The search for dark matter (2023)

Victoria’s Stawell goldmine is unassuming in its operation. Distant beeps from earthmovers disrupt the otherwise constant background drone of the rotating “ball mill” that processes the extracted raw materials. The buildings and mining artifices are rich with patination, a marker of the slow, uninterrupted nature of the work.

A sign over a small gravel ramp reads “Magdala Decline”, in honour of the fault that borders the Stawell goldfield lode. That’s the entrance. Between the gravel car park and the corrugated steel equipment sheds, the only other evidence of the multimillion-dollar facility buried more than a kilometre below ground is two weathered information panels strung to the security fence. It seems a fitting parallel to the hidden nature of the Stawell Underground Physics Laboratory’s primary research focus: dark matter.

In close to 100 years, since Swiss astrophysicist Fritz Zwicky made his case for the existence of an unseen mass in space, dunkle Materie, science has yet to provide aconclusive explanation for what is thought to account for a staggering amount of our whole picture: about 85 per cent of the matter in the universe. What is understood so far about dark matter is observed in the interactions between cosmological bodies in space, namely interactions that cannot be explained with what is visible using accepted theories of gravity. Dark matter’s presence is implied as the missing piece of these celestial dances.

The first potential evidence of dark matter was recorded by the DAMA/LIBRA experiment at the Laboratori Nazionali del Gran Sasso in Italy, situated underneath the Apennine Mountains – substantial evidence has eluded more than 50 major experiments in a century of research. DAMA/LIBRA’s observations note consistent, yearly patterns in data, with the team believing it could be a marker of Earth’s movement through a substrate of dark matter in our galaxy – on our yearly orbit around the sun, we effectively move with and against the current.

Sceptics have posited that the findings could be disproved by seasonal variation, and so the experiment is being replicated in the Stawell lab, the first of its kind in the southern hemisphere, in order to rule this out. If the Italian laboratory’s data can be replicated here, it will herald a new chapter in the fundamental understanding of our universe. The resulting joint experiment is known as SABRE – Sodium Iodide with Active Background Rejection Experiment – a fitting title for such cutting-edge research.

Dr Theresa Fruth is a specialist in astroparticle physics working closely on SABRE South (to Italy’s “North”). “If you look at it historically, there might be things out in space we cannot see at the moment,” she tells The Saturday Paper.

“Seems kind of reasonable, right? But the more we got access to, it really clarified over the past 100 years that there must be something actually quite new, something which does not fit into our current models and understanding.”

Professor Phillip Urquijo is the technical co-ordinator of SABRE South. Like many on the research team, he has an impressive resume of particle physics experience: most notably at the Large Hadron Collider as part of the ATLAS experiment, and more recently with the development of the new Hyper-Kamiokande neutrino observatory in Japan.

“Those experiments are some of the biggest in the world. I mean, the Large Hadron Collider is this 27-kilometre ring smashing together protons at close to the speed of light, re-creating the conditions of the big bang.”

Stawell’s lab is small by comparison, but the significance of the research is comparable: searching for the fundamental building blocks of the universe. The problem is, the building blocks SABRE is looking for are invisible. What we can see is the visitation of dark matter at a subatomic level. As our planet moves with the sun through the galaxy at 792,000 kilometres an hour, dark matter flows through us. As such, dark matter particles occasionally bump into ordinary matter particles.

Fruth uses a sports analogy: “Imagine the dark matter particles coming in and, just like a billiard ball, off the nucleus of an atom, and the recoil of this nucleus, that’s what we’re trying to detect.” The interactions are so rare that she suggests little more than a “handful” of collisions could be detected over the life of the project. The collisions give off heat, which is in turn converted to light. In an astonishing feat, the apparatus at the core of SABRE’s experiment is able to observe these infinitesimally small glints. The backdrop: large, transparent sodium iodide crystals, seemingly plucked directly from Superman’s Fortress of Solitude.

The extreme precision of the experiment’s design means that an incredible amount of countermeasure is invested in warding off false positives. As Phillip Urquijo explains, “We’re looking for such feeble and rare interactions that can really easily be mimicked by very tiny amounts of radiation inordinary matter.”

The layers of earth above the lab act as a shield, blocking the unwanted imposition of cosmic rays and background radiation – the eternal reverberation of our early primordial universe. Bananas are forbidden in the lab due to their high potassium levels, a small amount of which is radioactive. Even the steel used for the detector is of a bespoke purity, as a majority of steel produced from World War II onwards contains remnants of nuclear weapons testing fallout.

The main detector itself has a strategy of mitigation, with discrete ancillary detectors watching for interactions with muons – stray subatomic particles that can make their way through the one kilometre of earth shielding. It is an arresting amount of complication: fail-safe upon fail-safe, subsystem upon subsystem. The “muon paddles” are currently being installed, to the excitement of the team, with the full system expected to be operational next year.

SABRE represents the flagship project for the Stawell Underground Physics Laboratory, but the unique nature of the lab’s design presents an opportunity for other leading-edge research to be conducted in Australia. Deep-underground facilities have been used to study quantum computing and astrobiology; the extreme nature of these sites provides a way to observe how life operates without cosmic radiation.

Urquijo doesn’t rule out the possibility of additional discoveries. His face lights up as he describes the potential of “dark photons” and “a new fundamental force”. Above all, the Stawell lab presents the potential for Australia to make an enormous contribution to the broader scientific canon. “Because we’re searching so broadly everywhere, if we can finally say that [dark matter] interacts like this ... it changes everything.”

The tradition of Australian “sister cities” – locales bound by cultural exchange – has nurtured connections with more than 50 countries internationally. So now, a sleepy Victorian gold rush settlement shares bonds with the Italian Apennine mountain range. At both sites, the earth rises to protect the most precious of interactions. In Stawell, the pursuit of rare materials began 170 years ago, but the years ahead may bear witness to something far rarer.

This article was first published in the print edition of The Saturday Paper onMay 13, 2023 as "Their dark materials".