“Where did you come from, where did you go, where did you come from, neutrino?”
This 1880’s folk tune – Cotton-Eyed Joe – now popular with line dancers, could well have been adapted to be the theme of last week’s gathering of scientists from around the world who gathered in Sydney to grapple with the challenge of how to tell what direction a theoretical dark matter particle has come from.
That this is the topic of an entire gathering of dark matter scientists gives some clue as to the importance of this question. A decision from the conference could turbo-charge the hunt for the answer.
The reason directionality is such a focus is because it is currently the most promising way to tell the difference between a dark matter particle – also known as a Weakly Interacting Massive Particle, or WIMP – and a neutrino, which is similar in almost every way.
Dark matter is the term to describe the vast amount of matter – around 85 percent of all matter in the universe – that is effectively invisible, in that it does not interact electrically or emit any kind of radiation. The only clues to its existence come from its gravitational effects on matter, such as stars and galaxies.
Scientists have long searched for the elusive dark matter particle, building ever more powerful and sensitive underground detectors, such as the Jadugoda Underground Science Laboratory, the Sanford lab in the US, or the Stawell Underground Physics Laboratory, built one kilometre below the surface in an old Victorian gold mine to reduce interference from other sources of radiation.
Read: Search for dark matter: Stawell’s genius lair
But as these detectors become more and more sensitive, they are encountering the problem of ‘noise’ from other particles – specifically, neutrinos, which are equally enigmatic and hard-to-detect – that could drown out the very faint signal of dark matter particles.
“At some point neutrinos will dominate the background, so every time we build a larger detector we need to reduce this background,” says physicist Dr Theresa Fruth, from the University of Sydney. But these neutrinos can’t be eliminated from detectors, so scientists have had to find another way to distinguish between them and dark matter particles.
This is where directionality becomes important. Dark matter is thought to exist as disks or spheres around galaxies and galaxy clusters, as a ‘dark halo’. On Earth, that means the dark matter WIMPs are most likely to be detected coming from the direction of constellation Cygnus. In contrast, the biggest source of neutrinos on Earth is the Sun, which is in a different direction.
But directionality is only half the problem. The other half is how to detect a completely neutral particle that leaves no trace, at least not one that can be detected with current technologies
“Charged things typically leave ionisation in the detector so they’re easy to detect,” explains Professor Sven Vahsen, a physicist at the University of Hawaii in Honolulu. Dark matter particles and neutrinos have no charge, so instead physicists are looking for the echoes of their transit through the gas, liquid or solid medium of the detector.
“They bounce off something and then the other thing recoils,” Vahsen says. That recoil can be detected, because whatever is recoiling does have charge, and “that’s what creates the ionisation.” And, most importantly, the direction of that recoil can be used to work out the direction of the incoming dark matter particle.
The question is then, what is the thing that recoils? Depending on the size and energy of the dark matter particle, the thing it displaces might be as large as an atomic nucleus or as small as an electron.
As physicist Professor Elisabetta Baracchini describes it, it’s like billiard balls on a table.
“Like playing pool, when you have two balls of similar dimension or mass, they exchange momentum more efficiently,” says Baracchini, from the Gran Sasso Science Institute in L’Aquila, Italy.
Experiments to detect dark matter particles have to choose what mass of WIMP they’re going to look for, and therefore what sized recoil they need to be powered and designed to detect.
One of the topics of discussion at the conference is what is the best medium to show up this recoil effect. “There are some proposals to exploit solids, but the only real demonstration technology that can achieve this is the gaseous detector,” Baracchini says.
Vahsen says the fact that molecules are further apart in gas compared to liquid means that the recoiling nucleus or electron will likely travel a longer distance before it gets slowed or stopped by other particles in the medium.
Even then, it’s still only a few millimetres, but that’s easier to detect than the nanometre distances travelled in a liquid.
No one has reliably detected a dark matter WIMP yet, but another announcement made at the conference could bring that exciting development closer. Scientists working in the field of directional recoil detection are proposing to unite their efforts in a new international collaboration.
“At the moment, we’re a bunch of Institutes working on similar goals, but we’re not funded collectively,” Vahsen says. “What’s exciting now is we’re starting to think about actually building some experiments together.”
