Seventy years after the first attempt to detect them, physicists continue to deploy increasingly large and sophisticated instruments to study neutrinos, subatomic particles so elusive they can pass through the entire Earth unimpeded.
The Elusive Particle’s Origin
The quest to find neutrinos began in the 1930s when physicist Wolfgang Pauli proposed their existence to explain missing energy in radioactive beta decay. He famously described his hypothesis as postulating a particle that could not be detected. These particles, with almost no mass and no charge, were named neutrinos.
In 1956, Clyde Cowan and Frederick Reines, using a custom-built 10-ton detector near a nuclear reactor at the Savannah River Plant, confirmed Pauli’s theory. Their experiment, dubbed “Project Poltergeist,” successfully detected neutrinos, a feat that sent a telegram to Pauli announcing the discovery.
From Solar Neutrinos to Oscillations
The detection of neutrinos opened new avenues of inquiry, particularly the possibility of observing nuclear processes within stars, like the sun. This presented a significant challenge: how to detect particles that interact so rarely with matter. Scientists concluded that detecting neutrinos would require vast amounts of matter, carefully shielded from other radiation.
In the 1960s, Raymond Davis Jr. and colleagues at Brookhaven National Laboratory placed a detector filled with nearly 400,000 liters of perchloroethylene 1.5 kilometers underground in the Homestake mine in South Dakota. When a neutrino struck a chlorine nucleus, it produced a detectable radioactive form of argon. This experiment, running for 25 years, detected only a third of the predicted solar neutrinos, leading to the “solar neutrino problem.”
The mystery began to be solved decades later with experiments like Kamiokande, built by Masatoshi Koshiba in Japan’s Kamioka mine. This detector used 3 million liters of ultrapure water. Neutrinos interacting with the water’s atomic nuclei produced fast-moving electrons that generated flashes of Cherenkov light, which were then detected.
Kamiokande and its successor, Super-Kamiokande, along with Canada’s Sudbury Neutrino Observatory, helped explain the discrepancy. They revealed that neutrinos have three “flavors”—electron, muon, and tau—and can oscillate, or change, between these flavors. This oscillation implies that neutrinos must possess mass, a property not predicted by existing physics laws.
New Detectors, New Discoveries
The tradition of ambitious neutrino detection continues with modern observatories. The IceCube Neutrino Observatory, located beneath the Amundsen-Scott South Pole Station, uses Antarctic ice. It has created a neutrino-based map of the Milky Way and traced high-energy cosmic particles to active galaxies powered by supermassive black holes.
The Cubic Kilometer Neutrino Telescope (KM3NET) in the Mediterranean Sea has recorded the highest-energy cosmic neutrino ever detected, though its source remains unknown. China’s Jiangmen Underground Neutrino Observatory (JUNO), launched in 2025, has already provided precise measurements of neutrino oscillations. Future projects, including Japan’s Hyper-Kamiokande and the Deep Underground Neutrino Experiment (DUNE) in the American Midwest, are expected to begin operations later this decade.
These ongoing and planned experiments, building on the principle of using massive, deep detectors, are steadily revealing the secrets of the neutrino, the particle once thought to be undetectable.
Helene Elliott is the senior reporter for News Raise. She covers Science news. She also has a keen interest in photojournalism. Helene holds a nomination for the prestigious Red Smith Award. She is married to author Dennis D’Agostino, a former publicist with the New York Mets.
