The Axion Dark Matter Experiment (ADMX, also written as ''Axion Dark Matter eXperiment'' in the project's documentation) is an experiment that uses a resonant microwave cavity within a large superconducting magnet to search for cold dark matter

axions An axion () is a hypothetical elementary particle originally theorized in 1978 independently by Frank Wilczek and Steven Weinberg as the Goldstone boson of Peccei–Quinn theory, which had been proposed in 1977 to solve the strong CP problem in ...

in the local galactic

dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...

halo. Unusual for a dark matter detector, it is not located deep underground. Sited at the Center for Experimental Nuclear Physics and Astrophysics (CENPA) at the

University of Washington The University of Washington (UW and informally U-Dub or U Dub) is a public research university in Seattle, Washington, United States. Founded in 1861, the University of Washington is one of the oldest universities on the West Coast of the Uni ...

, ADMX is a large collaborative effort with researchers from universities and laboratories around the world.

Background

The

axion An axion () is a hypothetical elementary particle originally theorized in 1978 independently by Frank Wilczek and Steven Weinberg as the Goldstone boson of Peccei–Quinn theory, which had been proposed in 1977 to solve the strong CP problem ...

is a hypothetical elementary particle originally postulated to solve the

strong CP problem The strong CP problem is a question in particle physics, which brings up the following quandary: why does quantum chromodynamics (QCD) seem to preserve CP-symmetry? In particle physics, CP stands for the combination of C-symmetry (charge conjugati ...

. The axion is also an extremely attractive

candidate. The axion is the puzzle piece allowing these two mysteries to fit naturally into our understanding of the universe.

Strong CP problem

The axion was originally postulated to exist as part of the solution to the "strong CP problem". This problem arose from the observation that the

strong force In nuclear physics and particle physics, the strong interaction, also called the strong force or strong nuclear force, is one of the four known fundamental interactions. It confines quarks into protons, neutrons, and other hadron particles, an ...

holding nuclei together and the weak force making nuclei decay differ in the amount of

CP violation In particle physics, CP violation is a violation of CP-symmetry (or charge conjugation parity symmetry): the combination of C-symmetry (charge conjugation symmetry) and P-symmetry ( parity symmetry). CP-symmetry states that the laws of physics s ...

in their interactions.

Weak interaction In nuclear physics and particle physics, the weak interaction, weak force or the weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interaction, and gravitation. It is th ...

was expected to feed into the

strong interactions In nuclear physics and particle physics, the strong interaction, also called the strong force or strong nuclear force, is one of the four known fundamental interactions. It confines quarks into protons, neutrons, and other hadron particles, a ...

(

QCD In theoretical physics Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain, and predict List of natural phenomena, natural phenomena. This is in ...

), yielding appreciable

CP violation, but no such violation has been observed to very high accuracy. One solution to this Strong CP Problem introduces a new particle called the

. If the axion is very light, it interacts so weakly that it would be nearly impossible to detect but would be an ideal dark matter candidate. The ADMX experiment aims to detect this extraordinarily weakly coupled particle. 1e0657 scale

Dark matter

Although dark matter can't be seen directly, its gravitational interactions with familiar matter leave unmistakable evidence for its existence. The universe today would not look the same without dark matter. Approximately five times more abundant than ordinary matter, the nature of dark matter remains one of the biggest mysteries in physics. In addition to solving the

, the axion could provide an answer to the question "what is dark matter made of?" The axion is a neutral particle that is extraordinarily weakly interacting and could be produced in the right amount to constitute dark matter. If the dark matter accounting for the bulk of all matter in our universe is axions, ADMX is one of only a few experiments able to detect it.

History

Pierre Sikivie invented the axion haloscope in 1983. After smaller scale experiments at the

University of Florida The University of Florida (Florida or UF) is a public university, public land-grant university, land-grant research university in Gainesville, Florida, United States. It is a senior member of the State University System of Florida and a preem ...

demonstrated the practicality of the axion haloscope, ADMX was constructed at

Lawrence Livermore National Laboratory Lawrence Livermore National Laboratory (LLNL) is a Federally funded research and development centers, federally funded research and development center in Livermore, California, United States. Originally established in 1952, the laboratory now i ...

in 1995. In 2010 ADMX moved to the Center for Experimental Physics and Astrophysics (CENPA) at the

. Led by Dr. Leslie Rosenberg, ADMX is undergoing an upgrade that will allow it to be sensitive to a broad range of plausible dark matter axion masses and couplings.

