accumulate enough events to identify peaks in the energy spectrum. Moreover, gammas of low energy as would be required to calibrate the bottom range of the detector’s energy range cannot penetrate the cryostat shielding. This study investigates the possibility of using pulsed infrared LEDs mounted inside the cryostat as alternative calibration source.

Results from the Super Cryogenic Dark Matter Search Experiment at Soudan.

We report the result of a blinded search for weakly interacting massive particles (WIMPs) using the majority of the SuperCDMS Soudan data set. With an exposure of 1690kg d, a single candidate event is observed, consistent with expected backgrounds. This analysis (combined with previous Ge results) sets an upper limit on the spin-independent WIMP-nucleon cross section of 1.4×10^{-44} (1.0×10^{-44}) cm^{2} at 46 GeV/c^{2}. These results set the strongest limits for WIMP-germanium-nucleus interactions for masses >12 GeV/c^{2}.

The cryogenics design of the SuperCDMS SNOLAB experiment

The Super Cryogenic Dark Matter Search (SuperCDMS) experiment is a direct detection dark matter experiment intended for deployment to the SNOLAB underground facility in Ontario, Canada. With a payload of up to 186 germanium and silicon crystal detectors operating below 15 mK, the cryogenic architecture of the experiment is complex. Further, the requirement that the cryostat presents a low radioactive background to the detectors limits the materials and techniques available for construction, and heavily influences the design of the cryogenics system. The resulting thermal architecture is a closed cycle (no liquid cryogen) system, with stages at 50 and 4 K cooled with gas and fluid circulation systems and stages at 1 K, 250 mK and 15 mK cooled by the lower temperature stages of a large, cryogen-free dilution refrigerator.This paper describes the thermal design of the experiment, including details of the cooling systems, mechanical designs and expected performance of the system under operational conditions.

Thermal conductance modeling and characterization of the SuperCDMS SNOLAB sub-Kelvin cryogenic system

The detectors of the Super Cryogenic Dark Matter Search experiment at SNOLAB (SuperCDMS SNOLAB) will operate in a seven-layered cryostat with thermal stages between room temperature and the base temperature of 15 mK. The inner three layers of the cryostat, which are to be nominally maintained at 1 K, 250 mK, and 15 mK, will be cooled by a dilution refrigerator via conduction through long copper stems. Bolted and mechanically pressed contacts, flat and cylindrical, as well as flexible straps are the essential stem components that will facilitate assembly/dismantling of the cryostat. These will also allow for thermal contractions/movements during cooldown of the sub-Kelvin system. To ensure that these components and their contacts meet their design thermal conductance, prototypes were fabricated and cryogenically tested. The present paper gives an overview of the SuperCDMS SNOLAB sub-Kelvin architecture and its conductance requirements. Results from the conductance measurements tests and from sub-Kelvin thermal modeling are discussed.

Toward single electron resolution phonon mediated ionization detectors

Experiments seeking to detect rare event interactions such as dark matter or coherent elastic neutrino nucleus scattering are striving for large mass detectors with very low detection threshold. Using Neganov-Luke phonon amplification effect, the Cryogenic Dark Matter Search (CDMS) experiment is reaching unprecedented RMS resolutions of ∼14eVee. CDMSlite is currently the most sensitive experiment to WIMPs of mass ∼5GeV/c2 but is limited in achieving higher phonon gains due to an early onset of leakage current into Ge crystals. The contact interface geometry is particularly weak for blocking hole injection from the metal, and thus a new design is demonstrated that allows high voltage bias via vacuum separated electrode. With an increased bias voltage and a×2 Luke phonon gain, world best RMS resolution of sigma ∼7eVee for 0.25kg (d=75mm, h=1cm) Ge detectors was achieved. Since the leakage current is a function of the field and the phonon gain is a function of the applied voltage, appropriately robust interface blocking material combined with thicker substrate (25mm) will reach a resolution of ∼2.8eVee. In order to achieve better resolution of ∼ eV, we are investigating a layer of insulator between the phonon readout surface and the semiconductor crystals.

The SuperCDMS Soudan high threshold WIMP search and the planned SuperCDMS SNOLAB experiment

There is ample evidence that visible matter cannot account for a large component of the mass in the universe. Weakly Interacting Massive Particles (WIMPs) are one popular hypothesis to account for the missing mass. The Super Cryogenic Dark Matter Search (SuperCDMS) experiment is designed to directly detect WIMPs through interactions with a nucleus in a target crystal. The SuperCDMS detectors are instrumented with phonon and charge sensors, enabling excellent rejection of electron-recoil backgrounds. Approximately 3000 kg-days of exposure have been collected with the SuperCDMS Soudan experiment. We will describe the search for WIMPs with masses between 10-100 GeV and work towards the SuperCDMS SNOLAB experiment.

Low-threshold WIMP search at SuperCDMS

The Super Cryogenic Dark Matter Search (SuperCDMS) experiment aims to detecting nuclear recoils from weakly-interacting massive particles (WIMPs) by measuring phonon and ionization energy in crystalline Ge. It has been operating at the Soudan Underground Laboratory in Minnesota (USA) since March 2012 with improved background rejection capabilities with respect to CDMS II. A low-threshold analysis of the SuperCDMS data has been performed, allowing to explore WIMP masses below 30 GeV/c2. This is the first analysis using the full background rejection capabilities of SuperCDMS. In particular both phonon and ionization signals are used for defining a fiducial volume excluding events near any of the surfaces of the detectors. In addition, the background discrimination includes multivariate techniques optimized for several WIMP masses. The results are competitive with other low-threshold WIMP searches, and probe new parameter space for WIMP-nucleon scattering for WIMP masses between 4 and 6 GeV/c2.

Observation of Impact Ionization of Shallow States in Sub-Kelvin, High-Purity Germanium

We report on the observation of impact ionization processes involving shallow impurity states in a sub-Kelvin, high-purity n-type germanium detector similar to those used by direct detection dark matter experiments such as the Cryogenic Dark Matter Search. An optical fiber is used to generate packets of charge carriers near one surface of the detector. The charge carriers drift to the opposite surface by application of an electric field. The resulting drift current is measured by a high-speed charge amplifier. The onset of impact ionization for both electron and hole transport is clearly observed in the drift current as the applied electric field is increased above \(\approx \)5 V/cm. We present the effective charge collection efficiency and trapping length as a function of applied electric field for electrons and holes. We estimate the impact ionization cross section to be on the order of \(5\times 10^{-13}\,\mathrm {cm}^2\).