Huge neutrino detector detects first traces of particles from exploding stars
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Huge neutrino detector detects first traces of particles from exploding stars

Huge neutrino detector detects first traces of particles from exploding stars

The Super-Kamiokande underground reservoir must be emptied for major renovation.Author: Takumi Harada/Yomiuri Shimbun via AP/Alamy

Every few seconds, somewhere in the observable universe, a massive star collapses and unleashes a supernova explosion. Japan’s Super-Kamiokande observatory can now collect a steady stream of neutrinos from these cataclysms, physicists say — a multiple-detection rate per year.

These tiny subatomic particles play a key role in understanding the processes that occur inside a supernova. As they fly out of the collapsing star’s core and travel through space, they can provide information about any potentially new physical phenomena that might occur under extreme conditions.

At the Neutrino 2024 conference in Milan, Italy, last month, Masayuki Harada, a physicist at the University of Tokyo, revealed that the first signs of supernova neutrinos appear to emerge from the cacophony of particles that the Super-Kamiokande detector collects every day from other sources, such as cosmic rays hitting the atmosphere and nuclear fusion in the core of the sun. The result “indicates that we have started to see a signal,” says Masayuki Nakahata, a physicist at the University of Tokyo and a spokesman for the experiment, which is commonly referred to as Super-K. Nakahata cautions, however, that the supporting data — collected over 956 days of observations — is still very weak.

Elusive particles

Neutrinos are notoriously difficult to catch. Most of them pass through the planet like light through glass, and Super-K catches only a tiny fraction of those that do. Still, the detector has a good chance of catching neutrinos from supernovae, because the universe is supposed to be flooded with them. The collapse of a star releases a huge number of these particles (estimated at about 1058), which causes what astrophysicists call the supernova neutrino scattering background.

But so far, no one has been able to detect this background. Neutrinos have only been definitively traced once to a collapsing star—Nakahata was among researchers who spotted the particles using the Kamiokande-II detector, the predecessor to Super-K, in 1987. The detection was possible because the supernova occurred in the Large Magellanic Cloud, a dwarf galaxy that is close enough for the exploding star’s neutrinos to reach Earth in large numbers.

Between 2018 and 2020, the Super-K detector, a tank containing 50,000 tons of purified water located under a kilometer of rock near Hida in central Honshu island, underwent a simple but important upgrade to improve its ability to distinguish supernova neutrinos from other particles.

When a neutrino—or more precisely, its antiparticle, the antineutrino—collides with a proton in water, the proton can transform into a pair of other particles, a neutron and an antielectron. The antielectron produces a flash of light as it moves at high speed through the water, and that light is picked up by sensors embedded in the walls of the tank. The flash of light itself could be indistinguishable from flashes produced by neutrinos or antineutrinos from many other sources.

But during the upgrade, scientists added a gadolinium-based salt to the Super-K water. This allows the neutron produced when the antineutrino hits the water to be captured by the gadolinium core, which releases a second, telltale flash of energy. Super-K physicists looking for supernova neutrinos look for a rapid sequence of two flashes, one produced by the antielectron and the other by the captured neutron.

Solving cosmic mysteries

Nakahata says it will be a few years before true supernova signals clearly emerge, since double-blink signals could come from other neutrino sources, including those generated by cosmic rays hitting the atmosphere. But he adds that by the time Super-K is scheduled to close in 2029, it should have collected enough data to make a solid claim.

And an even larger experiment called Hyper-Kamiokande, scheduled for completion around 2027, could significantly improve on Super-K’s performance. Hyper-K will initially be filled with pure water, but “all the components of the detector are being tested for compatibility with gadolinium,” which may be added later, says Francesca Di Lodovico, a physicist at King’s College London and a co-spokesperson for the project.

Showing that neutrinos from distant supernovae that occurred billions of years ago still exist would confirm that neutrinos are stable particles and don’t decay into something else, Nakahata says. Physicists have long suspected this but haven’t been able to prove it conclusively.

Measuring the full energy spectrum of supernova neutrinos could also provide clues to how many supernovae have gone off at different times in cosmic history, Harada says. It could also reveal how many collapsing stars created a black hole—which would stop the neutrino emission—as opposed to leaving behind a neutron star.

The Super-K data are still too weak to declare a discovery, but the prospect of detecting scattered neutrinos is “extremely exciting,” says Ignacio Taboada, a physicist at the Georgia Institute of Technology in Atlanta and spokesman for the IceCube neutrino observatory at the South Pole. “Neutrinos would provide an independent measure of the star formation history of the universe.”