ORGAN Experiment Rules Out Dark Matter Candidate

The hunt for dark matter is now on in earnest in Australia. The ORGAN (Oscillating Resonant Group AxioN) experiment, hosted at the University of Western Australia’s (UWA) Quantum Technologies and Dark Matter Laboratory, has just completed its first comprehensive search for the mysterious substance, and it has already given scientists something to think about.

The reason is not because ORGAN has discovered what dark matter is. Instead, the first set of findings are a little more nuanced. This first discovery is all about what dark matter is not.

To explain, we’ll begin with what we know about dark matter. Its first property is that it has mass. In fact, if we were to weigh all the dark matter in the Universe, we would find it is about 5 times heavier than all the stars and planets and other regular stuff combined.

But despite there being such an enormous amount of it we can’t see it. This is its second property. It is completely invisible to us and our instruments, not because it is too far away, or that we don’t know where to look, but simply because it does not emit, reflect or interact with any light in any frequency, at all.

Of course, the fact that it is invisible makes dark matter very difficult to find, and we have been trying to work out what it is, and how it fits into the bigger picture, since around the 1930s when it was first postulated to exist. We know that it does exist because we see the gravitational effect it has on other objects (like stars and galaxies) as a result of its mass. If it turned out that it wasn’t there after all, it would mean that we have a fundamental misunderstanding of how gravity works. And that is something we thought we were pretty much on top of, thanks to the work of Newton and Einstein.

Being invisible has not deterred physicists from developing all sorts of theories about what dark matter might actually be, and we have a list of potential candidates ranging from incredibly small hypothetical particles to enormous unseen gobs of normal matter. And most theories, at least the credible ones, can be tested by means of experimentation.

Dark matter studies are designed around finding particles of a particular mass. The MACHO, EROS and OGLE projects that began in the 1990s, for example, worked on the assumption that there were enough planets and brown dwarf stars that were yet to be found that they could account for the missing mass attributed to dark matter. It seems unlikely now that dark matter could be explained this way.

The DAMA/LIBRA and the soon-to-begin Australian SABRE experiments are designed to look for something a bit smaller. They are searching for hypothetical dark matter particles that are just a bit more massive than a proton. If they exist, the theory says that they will occasionally bump into atomic nuclei and transfer a little bit of energy, enough so that super-sensitive instruments might be able to detect the collision. There have been a few odd results while searching for these particles that need further investigation, and Australia will shortly be able to shed some light on what is going on.

An even smaller hypothetical particle is the axion. So light that it can barely be classified as a particle, an axion is around a hundred billion times lighter than an electron. An experiment known as ADMX has been searching for axions at the lower end of the mass scale, and ORGAN, the Australian experiment run from UWA, has been searching for heavier axions. Neither experiment has had a positive detection as yet, but ADMX, and now ORGAN, have been able to rule out the existence of axions of certain masses.

Aaron Quiskamp is a PhD student at UWA and works with Dr Ben McAllister who designed and commissioned ORGAN. He explained why this is such an important result even though axions are still yet to be detected.

“We performed the most sensitive search so far for axion dark matter in a particular mass range,” he said.

“Although we didn’t find any, it’s very exciting because it’s Australia’s first large-scale, long-term direct dark matter detection experiment. It’s also given us useful information about what axion dark matter isn’t, which tells future axion searches across the globe where not to look.”

The experiment uses a type of detector that was invented in the 1980s called an axion haloscope. The haloscope is designed to detect the very weak conversion of dark matter axions into tiny flashes of light in the presence of a strong magnetic field.

“When we don’t see any little flashes, as was the case this time, we instead place exclusion limits, where we rule out axions that our experiment would have been sensitive to. Then, we tell the rest of the dark matter community ‘no dark matter here’ and move on to search for axions of a different mass,” said Dr McAllister.

No axion discovery yet then, but the team is currently making improvements to the detectors to achieve even greater sensitivity and to enable wider-ranging searches. When that’s complete, they’ll be back to probe an entirely new range of axion masses that are as yet unexplored.

ORGAN is Australia’s first major contribution to dark matter detection and will soon be followed by the Stawell Underground Physics Lab and the SABRE experiment. Together they are bringing us ever closer to solving one of the biggest mysteries of the universe.