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Particle Physics

Dark Matter

Working underground to learn about dark matter throughout the universe

Although invisible to our telescopes, dark matter is known by its gravitational effects throughout the universe. The nature of dark matter is unknown, but the consensus of the astrophysics and particle physics communities is that the dark matter is composed of new fundamental particles associated with an unknown sector of physics. PNNL is involved in the search for signals from dark matter particle interactions in the SuperCDMS, PICO, and MiniClean experiments. A group of international physicists collaborate to conduct experiments using extremely low-background radiation detectors in deep underground laboratories around the world.

PNNL's experience in ultra-low background materials helps reduce the natural and cosmically induced background that could overwhelm the extremely rare dark matter events—the common challenge for all these experiments. Our expertise in copper electroforming, uranium and thorium assay, and detector assembly are making order-of-magnitude improvements in sensitivity.


Custom detectors under development for the SuperCDMS at SNOLAB in Sudbury, Ontario, Canada.

Cryogenic Dark Matter Search, or CDMS—unique germanium detectors search for dark matter interactions and cryogenic bolometry to identify background events.

The SuperCDMS Collaboration is an international collaboration of nearly 100 scientists who are dedicated to experiments designed to investigate the hypothesis that particles outside the Standard Model of Particle Physics are the solution to the dark matter problem. These particles would be flowing through the laboratory and may, rarely, interact with a particle detector, causing nuclear recoils in the detector.

The SuperCDMS experiment aims to measure the recoil energy imparted to a nucleus from collisions with dark matter particles by employing detectors that are highly sensitive to the ionization and phonon signals that result from a dark matter-nucleus collision. Known as interleaved Z-sensitive Ionization Phonon, or iZIP detectors, feature state-of-the-art superconducting thin films deposited on germanium and silicon crystals to accurately measure information about particle interactions.

The SuperCDMS experiment is transitioning from the Soudan Underground Laboratory in Minnesota to the deeper SNOLAB facility in Sudbury, Canada. The SNOLAB location provides significantly improved shielding from cosmic rays, which are a source of background in the weakly interacting massive particle (WIMP) search.

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PICO revived the bubble chamber technique that is extremely sensitive to low-energy nuclear interactions indicative of dark matter. PICO has the world's best sensitivity to dark matter interactions that depend on nuclear spin. PICO experiments are conducted at the SNOLAB underground laboratory in Sudbury, Ontario, Canada.


A cross section of the MiniCLEAN detector

The MiniCLEAN detector deployed at SNOLAB Sudbury, Ontario, Canada.

The MiniCLEAN detector deployed at SNOLAB Sudbury, Ontario, Canada.

Mini-Cryogenic Low-Energy Astrophysics with Nobles, or MiniCLEAN, is a small experiment with about 15 members in the collaboration, led by Pacific Northwest National Laboratory. MiniCLEAN aims to detect weakly interacting massive particles, or WIMPs, a current favorite dark matter candidate. The MiniCLEAN detector is located in the SNOLAB facility in Sudbury, Canada. The SNOLAB location provides significantly improved shielding from cosmic rays, which are a source of background in the WIMP search.

The detector will be filled with over 500 kilograms of very cold, dense, ultra-pure materials—argon at first, and later neon. If a WIMP passes through and collides with an atom's nucleus, it will produce a pulse of light with a unique signature. The use of both argon and neon will allow MiniCLEAN to double-check any possible signals. Argon is more sensitive than neon, so a true dark matter signal would disappear when liquid argon is replaced with liquid neon. Only an intrinsic background signal from the detector would persist. Scientists would like to eventually scale this experiment up to a larger version (~200 tons) called CLEAN.

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  • Chris Jackson

The Axion Dark Matter eXperiment

All astrophysical and cosmological data point convincingly to a large component of cold dark matter (CDM) in the universe for which a light axion is a well-motivated candidate. It has long been known that axions constitute the dark matter of our own Milky Way galactic halo and may be stimulated to convert into a narrow-band microwave photon signal by an apparatus consisting of a microwave-cavity resonator permeated by a static magnetic field.

The Axion Dark Matter eXperiment (ADMX) is the first experiment sensitive to realistic dark-matter axion masses and couplings and the improved detector allows an even more sensitive search. ADMX uses a resonant microwave cavity within a large superconducting magnet to search for cold dark matter axions in the local galactic dark matter halo. Sited at the Center for Experimental Physics and Astrophysics at the University of Washington, ADMX is a large collaborative effort with researchers from universities and laboratories around the world.

ADMX detects the very weak conversion of dark matter axions into microwave photons. Axion conversion into photons is stimulated by an apparatus consisting of an 8 Tesla magnet and a cryogenically cooled high-Q tunable microwave cavity. When the cavity's resonant frequency is tuned to the axion mass, the interaction between nearby axions in the Milky Way halo and ADMX's magnetic field is enhanced. This results in the deposit of a very tiny amount of power (less than a yoctowatt) into the cavity.

An extraordinarily sensitive microwave receiver allows the very weak axion signal to be extracted from the noise. The experiment receiver features quantum-limited noise delivered by an exotic Superconducting QUantum Interference Device (SQUID) amplifier and lower temperatures from a 3He refrigerator.

PNNL joined the ADMX collaboration in 2015 and is responsible for the design, modeling, fabrication, and testing of the microwave receiver electronics for each frequency range and the design and integration of a coherent data acquisition and analysis framework that permits automated operation of the experiment and blind analysis of the data.

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Darkside-20k is a dual-phase argon time projection chamber (TPC) that combines the very powerful scintillation pulse-shape analysis and ionization information to discriminate against background events. Two unique aspects to the Darkside program are the use of an external neutron veto, and the use of low radioactivity argon from underground sources as the target. Scientists at PNNL were instrumental in the production of the low-radioactivity argon target for the Darkside-50 experiment. Combining all these techniques allows Darkside-20k to achieve a background-free 100 ton-year exposure accumulated in a 5-year run. The Darkside-20k detector will be located at the Laboratori Nazionali del Gran Sasso (LNGS) in Italy, and is scheduled to start taking data in 2021.

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