The experimental section of this workshop was broken up into two parts; general overview talks which describe problems common to different classes of experiments, and then specific experimental proposals. Of the latter, they were grouped into ‘current experiments’, either operating or under construction, and ‘new ideas’ for experiments not yet under construction but in the conceptual phase. We have summarized the broad overview talks.
Overview
- Fundamentals of Direct Detection – Scott Hertel (UMass Amherst) – Slides
Concise overview of direct detection experimental methods, highlighting the types of detector excitations that hold promise for detecting electron recoils from sub-GeV dark matter - Photon Detection: PMTs, SiPMs, Scintillators – Claudio Savarese (Princeton) – Slides
Comprehensive overview of scintillation mechanisms and single photon counting techniques used for dark matter detection with material on the evolution of these techniques for low mass dark matter searches - Charge Detection and Measurement – Alvaro Chavarria (U. Washington) – Slides
A thorough survey of charge detectors and measurement techniques for sub-GeV dark matter searches, exploring the successes and the issues that need to be addressed - Phonon Detection: TES, MKIDS, and Nanowires – Matt Pyle (UC Berkeley) – Slides
An overview of technologies used to detect phonons, or lattice vibrations, in cryogenic crystals. Readout techniques are dominated by different types of superconductors due to the ability to make them small (limit heat capacity) and the need operate them at cryogenic temperatures to minimize background thermal phonons. - Crystal Defects as Light Dark Matter Detectors – Ranny Budnik (Weizmann Institute) – Slides
Description of a novel type of search which uses defects in high-purity crystals to infer the interaction history in the crystal, and infer an energy spectrum from the distribution in size and number of defects found in the crystal. Utilizes a much different technique, looking at crystals in a semi-destructive way after an exposure, rather than observing a detector in real time. - Accelerator Prospects for Light Dark Matter – Torben Ferber (DESY) – Slides
Overview of how sub-GeV dark matter is probed by accelerator experiments, and summary of the prospects for collider searches over the next 5 years searching for dark photons as a stand in for more complex searches. Discusses prospects for Belle II, LDMX, and a potential e+/e- collider. - Quantum Detectors for Dark Matter Axions – Gianpaolo Carosi (Lawrence Livermore National Laboratory) – Slides
Comprehensive overview of advances in quantum sensing which have enabled limits on axion dark matter, and prospects for detecting higher-mass axion-like particles with ADMX. Covers a mass range below the focus of this workshop, but provides a complementary picture of another field with growth potential utilizing many of the same techniques to search for dark matter.
Current and Future Experiments
Current/future experiment speakers were given 15 minutes to cover material specific to their experiment, and were asked to cover the following topics:
- Brief introduction to the technology (if not covered in the overview talk) and proposed experiment
- Key (sensitivity-limiting) issues that need to be addressed and proposed R&D to accomplish them
- Estimate of sensitivity reach with assumptions clearly identified
- Estimate of resources and timescale needed for an experiment
They were also broken up into categories, as laid out below, to pair similar experiments and allow for direct contrast between technologies competing in the same space.
Scintillating Detectors
- Search for Light Dark Matter with CRESST-III – Florian Reindl (HEPHY & TU Vienna) – Slides
The CRESST experiment uses cryogenic CaWO3 and Sapphire (Al2O3) crystals with thermal readout to detector eV-scale energy deposits from dark matter. They have demonstrated thresholds down to 20 eV, and are currently limited by an unknown low-energy background which rises below 50 eV. These detectors use scintillation light to tag electron-recoils in order to reject them as background for nuclear recoil searches. - The NUCLEUS Experiment – Victoria Wagner (CEA-Saclay, DRF/Irfu) – Slides
The NUCLEUS experiment will take advantage of the technological improvements made by the CRESST experiment to deploy gram-scale calorimeters to study reactor CEvNS.
- Scintillating GaAs for Electron-Recoiling Dark Matter – Stephen Derenzo (Lawrence Berkeley National Laboratory) – Slides
GaAs scintillates cryogenically, and with a ~1.5 eV bandgap is similar to Si in its reach for electron-recoil dark matter. The direct gap, however, allows for better momentum transfer at low masses. Low-background scintillation spectra have yet to be demonstrated.
