Two Thousand Feet Underground, a Once-in-a-Century Discovery That Shaped Particle Physics

In a distant galaxy more than 150,000 years ago, a blue giant star exploded, spraying particles — including neutrinos, one of the most elusive subatomic particles known — across space.

Then, in the 1980s, neutrinos from this supernova were picked up by the Irvine-Michigan-Brookhaven detector deep underground in Ohio. The discovery marked one of the first measurements of neutrinos from beyond our solar system, helped kickstart the field of observational neutrino astronomy, and provided a starting point that next-generation neutrino detectors continue to build on.

But the discovery was also lucky: The detector was built primarily to study proton decay, rather than neutrinos. “When you build a new detector with new capabilities, you're sensitive to things that you never expected,” says Henry Sobel, a physics professor at the University of California, Irvine, and one of IMB’s original collaborators. The unexpected supernova would shape the legacy of IMB, which was recently recognized as an APS Historic Site for its role in neutrino science.

In the mid-1970s, teams of physicists were racing to build detectors that could measure proton decay, a hypothesized phenomenon that would confirm Howard Georgi and Sheldon Glashow’s new Grand Unified Theory, one that sought to unite three of the four fundamental forces of nature. The winner emerged in Painesville, Ohio, a small city northeast of Cleveland: The IMB detector, the world’s first kiloton-scale nucleon decay detector, began collecting data in 1982.

To look for proton decay, the IMB detector would need to track more than a nonillion (10³⁰) protons at once. If the lifetime of a proton is 10³⁰ years, scientists could expect to see one proton decaying each year; if no decay is observed, it means that a proton’s lifetime is longer than 10³⁰ years.

To measure that many protons, the IMB team designed and built an 8,000-ton tank for 2.5 million gallons of purified water surrounded by 2,000 photomultiplier tubes. This massive Cherenkov detector, which measures charged particles as they pass through water, was constructed in the Fairport Harbor Morton Salt Mine, nearly 2,000 feet underground, to avoid cosmic ray interference.

IMB quickly ruled out the Georgi-Glashow model and several other proton decay lifetimes, says Sobel, which helped theorists at the time “push the boundaries” and generate ideas for new experiments. “Theorists continuously come up with new predictions, so by trying to be sensitive to different modes of proton decay, we were able to inform the theorists of what's possible and what's not,” Sobel says.

Neutrinos, meanwhile, remained extremely difficult to study. Although common, these subatomic particles are chargeless and nearly massless, and they only interact through gravity and the weak force. The particles were first detected starting in the 1950s, but even by the 1970s, “the weak interaction was incredibly mysterious, and there was this huge hole in our understanding of the forces of nature,” says Lawrence Sulak, a physics professor at Boston University who was part of the team that designed and prototyped IMB. “It was clear that if you wanted to understand the fundamental forces, you had to understand neutrinos.”

Sobel and Sulak both say they and their IMB colleagues were aware that detecting a supernova was theoretically possible. “But the probability was a challenge,” says Sulak. “People knew that a supernova large enough to generate neutrinos that we could detect would only happen once a century, and with experiments typically only lasting for ten years, that means you would only have a 10% chance of seeing one.”

But the team got lucky. On Feb. 23, 1987, visible light reached Earth from SN 1987A, the explosion of a blue giant in the Large Magellanic Cloud that happened 166,000 years ago. After learning that the Kamioka Observatory, a Japanese neutrino and gravitational wave laboratory and IMB contemporary, had detected a burst of 11 neutrinos from the supernova, the IMB team took a closer look at their data and found evidence of another eight neutrinos, confirming theories that most of a supernova’s energy radiates away from its core in the form of neutrinos.

“Normally a neutrino interaction in the IMB detector happens once every five days, and here we saw eight in five seconds,” says Sobel, adding that the IMB team knew right away that they’d seen something significant.

IMB’s run ended in 1991, but its impact continues today. Not only did IMB and Kamioka influence the design of later detectors like Super-Kamiokande, their early observations on different neutrino “flavors” led to Nobel prize-winning findings on neutrino oscillation. Future efforts to understand neutrino oscillations, and the potential for neutrinos to break charge-parity symmetry, will be led by the next generation of detectors, including Hyper-Kamiokande and the Deep Underground Neutrino Experiment, or DUNE.

“Studying the neutrino has become a big physics business, and it all started with IMB and Kamioka and the properties that we discovered by operating those detectors,” Sobel says.

When it comes to neutrino astronomy specifically, experiments like the IceCube Neutrino Observatory are leading the charge in a field that, before IMB and Kamioka, had been an entirely theoretical enterprise but was considered essential for astronomy research.

“For really small wavelengths, you can only do physics with neutrinos,” says Francis Halzen, a physics professor at the University of Wisconsin-Madison, and the principal investigator of IceCube. “Neutrinos can also reach us from places in the universe where nothing else can get out — like close to black holes — so there was never any doubt that we wanted to do neutrino astronomy.”

As the world’s first gigaton neutrino observatory and Cherenkov detector, one that uses Arctic ice instead of purified water, IceCube has so far discovered high-energy neutrinos created by cosmic accelerators from across the universe and determined the sources of high-energy cosmic rays.

Halzen says that detectors like IMB directly inspired the methods employed by IceCube; this includes hanging the photomultiplier tubes into the ice cores from strings, which was modelled after IMB. “There was a long debate about the approach we would use at the time, and it’s clear we made the right choice to be inspired by IMB,” Halzen says.

While ongoing and future neutrino experiments are poised to help scientists understand these mysterious particles, both Halzen and Sobel also have their fingers crossed for yet another once-in-a-century supernova.

“Look at all of the physics we got from the 20 or so neutrinos that were detected from [SN1987A],” said Halzen. “The science we could do now if we observed another supernova would be incredible — those few seconds of physics would be the most important thing IceCube does.”