In June 1956, a team of physicists led by Clyde Cowan and Frederick Reines made a fundamental discovery that changed our understanding of the universe. At Los Alamos National Laboratory, they confirmed the existence of neutrinos, subatomic particles whose existence had been postulated decades earlier to solve a mystery in physics. Known for their “ghostly” nature, these particles barely interact with matter, requiring experiments on a monumental scale to observe.
The postulation of invisible particles and the challenge of initial detection
The idea of neutrinos emerged in 1930, when Austrian physicist Wolfgang Pauli proposed the existence of a particle to explain the apparent loss of energy during radioactive beta decay. He joked that he had done “a terrible thing” by postulating a particle that, he said, “cannot be detected.” However, scientific persistence has proven otherwise. Because they have no electrical charge and have minimal mass, neutrinos traverse space, the Earth and even our bodies with almost no interaction, making their detection an extraordinary feat.
To prove the existence of these particles, Cowan and Reines created the “Poltergeist Project” in early 1956. They positioned a 10-ton custom detector, protected by thick lead walls and wet sandbags, near a powerful nuclear reactor at the Savannah River Power Plant in South Carolina. In a message sent to Pauli in June of that year, physicists at Los Alamos National Laboratory excitedly announced: “We are pleased to report that we have detected neutrinos with certainty.” This was the first step in a seven-decade journey.
Solar enigma and the first underground observatories
After confirming the existence of neutrinos, the scientific community turned its attention to the possibility of using them to observe internal nuclear processes in stars, including the Sun. The big challenge was how to capture particles so elusive that they are emitted by distant stars and pass through almost everything without detection. The solution was to build gigantic detectors, protected from the noise of other forms of radiation and installed in deep locations.
In the 1960s, Raymond Davis Jr. and his colleagues at Brookhaven National Laboratory installed a tank a mile underground at the Homestake mine in South Dakota. This immense container was filled with nearly 400,000 liters of perchloroethylene, a chlorine-based cleaning fluid. The hope was that on the rare occasion that a neutrino collided with a chlorine nucleus, it would transform into a radioactive form of argon, capable of detection and counting. The experiment, which lasted 25 years, found only a third of the number of solar neutrinos predicted by theoretical models, giving rise to the “solar neutrino problem.”
The revelation of “flavors” and the mass of neutrinos
The mystery of the solar neutrino problem took decades to fully solve, requiring even more massive and sophisticated experiments. Deep in the Kamioka mine in Japan, physicist Masatoshi Koshiba built the Kamiokande detector, which used 3 million liters of ultrapure water. In this system, the occasional interaction of neutrinos with atomic nuclei in water generated an electron that moved so quickly that it produced a flash of Cherenkov light, detected by sensors.
The Kamiokande and Koshiba confirmed the discrepancy observed by Davis. Subsequent experiments, such as the even larger Super-Kamiokande in Japan and the Sudbury Neutrino Observatory in Canada, finally explained the failure. Neutrinos were found to come in three distinct “flavors” (electron, muon and tau) and are capable of oscillating, or switching, between them. This crucial discovery implied that neutrinos must have mass, something that the laws of physics had not previously predicted and which still raises fundamental questions today.
Exploring the universe with the new generation of telescopes
The tradition of grandiose ambitions and surprising results continues with the latest generation of neutrino detectors. These observatories explore extreme environments to maximize the chances of capturing and analyzing these elusive particles:
- IceCube Neutrino Observatory:Located under the Amundsen-Scott Station at the South Pole, this observatory uses Antarctic ice as a means of detection. He has already produced a detailed map of the Milky Way composed exclusively of neutrinos and tracked high-energy cosmic particles to active galaxies driven by supermassive black holes.
- Cubic Kilometer Neutrino Telescope (KM3NET):Installed at the bottom of the Mediterranean Sea, KM3NET has already recorded the highest-energy cosmic neutrino known. However, its exact origin remains one of the mysteries to be solved.
The Future Journey in Neutrino Detection
Neutrino oscillations and the countless enigmas they present continue to drive the development of ever more advanced detectors. International collaboration is key to these large-scale projects, which promise to reveal even more about the universe and the laws that govern it.
Among the future large observatories that are coming into operation or that have already provided initial data, the following stand out:
- Jiangmen Underground Neutrino Observatory (JUNO), China:Launched in 2025, JUNO has already published initial data in June 2026, offering the most accurate neutrino oscillation measurements to date.
- Hyper-Kamiokande (Hyper-K), Japan:An even larger and more sensitive version of the Kamiokande, expected to go into operation later this decade.
- Deep Underground Neutrino Experiment (DUNE), American Midwest:Another ambitious project that should begin operating in the coming years, seeking to deepen understanding of the properties of neutrinos.
Thanks to these and many other audacious experiments, the particle that Wolfgang Pauli believed to be undetectable is slowly revealing its deep secrets. The “recipe for discovery”, as seen in seven decades of research, remains the same: think big, dig deeper and have the patience to observe the invisible.
