LoaderSave StorySave this storyCommentLoaderSave StorySave this storySeventy years ago, the physicists Clyde Cowan and Frederick Reines took a custom-built 10-ton detector, surrounded it with thick lead walls and wet sandbags, and placed it near a powerful nuclear reactor at the Savannah River Plant in South Carolina. They called the experiment Project Poltergeist, designed as it was to catch a ghost.
More than a quarter of a century before, physicists had been puzzling over why energy appeared to be lost during a radioactive process called beta decay. Something was missing, and there was no known physics to explain it. Then in 1930, the Austrian physicist Wolfgang Pauli proposed a radical solution: A virtually undetectable particle was silently carrying the missing energy away. “I have done a terrible thing,” Pauli told a friend. “I have postulated a particle that cannot be detected.” It would come to be known as the neutrino. Having almost no mass and no charge, these particles can pass through Earth and everything on it, including our bodies, virtually unimpeded.
The massive device that Cowan and Reines deployed in early 1956 was meant to find what Pauli thought was impossible. That June, the pair of physicists from the Los Alamos National Laboratory sent Pauli a telegram: “We are happy to inform you that we have definitely detected neutrinos.”
Attention then shifted to a broader question. If nuclear reactions produce neutrinos, could we use them to peer at the nuclear fireworks inside stars, including the sun? This presented a huge challenge: How can you possibly catch particles shooting from distant stars if these particles can pass through almost anything undetected? The suspicion was that detecting a particle that rarely collides with matter requires a vast amount of matter for it to collide with. Moreover, the matter would have to be shielded from the noise of other forms of radiation. So the answer scientists came up with was to build some of the biggest, deepest, and most exotic experimental traps in scientific history … and then wait.
In the 1960s, Raymond Davis Jr. and colleagues at Brookhaven National Laboratory placed a tank 1.5 kilometers underground in the Homestake mine in South Dakota and filled it with nearly 400,000 liters of a chlorine-based cleaning fluid called perchloroethylene. On the rare occasion that a passing neutrino struck a chlorine nucleus, it would be transformed into a radioactive form of argon that could be detected and counted. The experiment, which would run for 25 years, found just one-third the number of neutrinos coming from the sun that had been predicted in theoretical models. This became known as the solar neutrino problem.
Decades passed before it was solved—by yet more massive experiments. Deep in the Kamioka mine in Japan, Masatoshi Koshiba built a different kind of detector called Kamiokande, which used 3 million liters of ultrapure water. In this setup, neutrinos occasionally interact with atomic nuclei in the water. The interaction creates an electron that moves so fast, it generates a flash of what’s called Cherenkov light. This light gets picked up by detectors.
Kamiokande and Koshiba confirmed Davis’ shortfall, and a second, even larger detector, Super-Kamiokande, as well as Canada’s Sudbury Neutrino Observatory, explained the discrepancy. Neutrinos come in three different “flavors” (electron, muon, and tau) and can oscillate, or switch, between them. To do so, neutrinos must have mass, which the laws of physics failed (and still fail) to predict.
Neutrino oscillations, and the myriad mysteries they give rise to, have driven the development of the newest wave of detectors. China’s Jiangmen Underground Neutrino Observatory (JUNO) launched in 2025; initial data published in June 2026 provided the most precise measurements of neutrino oscillation reported to date. Japan’s Hyper-Kamiokande (Hyper-K) and the Deep Underground Neutrino Experiment (DUNE) in the American Midwest are both expected to begin operation later this decade.
Because of these and other audacious experiments, the particle that Pauli was sure could never be caught has slowly been revealing its secrets. The recipe for discovery hasn’t changed in seven decades: Think big, go deep, and summon patience.
Different varieties of neutrinos passing through the Super-Kamiokande detector create telltale patterns on the walls of the detectors. Here we see the traces of a muon neutrino (left) and an electron neutrino (right), two of the fundamental particles in the Standard Model of particle physics.
Super-Kamiokande Collaboration/Science Photo LibrarySNO+ succeeded the Sudbury Neutrino Observatory in the Creighton mine and repurposed some of its predecessor’s equipment. But instead of heavy water, it was filled with linear alkylbenzene (LAB), which is used commercially to manufacture dish soap. LAB produces signals 50 times brighter than those produced by heavy water. Hanging from the top is the experiment’s director, Art McDonald.
The Borexino neutrino detector at the Gran Sasso National Laboratory in Abruzzo, Italy, provided the first direct evidence that the sun fuses hydrogen into helium in more than one way. Above, a researcher checks a photomultiplier tube, which amplifies light created by neutrino interactions.
Workers deploy digital optical monitors for the underwater KM3NET neutrino detector, which is still under construction. Nearly 200,000 optical sensors will be anchored to the floor of the Mediterranean Sea at a depth of around 3,500 meters off the coast of Sicily, as well as in a second, smaller array near Toulon, France. The sensors will detect the Cherenkov light generated by secondary particles produced by neutrino interactions with the surrounding water or rock.
The Deep Underground Neutrino Experiment (DUNE) will generate trillions of neutrinos and send them 1,300 kilometers through the Earth to study them when they interact with two detectors—a “near” one 60 meters underground at the Fermi National Accelerator Laboratory, and a “far” one 1.5 kilometers beneath the surface at the Sanford Underground Research Facility in Lead, South Dakota (left). Smaller-scale prototypes of the detectors (right), called ProtoDUNE, are being tested at CERN, the particle physics lab on the border of Switzerland and France.
Photograph: Maximilien Brice/CERNOriginal story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.
