This Australian experimenter is looking for elusive particles that could help unlock the mysteries of dark matter

Australian scientists are making strides to solve one of the universe’s greatest mysteries: the invisible nature of “dark matter”.

The ORGAN Experiment, Australia’s first major dark matter detector, recently completed the search for a hypothetical particle called an axion — a popular candidate among theories trying to explain dark matter.

ORGAN has placed new limits on the possible characteristics of axions and thus helped to narrow their search. But before we get ahead of ourselves…

Let’s start with a story

About 14 billion years ago, all the tiny bits of matter – the fundamental particles that would later become you, the planets and galaxies – were compressed into one very dense and hot region.

Then the Big Bang happened and everything flew apart. Particles combine to form atoms, which eventually clump together into stars, which explode and create all kinds of exotic matter.

After a few billion years came the Earth, which finally crawled on the little things called humans. Cool story, right? Turns out that wasn’t the whole story; it’s not even half.

Humans, planets, stars and galaxies are all made of “ordinary matter.” But we know ordinary matter is only one-sixth of all matter in the universe.

The rest is made of what we call “dark matter”. His name tells you almost everything we know about him. It emits no light (so we call it “dark”) and has mass (so we call it “matter”).

If it’s not visible, how do we know it’s there?

When we observe the way things move in space, we repeatedly find that we cannot explain our observations if we consider only what we can see.

The rotating galaxy is a great example. Most galaxies rotate at speeds that cannot be explained by the gravitational pull of visible matter alone.

So there must be dark matter in these galaxies, giving them extra gravity and allowing them to spin faster – without parts of it being thrown out into space. We think dark matter really holds galaxies together.

So there must be a huge amount of dark matter in the universe, pulling up everything we can see. It also passes by you, like some kind of cosmic ghost. You just can’t feel it.

How can we detect it?

Many scientists believe dark matter may consist of hypothetical particles called axions. Axion was originally proposed as part of a solution to another big problem in particle physics called the “strong CP problem” (which we can write about throughout the article).

However, after the axion was proposed, scientists realized that it could also form dark matter under certain conditions. That’s because the axion is expected to have very weak interactions with ordinary matter, but still have mass: two necessary conditions for dark matter.

So how do you look for axion?

Since dark matter is thought to be all around us, we can build detectors right here on Earth. And, fortunately, the theory that predicts the axion also predicts that the axion can turn into a photon (particle of light) under the right conditions.

This is good news, because we are great at detecting photons. And this is what ORGANS do. It engineered the right conditions for axion-photon conversion and looked for weak photon signals – tiny flashes of light produced by dark matter passing through the detector.

This kind of experiment is called the axion haloscope and was first proposed in the 1980s. There are several in today’s world, each slightly different in important ways.

Illuminating dark matter

An axion is believed to turn into a photon in the presence of a strong magnetic field. In a typical haloscope, we generate this magnetic field using a large electromagnet called a “superconducting solenoid”.

Inside the magnetic field, we place one or more hollow spaces of metal, which are meant to trap photons and cause them to bounce inside, making them easier to detect.

However, there was one hiccup. Everything that has a temperature constantly emits tiny random flashes of light (which is why thermal imaging cameras work). This random emission, or “noise,” makes it difficult to detect the faint dark matter signal we seek.

To get around this, we have placed our resonator in a “diluent refrigerator”. This luxury refrigerator cools the experiment down to a cryogenic temperature, around 273°C, which greatly reduces noise.

The colder the experiment, the better we can “hear” the faint photons produced during dark matter conversion.

Targeting mass territories

Axions with a certain mass will turn into photons with a certain frequency or color. But since the mass of the axion is unknown, the experimenters had to target their search to a different region, focusing on the regions where dark matter was thought to be more likely to exist.

If no dark matter signal is found, then the experiment is not sensitive enough to hear the signal above the noise, or there is no dark matter in the appropriate axion mass region.

When this happens, we set an “exceptional limit” – which is just a way of saying “we found no dark matter in this mass range, up to this level of sensitivity”. This tells the other dark matter research community to direct their search elsewhere.

ORGAN is the most sensitive experiment in the targeted frequency range. The process recently detected no dark matter signal. These results have set an important exception limit to the possible characteristics of the axion.

This is the first stage of a multi-year plan to find axion. We are preparing for our next experiment, which will be more sensitive and target new, unexplored mass ranges.

But why is dark matter important?

For example, we know from history that when we invest in basic physics, we end up developing important technologies. For example, all modern computing relies on our understanding of quantum mechanics.

We would never have discovered electricity, or radio waves, if we had not pursued what, at the time, seemed to be a strange physical phenomenon beyond our comprehension. Dark matter is the same.

Consider all that humanity has achieved by understanding only one-sixth of the matter in the universe – and imagine what we could do if we opened up the rest.

Ben McAllister works for The University of Western Australia. The work referenced in this article was funded by the Australian Research Council.

/Courtesy of The Conversation. Material from this original organization/author may be timely, edited for clarity, style and length. The views and opinions expressed are those of the author.

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