Cryogenic Dark Matter Search Detector (CDMS)

A crystal cooled to one-hundredth of a degree above absolute zero, waiting in a former iron mine for a particle that has never been confirmed to exist.

The CDMS detectors record two independent signals whenever a particle collides inside them: the phonon energy (heat vibrations spreading through the crystal lattice) and the ionisation energy (free electrons and holes pulled through the crystal by an electric field). The ratio of these two signals cleanly separates nuclear recoils — the expected signature of a WIMP — from electron recoils caused by ordinary radioactive backgrounds. This dual-channel discrimination lets the experiment reject nearly all backgrounds on an event-by-event basis, rather than relying on shielding alone.

Each detector is a disk of high-purity germanium or silicon, 100 mm in diameter and 33 mm thick, cooled to roughly 15–50 mK inside a dilution refrigerator. Superconducting tungsten Transition Edge Sensors (TES) patterned photolithographically on both faces of the disk measure athermal phonons with eV-scale resolution. Interleaved photolithographic channels on the same faces collect ionisation charge. An applied electric field of a few to ~100 volts drifts charge carriers toward the collection electrodes; in the high-voltage (HV) operating mode the drifting charges generate additional Neganov–Trofimov–Luke phonons, amplifying the phonon signal and pushing the recoil threshold down toward 40–100 eV. A coincidence veto from surrounding shields and a 2-km overburden of rock suppress cosmic-ray muons to negligible levels.

In plain terms

Imagine a super-cold gem buried deep underground that can feel the difference between a feather tickling it and a gust of wind. It is waiting for an invisible particle from space to tap a single atom inside — and the faint warmth and tiny electric spark from that tap together identify what kind of thing hit it.

Also called: CDMS, SuperCDMS, iZIP detector, CDMSlite.

Why it matters

First experiment to demonstrate event-by-event discrimination between nuclear and electron recoils in a solid-state detector, establishing a template for all subsequent cryogenic WIMP searches.
SuperCDMS iZIP detectors achieve ~10⁶:1 rejection of electron-recoil backgrounds at energies above 2 keV, one of the best background-suppression ratios of any dark matter detector technology.
The high-voltage CDMSlite and HVeV modes extend sensitivity into the sub-GeV dark matter mass range that liquid xenon detectors cannot access, making CDMS complementary to, not competitive with, xenon-based experiments.
A single 0.93-gram prototype HVeV device demonstrated sensitivity to single electron-hole pairs and set limits on dark photon absorption in 2018 — showing the technology scales down to gram-scale with sub-eV thresholds.
SuperCDMS SNOLAB, now being commissioned in 2025–2026, will deploy 24 full-size detectors totalling ~30 kg and is expected to probe WIMP–nucleon cross sections near 10⁻⁴³ cm² at 1 GeV/c².

Where it lives

CDMS I — Stanford Underground Facility (SUF), 17 mwe depth, 1998–2002
CDMS II — Soudan Underground Laboratory, 2090 mwe depth, 780 m, 2003–2009
SuperCDMS Soudan — 15 iZIP germanium detectors, 2011–2015
SuperCDMS SNOLAB — 24 iZIP + HV detectors (Ge and Si), 2 km depth, commissioning 2025–2026
NEXUS/CUTE — small-scale test facilities at SLAC and SNOLAB for R&D runs

Key numbers

Operating temperature: 15–50 mK (Dilution refrigerator; exact value varies by run mode)
Detector geometry (SNOLAB): 100 mm dia. × 33 mm thick (Ge: 1.39 kg each; Si: 0.61 kg each)
Total SNOLAB target mass: ~30 kg (24 detectors, mix of Ge and Si iZIP + HV types)
iZIP nuclear-recoil threshold: ~1 keV (With electron/nuclear recoil discrimination)
HV detector phonon threshold: ~40–100 eV (nuclear recoil) (SNOLAB goal; Soudan achieved ~170 eVee)
HVeV charge resolution: 0.1 electron-hole pairs (First demonstrated 2018; sub-eV phonon resolution achieved 2026)
Site depth (SNOLAB): 2 km (6010 mwe) (SNOLAB, Sudbury, Ontario — deepest CDMS site)
Muon flux reduction (Soudan vs. surface): ~50,000× (2090 mwe overburden)

Surprising facts

The iZIP detectors operate at roughly 15 mK — about 20 billion times colder than the surface of the Sun — so that thermal noise does not mask the sub-keV vibrations from a potential dark matter recoil.
The Soudan mine site sits 780 metres underground, where 2090 metres of water-equivalent overburden reduces the cosmic-ray muon flux by a factor of roughly 50,000 relative to sea level.
A SuperCDMS HVeV prototype detector containing less than one gram of silicon achieved sensitivity to the energy deposited by a single electron-hole pair — about 1.2 eV — making it among the most sensitive particle calorimeters ever built.
Despite its extraordinary sensitivity, CDMS has never confirmed a WIMP detection; three candidate events in the 2013 silicon dataset had only a 0.19% probability of being background-only under the best-fit WIMP hypothesis, but the statistics were insufficient to claim discovery.

Open questions

Low-energy event excess in HVeV and related detectors: Multiple SuperCDMS HVeV runs and similar cryogenic detectors have observed an excess of events at energies of order tens of eV above trigger threshold that exceeds known radioactive backgrounds. The excess is correlated across events and appears related to phonon-generating processes in or on the crystal surface. Its origin — surface radioactivity, infrared photons, or some other mechanism — remains under active investigation and limits sensitivity to sub-GeV dark matter.

A short history

The CDMS programme grew out of 1980s work at Stanford on cryogenic calorimeters. CDMS I ran at the shallow Stanford Underground Facility (17 mwe) from 1998 to 2002, demonstrating the dual phonon–ionisation technique in germanium and silicon. CDMS II relocated in 2003 to the Soudan Underground Laboratory in northern Minnesota (2090 mwe), operating a two-tower array of six germanium and six silicon detectors. Results published in 2004, 2009, and 2011 progressively set the world's best spin-independent WIMP–nucleon cross-section limits for WIMP masses above roughly 10 GeV/c². A 2013 silicon analysis with 140 kg-days of exposure found three WIMP-candidate events (expected background 0.41 events), stimulating substantial theoretical interest. SuperCDMS Soudan followed from 2011 to 2015 with improved 600-gram iZIP detectors, pioneering the CDMSlite low-threshold mode. The current phase, SuperCDMS SNOLAB, is being assembled 2 km underground at SNOLAB in Sudbury, Canada, with commissioning of 24 full-size detectors progressing in 2025–2026.

Sources

CDMS II Ge low-energy analysis, Phys. Rev. Lett. 106, 131302 (2011) (https://link.aps.org/doi/10.1103/PhysRevLett.106.131302)
CDMS II Si results — three WIMP candidates, Phys. Rev. Lett. 111, 251301 (2013) (https://link.aps.org/doi/10.1103/PhysRevLett.111.251301)
First SuperCDMS single-charge limits (HVeV), Phys. Rev. Lett. 121, 051301 (2018) (https://link.aps.org/doi/10.1103/PhysRevLett.121.051301)
Projected sensitivity of SuperCDMS SNOLAB, OSTI/arXiv (2017) (https://www.osti.gov/servlets/purl/1350517)
SuperCDMS SNOLAB status update, SNOLAB Indico (Feb 2025) (https://indico.snolab.ca/event/19/contributions/254/)
Fermilab CDMS II first underground results news release (2004) (https://news.fnal.gov/2004/05/first-data-deep-underground-experiment-narrow-search-dark-matter/)