When Michelle Galloway drives to work, she takes a tunnel into a mountain. Deep inside, at the entrance to the facility, guards ask her for a secret password. “Then, this James Bond-type door opens in the rock,” explains Galloway, a senior researcher at the University of Zurich. “It’s super cool.”
Behind the door lies the Gran Sasso National Laboratory in Italy. Sitting 1,400 metres below the surface, it’s the largest underground lab of its kind in the world. And in one of the cavernous halls carved into the rock of the Apennine Mountains lies a machine that could change our understanding of the entire universe.
Galloway and her colleagues on the XENONnT experiment have one goal in mind: find out what dark matter is made of. Dark matter, whatever it is, accounts for 85 per cent of the total mass of the Universe. It bends light and binds galaxies together, preventing them flinging themselves apart – gravitational effects such as these are the only reason physicists know it exists.
The other 15 per cent – everything else in the Universe, from the rings of Saturn to the cells of your stomach lining – is well covered by the Standard Model, the theory that describes all the known, essential particles of matter.
But dark matter troubles the Standard Model – it doesn’t fit. One idea, called supersymmetry, is that there is a whole range of other, difficult-to-detect particles that act as partners to the ones we already know about. “If we found some support for supersymmetry, then it would give us a way to expand the Standard Model,” explains Galloway.
She and her colleagues hope to find some answers with the help of 8.6 tonnes of liquid xenon, a noble gas sometimes used as a general anaesthetic. “It’s extremely rare, so it’s expensive, of course,” says Galloway. The last time the team purchased some, the price was roughly €12 per litre. At that price, 8.6 tonnes would cost around €17 million – but it was acquired gradually, and can be recycled.
Around five tonnes of the xenon, which is kept at -100°C, is pumped inside the smallest of three chambers that comprise the detector, which has recently undergone a massive upgrade. This inner compartment is called the time projection chamber, or TPC. It is designed to pick up the faint signal of dark matter particles breezing through Earth. One theoretical candidate particle that the team hopes to detect is called a weakly interacting massive particle – known as a “WIMP” for short. The XENONnT is effectively trying to pick up on a “wind of WIMPs” flying through the Universe, Galloway says.
If it works as planned, a WIMP will enter the cylindrical TPC and strike the nucleus of a xenon atom, causing a tiny amount of light to escape. During this “nuclear recoil” event, some electrons will also be released from the xenon. They’ll travel up to the top of the TPC and cause further emission of light upon interacting with a layer of xenon gas.
The problem is, the light detectors used in the experiment can pick up all kinds of physical interactions, including background radioactivity, which is not evidence of dark matter. But by pinpointing the precise location of light-emitting events, the research team can plot exactly where they occurred. And if multiple interactions at the appropriate energy level turn out to have happened in the heart of the TPC, right in the midst of that liquid xenon, the researchers can be confident these were caused by WIMPs.
Cancelling out the noise is one of the big challenges of an experiment like this. The xenon must be constantly purified, in order to extract substances that naturally accumulate in it from materials in the detector. Two outer chambers (pictured) filled with a special salt solution slow down passing particles that would disrupt WIMP detection, and additional light-sensing devices in these outer chambers detect non-WIMP interactions so they can be discounted. Think of it like trying to hear a baby bird faintly cheeping in a windy forest. If you can somehow block out the ambient noise, and listen very carefully, you’ll have a much better chance.
It’s possible that dark matter isn’t made up of WIMPs after all. It could be a mix of different particles, or something else entirely. But XENONnT should get us closer to the answer, one way or another. At the very least, the whole project is a reminder of how little we know about the Universe. “Even if we don’t figure out what it is in my lifetime,” says Galloway, “it just gives us a certain perspective, I think.”
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This article was originally published by WIRED UK
