19 July 2014 | NewScientist | 15

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Michael Slezak

“LV IN the hole!” Luke Norsworthy barks into the radio as our light vehicle bounces into a shaft in the Australian outback.

Norsworthy is the electrical supervisor at Stawell mine, 3 hours’ drive from Melbourne. The mine yields some 85 kilograms of pure gold every month. But it may also soon host one of the most important experiments in particle physics – one capable of confirming our best direct observations of dark matter.

“It could be a Nobel prize experiment,” says project leader Elisabetta Barberio at the University of Melbourne.

Dark matter is the mysterious substance thought to make up 80 per cent of the matter in the universe. We believe it exists because the motions of stars and galaxies hint at more mass than we can see. The leading theoretical candidates are weakly interacting massive particles, or WIMPs, which should occasionally collide with ordinary atoms and create a telltale signal.

Most WIMP detectors are placed deep underground to shield them from background radiation that can cloud that signal. Two such experiments, DAMA in Italy and CoGeNT in the US, have already reported whiffs of WIMPs. Both see more potential dark matter hits in June, which some physicists say is because Earth is barrelling through a dark matter “wind” created by the solar system’s motion around the galaxy (see diagram, right).

All the experiments attempting to measure WIMPs directly have so far been located in the northern hemisphere, and it is possible the June bump is caused by some other phenomenon that peaks during summer. So the Stawell team is racing against a South American group to build the first such experiment in the southern hemisphere. If a detector here also sees the signal in June, during winter, it could vindicate DAMA and CoGeNT.

On the face of it, Stawell mine is the perfect location. “The biggest cost in an underground lab is the hole,” says Matteo Volpi at the

University of Melbourne. Not only is the hole here already dug, but it has ventilation shafts and electrical and fibre-optic cables running right through it. What we are here to do is discover how well the mine blocks unwanted background radiation.

Sitting next to me in the vehicle, Volpi cradles a detector about twice the length of a shoebox. It measures muons, charged particles released when cosmic rays hit the atmosphere. In the back is a much bigger muon detector, about a metre square. The hope is that when we get deep

into the mine, we will find that most muons have been stopped by the rock above.

As we drive, we pass signs telling us how deep we are. Beyond the 881-metre marker, we pull into a small side tunnel and get out. As soon as I do, the

hot, thick air fogs up my glasses. We walk into a disused miners’ lunch room, where half-empty coffee jars and old fridges are covered with rust and dust. We set a few boxes of equipment on the lunch table, and get the muon counters up and running.

To Volpi’s surprise, the detectors seem to show quite high counts. We leave the equipment running, turn off the lights and drive back to the surface.

“The test was useful, because we realised that we were seated on the bottom of a Swiss cheese,” he tells me later. “There are a lot of voids above the room.” Although that particular space doesn’t seem suitable, he expects that another chamber in the mine will be.

Robert Foot at the University of Melbourne says that if the experiment goes ahead, it could not only find dark matter but also reveal properties not measurable in the northern hemisphere. Because of Earth’s orientation relative to the WIMP wind, particles must plough through much more of the planet before reaching a southern hemisphere detector. If they are slowed down by normal matter – or by other dark matter present inside Earth – that could be detectable, he says.

What if they don’t see WIMPs at all? Barberio is frank about the implications. Aside from DAMA and CoGeNT, most WIMP hunters are coming up empty and may be ready to rule out the long-sought particles, she says. “If we don’t see anything, those experiments will be right.” n

Golden chance to unmask WIMPs

FIELD NOTES Stawell mine, Victoria, Australia

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–It’s a rocky road to dark matter–

“ The Stawell team is racing to build the first dark matter detector in the southern hemisphere”

Windy seasonThe sun’s orbit around the galactic centre may drag the solar system past dark matter, which we experience as a “wind” of particles called WIMPs

Earth orbits the sun, so we battle the wind in June and ride with it in December, leading to a seasonal bump in suspected WIMP strikes

WIMP WIND

Earth’s orbit

GALACTIC PLANE

JUNEN

N

DECEMBER

SUN

60°

Galvanic lifeSolar system is travelling into the WIMP wind and orbiting the sun at the same time

WIMP WIND

SOLAR SYSTEM

CYGNUS GALACTIC PLANE

JUNE

DECEMBER

60°

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