Experiment

The experiment is designed to detect the weak conversion of dark matter axions into microwave photons in the presence of a strong magnetic field. If the hypothesis is correct, an apparatus consisting of an 8 tesla magnet and a cryogenically cooled high-Q tunable microwave cavity should stimulate the conversion of axions into photons. When the cavity's resonant frequency is tuned to the axion mass, the interaction between nearby axions in the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

halo HALO, halo, halos or haloes may refer to: Most common meanings * Halo (optical phenomenon) * Halo (religious iconography), a ring of light around the image of a head * ''Halo'' (franchise), a sci-fi video game series (2001–2021) Arts and en ...

and ADMX's magnetic field is enhanced. This results in the deposit of a tiny amount of power (less than a yoctowatt) into the cavity. An extraordinarily sensitive microwave receiver allows the weak axion signal to be extracted from the noise. The experiment receiver features quantum-limited noise performance delivered by a Superconducting QUantum Interference Device (SQUID) amplifier and lower temperatures from a ³He refrigerator. ADMX is the first experiment sensitive to realistic dark-matter axion masses and couplings and the improved detector allows a more sensitive search. ADMX magnet installation

Cavity

The microwave cavity within the magnet bore is at the heart of ADMX. It is a circular cylinder, 1 meter long and 0.5 meter diameter. ADMX searches for axions by slowly scanning the cavity resonant frequency by adjusting positions of two tuning rods within the cavity. A signal appears when the cavity resonant frequency matches the axion mass. The expected signal from axion decay is so small that the entire experiment is cooled to well below 4.2 kelvin with a liquid helium refrigerator to minimize thermal noise. The electric field within the cavity is sampled by a tiny antenna connected to an ultra-low-noise microwave receiver.

Receiver

The ultra-low noise microwave receiver makes the experiment possible. The dominant background is thermal noise arising from the cavity and the receiver electronics. Signals from the cavity are amplified by a Superconducting QUantum Interference Device (SQUID) amplifier followed by ultralow noise cryogenic

HFET A high-electron-mobility transistor (HEMT or HEM FET), also known as heterostructure FET (HFET) or modulation-doped FET (MODFET), is a field-effect transistor incorporating a junction between two materials with different band gaps (i.e. a heter ...

amplifiers. The receiver then downconverts microwave cavity frequencies to a lower frequency that can be easily digitized and saved. The receiver chain is sensitive to powers smaller than 10 rontowatts; this is the world's lowest-noise microwave receiver in a production environment.

Progress

In 2010, ADMX eliminated one of the two axion benchmark models from 1.9 μeV to 3.53 μeV, assuming axions saturate the Milky Way's halo. A 2016 upgrade should allow the ADMX to exclude or discover 1 μeV to 40 μeV dark matter axions.

SQUID amplifiers

In the first implementation of the experiment in 1996, the amplifier

noise temperature In electronics, noise temperature is one way of expressing the level of available noise power introduced by a component or source. The power spectral density of the noise is expressed in terms of the temperature (in kelvins) that would produce ...

was around 2 K. In 2009, the first stage amplifier was replaced by a SQUID amplifier, which greatly lowered the noise (to less than 100 mK) and vastly improved sensitivity. ADMX has demonstrated that the SQUID amplifier allows for quantum-limited-power sensitivity. In 2016, ADMX acquired Josephson Parametric Amplifiers which allow quantum noise limited searches at higher frequencies.

Dilution refrigerator

The addition of a dilution refrigerator was the main focus of the 2016 upgrade program. The dilution refrigerator allows cooling the apparatus to 100 mK or less, reducing the noise to 150 mK, which makes data taking 400 times faster. This makes it the "Definitive Experiment".

References

External links

ADMX web site
{{Dark matter Experiments for dark matter search