Based on theoretical considerations concerning events that occur spontaneously in the nuclei of atoms, the physicist Wolfgang Pauli (1900-1958), in 1930, postulated a new fundamental particle of matter, subsequently (1935) named by the physicist Enrico Fermi (1901-1954), the neutrino ("little neutral one", in Fermi's Italian). The neutrino is an electrically uncharged particle, initially conceived of as associated with the negatively electrically charged particle, the electron, specifically an electron that certain unstable atoms' nuclei generate and emit during a certain type of radioactive emission called beta decay, which occurs spontaneously in certain varieties (isotopes) of certain of Earth's naturally-occurring chemical elements.  Initially the electron-accompanying neutrino was presumed to have no mass.
Physicists subsequently confirmed the existence of the 'electron neutrino' and provided evidence that it has a "vanishingly small" but finite mass. Physicists have also provided evidence of two additional types, or 'flavors', of neutrinos, the muon neutrino, associated with muon particles, and the tau neutrino, associated with tau particles, and they provided evidence that an antineutrino exists for each of the three flavors of neutrinos.
The study of neutrinos has begun to reveal important information about the nature of the universe. This article will discuss what physicists currently know about neutrinos, and to some extent, how they came to that knowledge, its implications regarding the nature of atoms and the universe, and its applications for discovery and for human use.
In September, 2011, the European Organization for Nuclear Research (CERN), Geneva, Switzerland, announced experimental results appearing to show neutrinos traveling faster than light, and invited the scientific community to check them. The cause of this apparent anomaly was later traced to problems with a fiber optic cable used to transmit timing signals. Once these problems were corrected, it was found that the neutrinos did not travel faster than the speed of light after all.
At the time of Pauli's postulated existence of the neutrino, physicists had already discovered that atoms, previously thought of as homogeneous and indivisible, consisted of sub-particles, called subatomic particles, specifically, protons, localized in a center-of-the-atom nucleus, the major location of the atoms' mass, and electrons, by comparison tiny, surrounding the nucleus. The protons each carried a unit of positive electric charge and the electrons, equal in number to the number of protons, each carried a unit of negative electric charge, rendering the atom as a whole electrically neutral.
They had also discovered that some atoms were unstable, in that their nuclei might emit energy (electromagnetic and kinetic) and one or more subatomic particles, a process called radioactivity.
Basing his thinking on the widely accepted law of conservation of energy, Pauli postulated the existence of an electron-associated, presumably massless, neutrino, postulated in order to reconcile an observed discrepancy between the energy emitted from the nucleus during the radioactive process called 'beta decay' — energy too small — and the energy change of the nucleus itself, the missing energy, the energy lost by the nucleus during the beta decay process, carried off by the postulated chargeless neutrino particle.
- Biography of Wolfgang Pauli on the Nobel Prize website.
- Biography of Enrico Fermi on the Nobel Prize website.
- Close F. (2010) Neutrino. Oxford: Oxford University Press. ISBN 9780199574599. | Google books preview.
- What's a Neutrino.
- Neutrino Mass.
- "The current limits from cosmological considerations are less than about 0.5 eV (one millionth of the electron mass!) for the sum of the masses of all three neutrino types. The known values of the mass-squared differences imply that the heaviest neutrino type cannot be less massive than about 0.05 eV. Thus the heaviest neutrino must be between about a ten-millionth and a millionth of the electron mass. The lightest neutrino could in principle be massless, although this would be surprising.
- "Web page copyright © 2007 Kevin McCarty, on behalf of the Borexino Princeton group".
- Franklin A. (2001) Are There Really Neutrinos? An Evidential History. Cambridge, MA: Perseus. ISBN 0738202657. |Google Books preview.
- Adama T et al. (2011) Measurement of the neutrino velocity with the OPERA detector in the CNGS beam.
- The Story of the Neutrino. NuMI-MINOS Homepage. Fermi National Accelerator Laboratory. | NuMI = Neutrino at Main Injector, a FermiLab project to generate an intense beam of neutrinos for study; MINOS = Main Injector Neutrino Oscillation Search, experiments to study the phenomenon of 'neutrino oscillations'.