In 1930, Wolfgang Pauli proposed the existence of a new tiny particle with no electric charge. The particle was hypothesized to be very light—or possibly have no mass at all—and hardly ever interact with matter. Enrico Fermi later named this mysterious particle the “neutrino” (or “little neutral one”).

Although neutrinos are extremely abundant, it took 26 years for scientists to confirm their existence. In the 60 years since the neutrino’s discovery, we’ve slowly learned about this intriguing particle.

“At every turn, it seems to take a decade or two for scientists to come up with experiments to start to probe the next property of the neutrino,” says Keith Rielage, a neutrino researcher at the Department of Energy’s Los Alamos National Laboratory. “And once we do, we’re often left scratching our heads because the neutrino doesn’t act as we expect. So the neutrino has been an exciting particle from the start.”

We now know that there are actually three types, or “flavors,” of neutrinos: electron, muon and tau. We also know that neutrinos change, or “oscillate,” between the three types as they travel through space. Because neutrinos oscillate, we know they must have mass.

However, many questions about neutrinos remain, and the search for the answers involves scientists and experiments around the world.

The mystery of the missing energy

Pauli thought up the neutrino while trying to solve the problem of energy conservation in a particular reaction called beta decay. Beta decay is a way for an unstable atom to become more stable—for example, by transforming a neutron into a proton. In this process, an electron is emitted.

If the neutron transformed into only a proton and an electron, their energies would be well defined. However, experiments showed that the electron did not always emerge with a particular energy—instead, electrons showed a range of energies. To account for this range, Pauli hypothesized that an unknown neutral particle must be involved in beta decay.

“If there were another particle involved in the beta decay, all three particles would share the energy, but not always exactly the same way,” says Jennifer Raaf, a neutrino researcher at DOE’s Fermi National Accelerator Laboratory. “So sometimes you could get an electron with a high energy and sometimes you could get one with a low energy.”

In the early 1950s, Los Alamos physicist Frederick Reines and his colleague Clyde Cowan set out to detect this tiny, neutral, very weakly interacting particle.

At the time, neutrinos were known as mysterious “ghost” particles that are all around us but mostly pass straight through matter and take away energy in beta decays. For this reason, Reines and Cowan’s search to detect the neutrino came to be known as “Project Poltergeist.”

“The name seemed logical because they were basically trying to exorcise a ghost,” Rielage says.

Catching the ghost particle

“The story of the discovery of the neutrino is an interesting one, and in some ways, one that could only happen at Los Alamos,” Rielage says.

It all started in the early 1950s. Working at Los Alamos, Reines had led several projects testing nuclear weapons in the Pacific, and he was interested in fundamental physics questions that could be explored as part of the tests. A nuclear explosion was thought to create an intense burst of antineutrinos, and Reines thought an experiment could be designed to detect some of them. Reines convinced Cowan, his colleague at Los Alamos, to work with him to design such an experiment.

Reines and Cowan’s first idea was to put a large liquid scintillator detector in a shaft next to an atmospheric nuclear explosion test site. But then they came up with a better idea—to put the detector next to a nuclear reactor.

So in 1953, Reines and Cowan headed to the large fission reactor in Hanford, Washington with their 300-liter detector nicknamed “Herr Auge” (German for “Mr. Eye”).

Although Reines and Cowan did detect a small increase in neutrino-like signals when the reactor was on versus when it was off, the noise was overwhelming. They could not definitively conclude that the small signal was due to neutrinos. While the detector’s shielding succeeded in blocking the neutrons and gamma rays from the reactor, it could not stop the flood of cosmic rays raining down from space.

Over the next year, Reines and Cowan completely redesigned their detector into a stacked three-layer configuration that would allow them to clearly differentiate between a neutrino signal and the cosmic ray background. In late 1955, they hit the road again with their new 10-ton detector—this time to the powerful fission reactor at the Savannah River Plant in South Carolina.

For more than five months, Reines and Cowan collected data and analyzed the results. In June 1956, they sent a telegram to Pauli. It said, “We are happy to inform you that we have definitively detected neutrinos.”