The neutrino and its friends

Neutrinos are one of the fundamental particles which make up the universe. They are also one of the least understood.

Neutrinos are similar to the more familiar electron, with one crucial difference: neutrinos do not carry electric charge. Because neutrinos are electrically neutral, they are not affected by the electromagnetic forces which act on electrons. Neutrinos are affected only by a "weak" sub-atomic force of much shorter range than electromagnetism, and are therefore able to pass through great distances in matter without being affected by it. If neutrinos have mass, they also interact gravitationally with other massive particles, but gravity is by far the weakest of the four known forces.

Three types of neutrinos are known; there is strong evidence that no additional neutrinos exist, unless their properties are unexpectedly very different from the known types. Each type or "flavor" of neutrino is related to a charged particle (which gives the corresponding neutrino its name).  Hence, the "electron neutrino" is associated with the electron, and two other neutrinos are associated with heavier versions of the electron called the muon and the tau (elementary particles are frequently labelled with Greek letters, to confuse the layman). The table below lists the known types of neutrinos (and their electrically charged partners).

A Brief History of the Neutrino

1931 - A hypothetical particle is predicted by the theorist Wolfgang Pauli. Pauli based his prediction on the fact that energy and momentum did not appear to be conserved in certain radioactive decays. Pauli suggested that this missing energy might be carried off, unseen, by a neutral particle which was escaping detection.
• 1934 - Enrico Fermi develops a comprehensive theory of radioactive decays, including Pauli's hypothetical particle, which Fermi coins the neutrino (Italian: "little neutral one"). With inclusion of the neutrino, Fermi's theory accurately explains many experimentally observed results.
• 1959 - Discovery of a particle fitting the expected characteristics of the neutrino is announced by Clyde Cowan and Fred Reines (a founding member of Super-Kamiokande; UCI professor emeritus and recipient of the 1995 Nobel Prize in physics for his contribution to the discovery). This neutrino is later determined to be the partner of the electron.
• 1962 - Experiments at Brookhaven National Laboratory and CERN, the European Laboratory for Nuclear Physics make a surprising discovery: neutrinos produced in association with muons do not behave the same as those produced in association with electrons. They have, in fact, discovered a second type of neutrino (the muon neutrino).
• 1968 - The first experiment to detect (electron) neutrinos produced by the Sun's burning (using a liquid Chlorine target deep underground) reports that less than half the expected neutrinos are observed. This is the origin of the long-standing "solar neutrino problem."  The possibility that the missing electron neutrinos may have transformed into another type (undetectable to this experiment) is soon suggested, but unreliability of the solar model on which the expected neutrino rates are based is initially considered a more likely explanation.
• 1978 - The tau particle is discovered at SLAC, the Stanford Linear Accelerator Center. It is soon recognized to be a heavier version of the electron and muon, and its decay exhibits the same apparent imbalance of energy and momentum that led Pauli to predict the existence of the neutrino in 1931. The existence of a third neutrino associated with the tau is hence inferred, although this neutrino has yet to be directly observed.
• 1985 - The IMB experiment, a large water detector searching for proton decay but which also detects neutrinos, notices that fewer muon-neutrino interactions than expected are observed. The anomaly is at first believed to be an artifact of detector inefficiencies.
• 1985 - A Russian team reports measurement, for the first time, of a non-zero neutrino mass. The mass is extremely small (10,000 times less than the mass of the electron), but subsequent attempts to independently reproduce the measurement do not succeed.
• 1987 - Kamiokande, another large water detector looking for proton decay, and IMB detect a simultaneous burst of neutrinos from Supernova 1987A.
• 1988 - Kamiokande, another water detector looking for proton decay but better able to distinguish muon neutrino interactions from those of electron neutrino, reports that they observe only about 60% of the expected number of muon-neutrino interactions.
• 1989 - The Frejus and NUSEX experiments, much smaller than either Kamiokande or IMB, and using iron rather than water as the neutrino target, report no deficit of muon-neutrino interactions.
• 1989 - Experiments at CERN's Large Electron-Positron (LEP) accelerator determine that no additional neutrinos beyond the three already known can exist.
• 1989 - Kamiokande becomes the second experiment to detect neutrinos from the Sun, and confirms the long-standing anomaly by finding only about 1/3 the expected rate.
• 1990 - After an upgrade which improves the ability to identify muon-neutrino interactions, IMB confirms the deficit of muon neutrino interactions reported by Kamiokande.
• 1994 - Kamiokande finds a deficit of high-energy muon-neutrino interactions. Muon-neutrinos travelling the greatest distances from the point of production to the detector exhibit the greatest depletion.
• 1994 - The Kamiokande and IMB groups collaborate to test the ability of water detectors to distinguish muon- and electron-neutrino interactions, using a test beam at the KEK accelerator laboratory. The results confirm the validity of earlier measurements. The two groups will go on to form the nucleus of the Super-Kamiokande project.
• 1996 - The Super-Kamiokande detector begins operation.
• 1997 - The Soudan-II experiment becomes the first iron detector to observe the disappearance of muon neutrinos. The rate of disappearance agrees with that observed by Kamiokande and IMB.
• 1997 - Super-Kamiokande reports a deficit of cosmic-ray muon neutrinos and solar electron neutrinos, at rates agreeing with measurements by earlier experiments.
• 1998 - The Super-Kamiokande collaboration announces evidence of non-zero neutrino mass at the Neutrino '98 conference.