The first dark matter detector in the Southern Hemisphere has been officially opened. The Stawell Underground Physics Laboratory (SUPL) is built in a disused gold mine in Australia, and the aim is to use its unique position on the globe to finally pick up signals from the mysterious material thought to pervade the universe.
Decades of astronomical observations indicate that there’s much more to the cosmos than meets the eye. Gravitational effects don’t make sense based on the amount of mass from matter we can see, leading astrophysicists to infer that there’s far more matter out there that we can’t see. This so-called dark matter doesn’t emit or interact with light, and rarely interacts with normal matter.
Occasionally though, a dark matter particle might bump into an atom of regular matter and produce a detectable signal – although they’d usually be impossible to differentiate from all the other interactions constantly going on around us. But if you removed all those distractions, theoretically you should be able to detect dark matter.
And that’s the idea behind the new facility. SUPL was constructed 1 km (0.6 miles) underground to block cosmic rays from reaching the instruments. The detector is also surrounded by about 100 tonnes of steel and polymer shielding, as well as a liquid scintillator system that helps eliminate false positives.
The detector itself is a tank containing 50 kg (110 lb) of ultra-pure sodium iodide crystals, which will give off flashes of light if struck by a particle whizzing through. Extremely sensitive light detectors are constantly watching the tank for any such signals. Other particles like neutrinos can produce similar flashes, but these would usually also produce signals in the liquid scintillator at the same time. Any flashes that only occur in the sodium iodide could turn out to be the elusive dark matter.
The overall design is common for dark matter detectors like XENON1T and LUX, which use liquid xenon as the detector volume, and other proposed designs using superfluid helium or supercooled water work on similar principles.
But SUPL’s main advantage is its location. Previous dark matter detectors have been concentrated in the Northern Hemisphere, so building a detector at the other end of the world could help confirm or rule out some intriguing signals that these earlier experiments have reported.
For instance, SUPL’s sister facility, the Gran Sasso Laboratory in Italy, has found that dark matter candidate signals seem to peak in June each year. The optimistic interpretation is that this is corresponds with a point in the Earth’s orbit where the planet is flying head-on through a “wind” of dark matter particles as the solar system moves through the galaxy. The pessimistic interpretation, of course, is that other seasonal factors, like humidity fluctuations, are at play.
SUPL, with its inverse seasons, could help answer that question. If it also detects an influx of signals every June, it would be very strong evidence of the dark matter wind hypothesis. If, on the other hand, its peaks occur around December, that would indicate interference from the summer weather.
However it plays out, SUPL will be a fascinating experiment to watch.