Ghost particle probe marks end of an era

Seeing the invisible

To detect something as elusive as a neutrino requires extraordinary engineering, bordering on science fiction, and physicists often describe such projects as extreme precision measurement on an extreme scale.

One notable example is the Super Kamiokande detector, located 1,000 meters underground inside Mount Ikenoyama in Japan, which was launched in 1996. It features a 15-story-tall, stainless-steel tank holding 50,000 metric tons of ultrapure water, which is monitored by around 13,000 photomultiplier glass bulbs, each half a meter in diameter, installed on the tank's wall.

The facility is built underground to eliminate interference from other particles. When a high-energy neutrino collides with the atomic nucleus of the water, the detectors will register a faint light known as Cherenkov radiation, signaling that a neutrino has been found. But even with this degree of engineering, identifying a neutrino is still exceedingly rare.

JUNO will use an equally awe-inspiring design. Its central detector will consist of a 35.4-meter diameter, 12-centimeter-thick transparent acrylic sphere, the biggest of its kind in the world, holding 20,000 tons of liquid scintillator, a solution that exhibits flashes of light when excited by ionizing radiation.

The sphere will be submerged in a tank holding 30,000 tons of ultrapure water and be surrounded by tens of thousands of detectors. The cave to hold all the instruments will need to be 50 meters in diameter and 70 meters tall.

It is hoped JUNO will discover more about the masses of neutrinos, including their ordering, Wang said. It will also be able to measure the three types of neutrino oscillations with greater precision.

"For a long time, physicists thought neutrinos had no mass, but if that is the case, then the universe cannot form the structure as it is today," he said. "Because of neutrinos, we exist.

Wang said JUNO will make China one of the world's front-runners in neutrino research technology, comparable only to the United States' Deep Underground Neutrino Experiment and Japan's Hyper-Kamiokande, both of which are set to be operational in 2027.

"JUNO will complement the other neutrino observatories both in their mission objectives and timeline," he said. In the past, Chinese scientists often took part in big scientific experiments hosted by other countries, but now they are capable of launching, organizing and managing major scientific projects that can attract global participation, which speaks volumes for China's scientific development, he added.

However, it is hard to predict when the next big discovery will occur since neutrinos rarely interact with anything and are so hard to detect. "There is quite a bit of luck involved," Wang said.

Over its estimated 30-year operational life, JUNO hopes to record a supernova explosion in our galaxy, which happens only around once a century.

A supernova is the explosion of a massive star at the end of its life and is capable of releasing more energy than the total output of the sun in 10 billion years, making it one of the brightest and most powerful events in the universe.

"Around 99 percent of the supernova's energy is released in the form of neutrinos within a dozen seconds, some of which will show up on our detectors as a burst of events," Wang said. "This will give us more opportunities to study the supernovae explosion mechanism and potentially lead to new discoveries about the cosmos."