The Goal of ADMX

The Axion Dark Matter Experiment searches for the Axions that may make up the dark matter halo in our galaxy. ADMX will help us understand both the mysterious dark matter that permeates our universe and the strong force that holds atomic nuclei together.

The Detection Process

Some Dark Matter Axions passing through the ADMX detection cavity will interact with the magnetic field generated by the superconducting magnet and decay into microwave photons with a frequency proportional to the Axion mass. These microwaves will be picked up by an antenna, amplified, and seen as an excess of power at a frequency corresponding to the mass of the Axion.

Why Detect Microwave Photons?

ADMX uses the prediction that incoming Axions would convert into photons when in the presence of a strong magnetic field (by smashing into the photons of the magnetic field itself). The Axion is also coupled to other matter, but the predictions for that kind of interaction are not as uncertain. This means that we are fairly sure what the rate of conversion of Axions to photons would be under some given magnetic field. Axions also interact with other particles, like with nucleons and electrons (this is thought to be happening in the Sun), but the rate of interaction with them is a lot more uncertain, so it would be more difficult to tell whether or not we should move on to a different frequency. For example, if the coupling rate of an Axion to an electron is very small, we could have just gotten tired of waiting and moved on, when in reality a detection was right around the corner. Therefore trying to detect Axions via photon coupling is a better choice, since we are much more certain of when to move on. Besides, this method has the potential to kill two birds with one stone (the dark matter problem and the strong CP problem).

The Apparatus

Parts
Phase 1

The resonant microwave detection cavity is the heart of ADMX. The cavity is only sensitive to a very small frequency range at any given time, so two rods are used to slowly scan over the frequencies corresponding to possible Axion masses, like tuning a radio to find a specific station. Axion decay is stimulated by a magnetic field, so to see as strong a signal as possible, the detection cavity is placed inside a 8 Tesla superconducting magnet.

The expected signal from Axion decay is so small that even if perfectly tuned at our full magnet field strength, the signal would be overwhelmed by the thermal microwaves emitted by the walls of the cavity at room temperature. Therefore, the entire experiment is cooled to below 4.2 degrees Kelvin with liquid helium to provide a quiet listening environment.

The small Axion decay signal must be amplified to a level where it can be recorded by the data aquisition electronics. The speed at which we can scan over Axion masses is determined by how cold we can make our cavity and the quality of our amplifiers, so experimental upgrades focus on improving our amplifier noise Phase 1 and cooling the cavity better (Phase 2).