We have long had a seemingly simple question about neutrinos: how much do they weigh? Now, a gigantic experiment in China is poised to find out – and the answer could transform particle physics forever The funicular railway carries me downwards into the mountain, and the familiar, above-ground world vanishes. The tunnel is steep, bored through solid granite, and we gradually descend – down, down, down – for nearly a quarter of an hour.
After a while, the pressure makes my ears pop. When the passageway finally ends, we are 700 metres underground, and I can see a vast subterranean complex of caverns and pipes spread out beneath me. But then again, you would expect nothing less from an experiment that could change particle physics forever.
I have come to the Jiangmen Underground Neutrino Observatory, better known as JUNO, a machine in southern China designed to catch ghostly particles called neutrinos. These are the tiniest fragments of reality we know of, so fleeting that they barely exist at all. Studying them has already overturned some of our most basic assumptions about the universe – and there are good reasons to think they hold the key to some of its deepest remaining secrets.
When it comes to the fundamental ingredients of reality, we know an awful lot about what is out there – but also know that we aren’t yet seeing the whole picture. The standard model of particle physics is a list of these ingredients, the fundamental particles and forces that make up the cosmos. There is an elegant symmetry to it, and it has been nothing short of a spectacular success.
But it doesn’t include gravity or explain the nature of dark matter. Then there is antimatter. Every kind of matter particle has an antimatter counterpart, with the same mass but opposite properties, such as electric charge.
When matter and antimatter meet, they annihilate each other in a burst of energy. That creates a puzzle because, as far as we understand, the early universe should have produced matter and antimatter in equal amounts – raising the question of why matter now dominates.
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