When Michelle Galloway drives to work, she takes a tunnel into a mountain. thorough inside, at the entrance to the facility, guards ask her for a secret password. “Then, this James Bond-kind 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 complete universe.
Galloway and her colleagues on the XENONnT experiment have one goal in mind: find out what dark matter is made of. Dark matter, in any case 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 form, the theory that describes all the known, basic particles of matter.
But dark matter troubles the Standard form – 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 sustain for supersymmetry, then it would give us a way to expand the Standard form,” 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 scarce, so it’s expensive, of course,” says Galloway. The last time the team purchased some, the price was approximately €12 per litre. At that price, 8.6 tonnes would cost around €17 million – but it was acquired little by little, 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 experienced a enormous 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 by Earth. One theoretical candidate particle that the team hopes to detect is called a weakly interacting enormous particle – known as a “WIMP” for short. The XENONnT is effectively trying to pick up on a “wind of WIMPs” flying by the Universe, Galloway says.
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