Dark matter might be ‘light’

Most of the matter in the universe is not in the form of atoms and molecules, but is something else entirely. We call it dark matter: an unknown substance that we cannot see because it does not emit or reflect light.

We know that dark matter exists because of the ‘fingerprints’ it leaves on the visible matter of the universe. Without it there would be no galaxies like our Milky Way and the universe would be a very different place.

We don’t know what dark matter is yet, but we have good reasons to believe that it is made up of new elementary particles. One of the biggest challenges in science today is to directly observe these particles for the first time.

But dark matter particles very rarely interact with ordinary matter. Experiments searching for collisions between dark matter and atoms must be protected from interference that would otherwise overwhelm this very rare signal.

For this reason, detectors are shielded by huge amounts of rock in deep underground laboratories.

Another major challenge is that we don’t know the mass of the dark matter particles we are searching for. Most experiments are sensitive to dark matter that is roughly 10-1000 times heavier than a hydrogen atom.

But it’s possible that dark matter could turn out to be much lighter, which makes detecting it even more challenging.

Imagine the collision of two balls, but one of them is invisible: If a visible bowling ball were to bump into an invisible one, they would both recoil, and we could infer the existence of the invisible ball by the movement of the other.

But if an invisible ping-pong ball were to bump into a bowling ball, the heavier ball would barely move – and we would have no idea that the ping-pong ball even existed.

So too, if a light dark matter particle bumps into a much heavier atom in our detector, we would not notice that anything had happened.

Our team of theoretical particle physicists at the University of Melbourne’s School of Physics and the ARC Centre of Excellence for Dark Matter Particle Physics have been exploring several ideas to overcome this problem.

First, we could use a detector made of light atoms instead of heavy ones. In other words, replace some of the bowling balls with ping-pong balls.

The current state-of-the-art dark matter detectors are made from heavy xenon atoms. Adding a small amount of hydrogen (the lightest element) to these detectors would be enough to do the trick.

This is an idea that the HydroX group is pursuing – a proposed upgrade to the LUX-ZEPLIN dark matter experiment located at the Sanford Underground Research Facility in the USA.

Second, we could look for a different signal that is easier to detect. If a dark matter particle hits an atomic nucleus, it could occasionally cause the atom to shake off an electron – the so-called ‘Migdal effect’.

This ejected electron can be seen even if the recoil of the nucleus itself is undetectable. The most sensitive searches for light dark matter currently use this technique, using theoretical predictions calculated by researchers at the University of Melbourne.

Our team has recently demonstrated that combining these two techniques could be especially effective. It would enable the detection of dark matter 200-times lighter than a hydrogen atom.

A third way to hunt for light dark matter is to look for ‘boosted’ dark matter – meaning dark matter that is much more energetic than usual.

The dark matter in our galaxy moves around relatively slowly (about one million kilometres per hour – which is actually very slow for an elementary particle). However, a small amount of the dark matter is likely to be moving much faster.

There are lots of ways we might get some higher-energy, high-speed dark matter, but the most compelling is ‘cosmic ray dark matter’.

The galaxy is awash with high-energy cosmic ray particles that zip around at close to the speed of light. These particles traverse the large distances between stars, which is filled with a background of slowly moving dark matter particles.

If a high-energy cosmic ray hit a slow-moving light dark matter particle it would transfer a large amount of energy to it – giving it a kick that would send the dark matter particle off at close to the speed of light.

This faster, higher energy dark matter would be much easier to observe in our detectors, even if the dark matter particles are very light. Even a ping-pong ball, if travelling fast enough, can cause a bowling ball to change direction.

This extra kick of energy can even allow the boosted dark matter to be seen in detectors designed to measure neutrinos, another common but incredibly hard to detect particle. Neutrino detectors are usually blind to the tiny energy deposits from normal low-energy dark matter.

The ability to see energetic dark matter in neutrino detectors would be a big advantage, because they are typically thousands of times larger than dark matter detectors.

Our team has shown in a recent preprint article that the future neutrino experiment JUNO would be an excellent probe of cosmic-ray dark matter.

All of these proposed techniques can be realised with existing and in-development experiments. They allow us to enhance or obtain extra information from existing dark matter experiments or planned neutrino experiments.

Ultimately, by expanding the types of dark matter that can be detected, these ideas will enable us to learn more about this elusive ghost.

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