Always elusive, Fermi's "little neutral one" turns out to be a quick-change artist as well, offering answers and new questions for physics and cosmology
It is a story told fondly in histories of physics. "Dear Radioactive Ladies and Gentlemen," wrote Wolfgang Pauli from Zürich in 1930, " . . . I have hit upon a desperate remedy . . . ." Pauli was working out some kinks in the emerging theory of radioactivity. One problem involved the beta decay of an atomic nucleus, in which a neutron turns into a proton and spits off an electron. By the law of conservation of energy, what emerges from such a process must equal what went into it. Each of the final two particles must end up with a fixed fraction of the total energy.
But Pauli observed that the electrons exit with less than this fixed energy, apparently violating the law of conservation of energy. His desperate remedy was a new, invisible particle to carry off the "missing" energy in the decay. He required this particle to have certain properties: a zero electric charge (to conserve charge), a spin of one-half and a very small mass. A few years later, after incorporating Pauli's "desperate remedy" into his famous theory of beta decay, Enrico Fermi named the new particle a neutrino, meaning "little neutral one."
Soon afterward, in the 1940s, physicists observed that there was also energy missing from the decay of high-energy particles found in cosmic rays. Some of these particles, called muons, are members (along with the electron) of the class of subatomic particles called leptons, and they decay into neutrinos and other particles during the particle showers that happen when cosmic rays encounter the earth's atmosphere. A consistent explanation of cosmic-ray decay patterns required the existence of different "flavors" of neutrino. Thus electrons and electron neutrinos have electron flavor; muons and muon neutrinos have muon flavor.
The story of the neutrino slows down at this point. Many years passed before experimenters were able to observe any kind of interaction involving a neutrino and confirm the existence of the two flavors. The explanation for their slow progress lies in the famous elusiveness of the neutrino. Neutrinos interact with other particles via the "weak force," one of the four fundamental forces of nature. Weak indeed is this interaction. Imagine looking for the imprint of a high-energy neutrino in a 10-meter-square cube of water. More than 1011 neutrinos—100 billion—would pass right on through for each interaction detected. Happily, a few neutrinos do leave traces for experimenters who make the effort to find them. In 1956 Clyde Cowan, Jr. and Frederick Reines directly observed neutrino interactions for the first time in an experiment at the Savannah River nuclear reactor.
Since then particle physics has come a long way, building a "Standard Model" describing the fundamental subatomic particles and the forces between them. In this model, there are six types of quarks, which bind together to form the familiar proton and neutron and several less-familiar composite particles. The lighter leptons, also six, are found outside the atomic nucleus. The quarks and the leptons are each grouped in three "generations" containing paired particles of successively higher masses; the most massive of these have been tracked down in accelerator experiments in recent years. Physicists commonly say leptons come in three flavors (another word for generations)—electron, muon and tau (whose existence was not confirmed until the 1970s). The Standard Model asserts that there is one neutrino per lepton flavor, each paired with its antiparticle, or antimatter counterpart.
Part of the fascination with the neutrino comes from the question of whether it has mass; this question has special importance to cosmologists studying the "dark matter problem," the fact that there appears to be more gravitationally interacting matter in the universe than can now be seen. In its present formulation, the Standard Model has no explanations for neutrino mass. In fact, until very recently it has not been known whether neutrinos have mass at all. This question must be answered by observation and experiment. And indeed, although the masses of neutrinos remain unmeasured, physicists now know from direct detection that some, at least, must have mass, and that they oscillate: They change from one flavor to another. In order for particles to oscillate, there must be differences between the masses of the different flavors; these differences are related to the frequency of the oscillation, and so the new oscillation measurements begin to suggest how large the differences might be. The new evidence that the neutrino may have mass, and oscillate, is a finding with the potential to advance astrophysics, cosmology and high-energy physics in general. We are pleased to have been among the large group of collaborators who joined in publishing the finding, and are happy to share the story.