 |
| Wolfgang Pauli, who first hypothesized the neutrino |
A tau neutrino event as recorded by DONUT
“Neutrinos, they are very small/ They have no charge and have no mass/ And do not interact at all,” John Updike wrote in his 1960 poem, “Cosmic Gall.” Neutrinos were a fairly recent discovery then, and within two years physicists would discover that they were only just beginning to understand this mysterious “ghost particle.” For instance, there was more than one kind of neutrino, and it would take physicists another 40 years to find them all.
“Neutrinos, they are very small/ They have no charge and have no mass/ And do not interact at all,” John Updike wrote in his 1960 poem, “Cosmic Gall.” Neutrinos were a fairly recent discovery then, and within two years physicists would discover that they were only just beginning to understand this mysterious “ghost particle.” For instance, there was more than one kind of neutrino, and it would take physicists another 40 years to find them all.
Wolfgang Pauli first proposed the existence of neutrinos in 1930 while investigating the conundrum of radioactive beta decay, in which some of the original energy appeared to be missing after an electron was emitted from an atomic nucleus. He hypothesized that in order to abide by the laws of energy conservation, another, as-yet-undetected neutral particle might also be emitted, accounting for the missing energy.
Pauli was reluctant to publish a paper on this unusual hypothesis, but he penned a letter to a group of prominent nuclear physicists gathering for a conference in Tuebingen, Germany in December asking for input regarding means of detecting such a particle experimentally. “I have done something very bad today by proposing a particle that cannot be detected; it is something no theorist should ever do,” he wrote, describing his idea as “a desperate remedy.”
 |
| A tau neutrino event as recorded by DONUT |
Among the physicists who took Pauli’s idea seriously was Enrico Fermi, who developed the theory of beta decay further in 1934, coining the name “neutrino” (“little neutral one”) in the process. It became clear that if such a particle existed, it must be both very light–less than 1% the mass of a proton–and interact very weakly with matter, making it very difficult to detect. But in 1956, Clyde Cowan and Frederick Reines succeeded in doing just that, sending a telegram to Pauli informing him of their discovery. “Thanks for message,” Pauli telegrammed back. “Everything comes to him who knows how to wait.”
Pauli died two and a half years later, and thus missed the discovery in 1962 of a second type of neutrino, dubbed the muon neutrino, corresponding to the charged muon lepton. (The latter caused I.I. Rabi to famously exclaim, “Who ordered that?”) In 1975, a third charged lepton, tau, was discovered, and subsequent experiments hinted strongly that there should also be a third kind of neutrino. While scientists at CERN uncovered further proof in 1989 of the tau neutrino’s existence, it would be 25 years from the discovery of the tau before the technology was available to actually detect its neutrino directly.
In the 1990s, Fermilab designed the DONUT (Direct Observation of the NU Tau) experiment to search specifically for tau neutrino interactions. The scientists used the Tevatron to produce an intense neut
