Underground Detector Hunt for Ghostly Particles Could Rewrite Physics

A funicular railway descends 700 metres into solid granite, leaving the surface world far behind. At the bottom lies JUNO, the Jiangmen Underground Neutrino Observatory, a machine designed to catch some of the universe’s most elusive particles. These aren’t flashy discoveries waiting to happen. Neutrinos are the tiniest fragments of reality we know, so fleeting they barely exist at all. Trillions pass through your body every second without leaving a trace.

Yet studying them has already overturned fundamental assumptions about the universe. Physicist Silvia Pascoli from the University of Bologna notes that neutrino mass provides the only particle physics evidence that the standard model is incomplete. That’s huge. It suggests we’re missing something essential about how reality works.

The Standard Model’s Missing Pieces

The standard model of particle physics is our best map of the cosmos. It lists the fundamental particles and forces that make up everything. It’s elegant, successful, and spectacularly wrong in important ways. Gravity doesn’t fit. Dark matter remains mysterious. And then there’s the matter-antimatter problem that keeps physicists awake at night.

Every matter particle has an antimatter twin with opposite properties. When they meet, they annihilate in bursts of energy. The early universe should have created matter and antimatter in equal amounts, yet matter now dominates. Something tipped the scales, and physicists suspect neutrinos hold a clue.

The story begins in a gold mine in South Dakota during the late 1960s. Physicist Raymond Davis Jr. filled a tank with chlorine-containing cleaning fluid to detect neutrinos streaming from the sun. His detector was buried deep to filter out cosmic interference. Yet when he ran the numbers, two-thirds of expected neutrinos never showed up.

Then came the revelation: neutrinos come in three flavours. Electron neutrinos, muon neutrinos, and tau neutrinos. As they travel, they change flavour, a metamorphosis called neutrino oscillation. Those missing solar neutrinos hadn’t vanished. They’d simply transformed into different types during their journey to Earth.

The Mass Mystery

Here’s where it gets strange. The three flavours don’t have distinct masses. Each neutrino mass state is a quantum mixture of all three flavours simultaneously. As they travel, the balance shifts, changing which flavour we observe. But those exact masses? Still unknown.

The disparity between electron and neutrino mass is roughly the difference between a fully loaded station wagon and a single playing card. This vast gulf raises profound questions. Do neutrinos acquire mass differently from other particles, which get mass through interactions with the Higgs boson? If so, that opens windows onto deeper reality.

Some proposed mechanisms for generating neutrino mass would violate fundamental rules physicists currently assume are true. One intriguing possibility is the seesaw mechanism, proposed in the 1980s. It suggests undiscovered, enormously heavy neutrinos exist, linked to the tiny masses we observe. Push one end of the seesaw up, the other drops down.

JUNO’s centrepiece is a vast acrylic sphere containing 20,000 tonnes of scintillator fluid. Fifty-three kilometres away, nuclear power plants produce electron antineutrinos piped toward the detector. When an antineutrino interacts with the fluid, it produces a tiny flash, amplified by thousands of golden orbs positioned around the sphere’s edges.

The engineering scale is breathtaking. The cavern could swallow a 14-storey building. Yet this machine hunting the universe’s most elusive particles was itself hidden under black plastic during the visit. Perhaps that’s fitting.

JUNO aims to resolve whether neutrino mass states follow a normal or inverted hierarchy, which would help constrain the actual masses and narrow competing explanations. These initial results lean toward normal ordering, but more data will follow.

Even if JUNO succeeds, we probably won’t get bulletproof answers about how neutrinos acquire mass. Instead, the detector acts as a sieve, making some theories more plausible while straining others. Coming soon are Hyper-Kamiokande in Japan and DUNE in the US, experiments that can create controlled neutrino beams and search for differences between neutrinos and antineutrinos. Working together, these three detectors could finally crack riddles that have haunted physics for decades.

Original source BBC Science Focus Magazine

What if neutrinos carry the key not just to understanding matter, but to the very origin of our existence itself?