Charge Detectors
- Single Charge (Skipper) CCDs: SENSEI – Guillermo Fernandez-Moroni (Fermi National Accelerator Laboratory) – Slides
The SENSEI experiment demonstrates the first realization of sub-electron readout noise using silicon CCDs. Data using a small (10g) detector is currently being taken in a shallow site (MINOS) while commissioning of a full-scale (100g) detector is underway at SNOLAB.
- Single Charge (Skipper) CCDs: DAMIC-M – Daniel Baxter (U Chicago) – Slides
The next phase of the DAMIC experiment will combine the established sub-electron resolution of silicon CCDs with improved background controls (0.1 dru) and a larger (1kg) detector to set strong limits on sub-GeV dark matter.
- Single-Charge DEPFETs: DANAE – Alexander Bahr (MPG HLL) – Slides
The DANAE experiment is applying the advantages of non-destructive pixelated readout to new DePleted p-channel Field Effect Transistors (DePFETs) constructed from CMOS. These detectors include individual readout for each pixel, rather than by charge transfer, thus allowing for uniquely fast readout times while maintaining excellent energy and position resolution.
- Status and Prospects of the NEWS-G Experiment – Alexis Brossard (Queen’s University) – Slides
NEWS-G uses a pressurized gas detector to search for sub-keV point-like energy depositions. Recent limits from the Laboratoire Souterrain de Modane using methane-doped neon are competitive with other experiments. Future operation at SNOLAB are expected next year with lower backgrounds and improved shielding.
Heat Detectors
- SuperCDMS HVeV and Low Threshold R&D – Noah Kurinsky (FNAL) – Slides
SuperCDMS uses cryogenic crystals (Si and Ge) with athermal phonon readout, employing superconducting sensors to achieve eV-scale thresholds. When a large electric field is places across these crystals, they can also be operated as sub-electron resolution detectors. SuperCDMS has put the best surface limits to date on electron-recoiling dark matter from a surface site, and is limited (like SENSEI and DAMIC) by dark current. Efforts are underway to improve sensor resolution and utilize new materials such as sapphire and diamond. - EDELWEISS-SubGeV – Julien Billard (CNRS) – Slides
EDELWEISS employs cryogenic germanium crystals with thermal and charge readout to achieve 10eV-scale resolutions, and is largely focused on nuclear recoil channels and scaling small detectors to high mass through crystal arrays. They have made strides in reducing vibrational noise, and are working on lowering the charge resolution of their detectors by employing improved amplifiers and fully-integrated cryogenic readout.
Liquid Noble Detectors
- Light Dark Matter Searches with Liquid Argon – Masayuki Wada (INFN Cagliari) – Slides
The next phase of liquid argon detectors will reach new levels of radioactive purity, which will help enable lower thresholds. This, combined with the kinematic advantage from the relatively lighter nucleus of argon compared to xenon will allow for improved sensitivity to low energy nuclear recoils.
- LBECA: Pushing Xenon TPCs to Single Electrons – Rafael Lang (Purdue) – Slides
One way to capitalize on the investment into large-scale xenon TPCs is to perform an S2 (charge-only) analysis to push down to very low thresholds. The LBECA detector will be the first XENON TPC optimized for such an analysis, including single-electron resolution and thresholds as low as two electrons.
- Scintillating Bubble Chambers – Eric Dahl (Northwestern) – Slides
Bubble chamber detectors have previously demonstrated an unparalleled ability to reject backgrounds due to electron recoils, which do not deposit sufficient heat to nucleate a bubble, an effect most strongly demonstrated in noble liquids. The addition of a scintillation channel to the existing acoustic signal allows sufficient energy reconstruction to enable a background-free sub-keV nuclear recoil analysis.
New Ideas
The talks in this section cover techniques that have yet to be demonstrated in a typical dark matter search setting, ranging from modifications to existing techniques, to emerging technologies, and proposals from theorists for entirely new methods as-yet untried by experimental physics. We briefly summarize the ideas of each talk below.
