To catch an axion

As with WIMP dark-matter candidates, one of the main points about axion or axion-like dark-matter candidates is that the proposed particles are hard to detect.

This is due to their inherent nature- they interact only very weakly with ordinary matter. Even though they are swarming through us- as you read this, for example- we simply don’t notice them, because of that propensity for what is more formally termed “a very small cross-section”.

So we must come up with a way to make such an elusive thing detectable. And, as with WIMP candidates, the main method used for axions is a little indirect- we look not for the axion itself, but for the result of the axion interacting with something, with that interaction producing something that we can detect. It’s still classed as “direct detection”, though, since the particle itself interacts directly with our equipment in some way in order to produce a detectable effect.

In this case, the method is based in what is known as the inverse Primakoff effect. In essence, this says that an axion may interact with a strong magnetic field in order to produce a photon of a specific frequency. This frequency is related to the mass of the axion (whatever that may be). Thus, if we can detect and determine the frequency of the resultant photon, we can determine the mass of the originating axion (not to mention confirm that it exists!).

Although we may not know the exact mass of an axion, we do have an idea of the range of masses that they might have. The lower and upper limits of that range come from astronomical observation and cosmological theory. That mass range can be used to determine a practical frequency range for the anticipated photons- this happens to be in the microwave region of the electromagnetic spectrum, and we do know how to work with that!

(When I say “practical frequency range”, it is to be noted that the theoretical limits for the wavelength of the resulting photons range from a wavelength on the scale of an entire galaxy (so a very, very, very low frequency…) to wavelengths in the visible part of the electromagnetic spectrum. In practical terms, we can’t explore all of that, so we meed to narrow the range somewhat!)

The most common axion detection concept is as follows: a cylindrical metal enclosure (called a cavity) is encased by an electromagnet, which provides a strong and uniform magnetic field throughout the cavity. The dimensions and internal structure of the cavity are tuned to be resonant with a specific microwave frequency. An antenna within the cavity samples the amount of microwave energy within the cavity.

If the resonant frequency of the cavity happens to match the frequency associated with the axion mass, excess power will be detected within the cavity (as a result of axion-photon conversion), as compared with the conditions when the magnet is off, or when the cavity is tuned to a different frequency.

Such a device is known as a “haloscope” (since it investigates the dark matter halo of our galaxy), and was proposed by the physicist Pierre Sikivie.

Since we don’t know the axion mass, we don’t know the exact required frequency… so the cavity is designed to be able to be tuned to different microwave frequencies. Over time, that resonant frequency is changed, and thus the frequency space associated with the proposed axion mass range is sampled over time.

If a peak is detected in the antenna’s output, then the frequency to which the cavity is tuned at that time may be used to estimate the axion mass.

That’s the essence of it. There are (unsurprisingly) some caveats. The expected power from an axion signal is extremely small… so many of the design requirements are correspondingly extreme- sections of the apparatus need to be cooled to only a few degrees above absolute zero; the electronics must introduce only the minutest amounts of noise into the signal path; the magnetic field strength must be high (around 160,000 times that of the Earth) but very uniform; and more besides.

But all these requirements have been achieved, and are even being improved upon. People are searching for dark-matter axions using such haloscopes.

In a coming post, we’ll have a look at the ORGAN Experiment at the University of Western Australia and its continuing mission to explore the axion photon coupling-mass phase space, going boldly where no physicists have gone before.

[With thanks to Ben McAllister of UWA for comments and improvements.]

An axion enters the cavity within the haloscope, interacting with the magnetic field and converting into a microwave photon. The resonant frequency of the cavity is changed by moving tuning rods (or some other structure) within the cavity. When the resonant frequency happens to match the axion-produced photon’s frequency, a peak in the microwave power within the cavity is noted. (Image by C. Boutan/Pacific Northwest National Laboratory; adapted by the American Physical Society/Alan Stonebraker.)