GoKawiilBy 1970, fourteen years after Los Alamos scientists first proved the existence of the neutrino, they knew something was amiss: an underground neutrino experiment built inside the Homestake Mine, an enormous active gold mine in South Dakota, was detecting way fewer neutrinos coming from the Sun than scientists had predicted, and they didn’t know why.
Neutrinos are famously elusive elementary particles. They are electromagnetically neutral and nearly massless, making them extremely hard to detect. These particles continuously and innocuously rain down on Earth from the Big Bang and sources in space. And they come in three varieties, called flavors—electron, muon, and tau.
“Either we didn’t understand the Sun, or we didn’t understand neutrinos,” recalls Thomas Bowles, retired Los Alamos physicist and Laboratory Fellow.
Neutrinos are doubly elusive: not only do they hardly interact with anything, including detectors, but they also shapeshift.
Bowles was lead on the Soviet-American Gallium Experiment (SAGE), a government-to-government collaboration launched in 1987 to figure out what was going on with the Homestake experiment’s data. As it turns out, the Sun was behaving as expected; the neutrinos were playing tricks. SAGE data showed that something was happening to solar neutrinos after production and before detection that caused the detected number to be lower than expected. Scientists proposed that the data could be explained by oscillation—neutrinos changing between flavors as they raced through space.
This would mean neutrinos are doubly elusive: not only do they hardly interact with anything, including detectors, but they also shapeshift. “It’s like a ghost in a white coat enters a room then suddenly the coat flips to black,” laughs Bowles.
Catching the ghost
Known physics was breaking down in the 1920s, and the neutrino fixed it. Physicists studying radioactive decay of atomic nuclei were seeing data that violated the principle of energy conservation. They theorized an undetected particle with specific properties—electrically neutral and nearly massless—that would make sense of the data. Inventing an invisible thing to precisely fill a specific gap was, they admitted, a desperate remedy. But once a new theory was drafted that included the neutrino, suddenly the math worked and the data made sense—the cosmic books were balanced.
Known physics was breaking down in the 1920s, and the neutrino fixed it.
Just because neutrinos fit the data did not make them real. Neutrinos, if they existed, were predicted to interact so weakly that tens of quadrillions would need to pass through a detector before just one was detected. At Los Alamos in the 1950s, nuclear weapons and nuclear reactors were central areas of research, either of which, reasoned Lab physicists Fred Reines and Clyde Cowan, might produce enough neutrinos to catch one. Reines and Cowan explored both areas and in the end took a custom-built detector to the nuclear reactor at South Carolina’s Savannah River Plant in 1956, and made the catch. Specifically, they detected electron antineutrinos—the antiparticle of the electron neutrino, whose very existence proved the existence of the other. Cowan passed away in 1974, and Reines received the Nobel Prize in Physics in 1995 for their discovery.
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