Liquid Detectors
- HydroX: Hydrogen Doped LXe – Alissa Monte (FNAL) – Slides
Expanding the reach of LXe TPCs (such as LZ) by doping with hydrogen. This allows for more efficient transfer of energy to the target. Need to demonstrate whether the energy deposits can even be seen, filtration issues, possible damage to phototubes. - HeRALD: Direct Detection with Superfluid 4He – Doug Pinckney (UMass) – Slides
Use quantized roton states in superfluid 4He to detect meV-scale energy deposits due to nuclear recoils. Relies on adhesion gain, where unpaired nuclei gain energy as they attach to a surface, to increase energy deposits in a low-threshold solid-state detector. Requires low signal losses; prototypes in development. - Hydrogenated SIMP and WIMP Targets – Juan Collar (U Chicago) – Slides
Hydrogen-rich scintillators show good light yield below 1 keV, and hydrogen has the best kinematic matching to light dark matter. Can be used both for NR and ER dark matter as a scintillating detector or bubble chamber, and may be competitive with Si in the single-photon regime. Unclear what the background levels are or if there’s discovery potential at the lowest masses due to dark count readout.
Solid State Detectors
- Germanium CCDs – Chris Leitz (MIT Lincoln Laboratory) – Slides
Germanium has a bandgap twice as low as silicon, and thus can probe lower masses than silicon-based charge experiments via electron scattering. It also suffers from higher dark rates and is a less well established CCD technology. Rapid progress is being made to reduce dark counts to a level competitive with current Si experiments. - Graphene Nanotubes – Antonio Polosa (Sapienza University of Rome) – Slides
Graphene has unique directional properties, and recoils can eject electrons and ions from nanotubes which can be readout via conventional technologies (e.g. APDs). They can also be made into photo-transistive elements to directly readout energy. Low thresholds with low backgrounds have not yet been achieved, but work on different experimental designs is progressing. - Dirac Materials and Narrow-Gap Semiconductors for keV-MeV Freeze-in Dark Matter – Yoni Kahn (U Chicago/UIUC) – Slides
Dirac materials, such as ZrTe5, have uniquely high directional dark matter signals, but have not been fabricated on mass scales before. Other low-gap materials, such as InSb, have been used as photon detectors, but have not been run cryogenically to reduce the dark rates to the point that they would be useful for dark matter. Both make excellent keV-scale dark matter target candidates.
Superconducting Sensors
- Superconducting Nanowires – Karl Berggren (MIT) – Slides
Small superconductors, held near their critical supercurrent, can be driven normal by very small energy deposits, and are therefore robust low-threshold detectors. Their small mass is one large obstacle to dark-matter search scaling, but they hold the record for dark rate performance (1e-3 Hz/cm2). - Low Mass Dark Matter Searches Using Quantum Sensing and and Readout with MKIDs and Paramps – Ritoban Basu Thakur (Caltech) – Slides
Superconductors have an inductance that changes depending on the number of quasiparticles generated by non-thermal events. These sensors are only limited by the amplifiers used to read them out, and don’t have a thermal noise floor like competing readout technologies. Tuning resonance and efficiently coupling signal to them is a work in progress, and the amplifier noise temperatures are steadily falling.
Exotic Ideas
- Optical Haloscope Searches for Dark Photons and Axions – Masha Baryakhtar (NYU) – Slides
Stacked dielectric layers can be used to convert dark photons and axion-like particles to photons, which can then be focused by a lens onto a sensitive, low-threshold photodetector. It’s unclear whether each stack is narrow-band and whether lenses exist for all of the proposed parameter space. Work is underway to make a proof of concept haloscope. - Resonant Absorption of Dark Matter in Molecules – Ken Van Tillburg (NYU & IAS) – Slides
Molecular transitions have energies on the order of 0.2-20 eV, and can be used as resonant dark photon and ALP detectors in combination with single photon, low dark rate detectors. Properties of large molecular crystals need to be demonstrated, unclear if photon detectors with desired properties can be coupled to molecular crystals under the proposed conditions. - Direct Detection of Light Dark Matter with Magnons – Kevin Zhang (UC Berkeley) – Slides
This was the first presentation of this concept, and is bleeding edge. The proposal is to probe spin-dependent DM-electron interactions in materials that are highly sensitive to electron spin via detection of magnons, the spin-polarized version of phonons in magnetic materials. More work needed on which materials would be good candidates for this technique. - Detecting Very Low Mass Dark Matter via Paleo Detectors – Andzej Drukier – Slides
This proposal is to detect signatures similar to crystal defects, but in specific crystals with known formation history. This trades off size of single crystals for long exposure. There is a precedent for this technique; it was demonstrated with Mica in 1995. The authors propose more materials with different defect thresholds that can be extracted, and their ages estimated. Scaling from the Mica paper, many separate demonstrations are required to prove the longevity of defects for this technique to be viable; it’s possible Mica is unique in its suitability for this technique.