ORGAN: WISP Direct Detection Research

The Centre’s main effort in WISP detection is what is known as The Oscillating Resonant Group AxioN Experiment, or ORGAN for short.

ORGAN is a type of detector known as a “haloscope”, which tries to detect dark matter axions as they pass through the laboratory.

Rather than relying on energy imparted when an axion collides with a standard model particle, haloscopes like ORGAN rely on what is called the axion-photon coupling. Essentially, if we engineer the correct conditions inside the detector, and axions are passing through, we can force a small number of the axions to convert into photons, which are particles of light.

So, we can take the invisible dark matter and convert it into a tiny flash of light which we can then detect.

Fortunately, while we cannot see or touch dark matter directly, we are fantastic at detecting light – it is the thing we are best at detecting as a species. So, this technique of converting something invisible into something we can readily observe is promising.

There are, of course, many technical design considerations. Chiefly, the whole experiment needs to be well shielded from any other source of light, since the number of photons produced by axion conversion is very small, and any background would wash out the signal. This is similar to the need for shielding by going underground in SABRE and other WIMP experiments.

UWA Group in front of the dilution refrigerator which houses The ORGAN Experiment

However, as the nature of the background for WIMP and axion detectors are different - the ORGAN experiment does not need to be located underground. Instead, it must be cooled to cryogenic temperatures inside a special tool called a dilution refrigerator. Housing the experiment in the fridge and cooling it reduces not only external background light interference, but the thermal radiation which is given off by all of the components in the detector itself.

To stimulate axion-photon conversion, we need a strong magnetic field. Axions interact with the magnetic field, and convert into flashes of light. To enable this, our cryogenic setup is fitted with a 12.5 Tesla superconducting solenoid, which is about 12 times as strong as a magnet that might be used in a scrap yard to pick up a car.

With the very cold, strong magnetic field in place, reducing the background and stimulating axion-photon conversion, all we need is the detector designed to catch the photons, and measure them.

In ORGAN this detector takes the form of a resonant cavity – essentially a metallic can – which traps the photons which are generated, and causes them to “resonate”, or bounce around for a while, thus enhancing our prospects for observing them.

The number of photons inside the resonator is monitored carefully over time, and any spikes above the expected background, and which have the expected signal characteristics, can be attributed to axions!

Unfortunately, we don’t know the mass of the axion, which means we don’t know the frequency (or equivalently, colour) of the photons that are expected to be generated. As a result, we need to build detectors like ORGAN to be “scannable”, meaning we can set them to search for a specific axion mass, and then scan to a different mass if no axions are observed.

ORGAN is commencing operation with Phases 1a and 1b in 2021, and will search for axions in a two targeted mass ranges, before being upgraded several times throughout Phase 2, commencing later in the life of the Centre.