Axion Dark Matter Experiment

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 holding nuclei together and the weak force making nuclei decay differ in the amount of CP violation in their interactions. Weak interaction was expected to feed into the strong interactions (QCD), yielding appreciable QCD CP violation, but no such violation has been observed to very high accuracy. One solution to this Strong CP Problem ends up introducing a new particle called the axion. 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.

The Bullet Cluster: HST image with overlays. The total projected mass distribution reconstructed from strong and weak gravitational lensing is shown in blue, while the X-ray emitting hot gas observed with Chandra is shown in red.

Although dark matter can't be seen directly, its gravitational interactions with familiar matter leave unmistakable evidence for its existence. The universe we see today simply wouldn't look the way it does without dark matter. Approximately five times more abundant than ordinary matter, the nature of dark matter remains one of the biggest mysteries in physics today. In addition to solving the strong CP problem, 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.

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.

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 an exotic cryogenic Superconducting QUantum Interference Device (SQUID) amplifier followed by ultralow noise cryogenic HFET 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 0.01 yoctowatts; this is the world's lowest-noise microwave receiver in a production environment.

Several years ago, the ADMX amplifier noise temperature was around 2 K. Recently the amplifiers were replaced by SQUID amplifiers, 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. More recently, ADMX has acquired Josephson Parametric Amplifiers which allow quantum noise limited searches at higher frequencies.

The addition of a dilution refrigerator is the main focus of the ADMX upgrade program. The dilution refrigerator allows cooling the apparatus down to 100 mK or less, reducing the noise to 0.15 K, which makes data taking 400 times faster. This makes it the "Definitive Experiment".

The Haloscope at Yale Sensitive to Axion CDM, or HAYSTAC (formerly known as ADMX-High Frequency), hosted at Yale University, is using a Josephson Parametric Amplifier, 9 T magnet, and microwave cavity with radius of 5 cm and height 25 cm to search masses 19–24 µeV.