¤ Majorana vs Dirac
The following is a short history of neutrinos as it relates to neutrino oscillation studies.
1920-1927 Charles Drummond Ellis(along with James Chadwick and colleagues) establishes clearly that the beta decay spectrum is really continous, ending all controversies.
1930 Wolfgang Pauli hypothesizes the existence of neutrinos to account for the beta decay energy conservation crisis.
1932 Chadwick discovers the neutron.
1933 Enrico Fermi writes down the correct theory for beta decay, incorporating the neutrino.
1946 Shoichi Sakata and Takesi Inoue propose the pi-mu scheme with a neutrino to accompany muon. (There is a long story about the confusion of mu for pi etc. They were the first to straighten it out and get the spins right, and write down the correct decay scheme completely: pi -> mu + nu_mu, mu -> e + nu_e + nu_mu, and noticed that both numu and nue are light, and neutral with spin 1/2, and suggested that they might be "different".)
1956 Fred Reines and Clyde Cowan discover (electron anti-) neutrinos using a nuclear reactor.
1957 Neutrinos found to be left handed by Goldhaber, Grodzins and Sunyar.
1957 Bruno Pontecorvo proposes neutrino-antineutrino oscillations analogously to K0-K0bar, leading to what is later called oscillations into sterile states.
1962 Ziro Maki, Masami Nakagawa and Sakata introduce neutrino flavor mixing and flavor oscillations.
1962 Muon neutrinos are discovered by Leon Lederman, Mel Schwartz, Jack Steinberger and colleagues at Brookhaven National Laboratories and it is confirmed that they are different from nues.
1964 John Bahcall and Ray Davis propose feasability of measuring neutrinos from the sun.
1965 The first natural neutrinos are observed by Reines and colleagues in a gold mine in South Africa, and by Goku Menon and colleagues in Kolar Gold fields in India, setting first astrophysical limits.
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 on which the expected neutrino rates are based is initially considered a more likely explanation.
1968 Ray Davis and colleagues get first radiochemical solar neutrino results using cleaning fluid in the Homestake Mine in North Dakota, leading to the observed deficit known as the "solar neutrino problem".
1976 The tau lepton is discovered by Martin Perl and colleagues at SLAC in Stanford, California. After several years, analysis of tau decay modes leads to the conclusion that tau is accompanied by its own neutrino nutau which is neither nue nor numu.
1976 Designs for a new generation neutrino detectors made at Hawaii workshop, subsequently leading to IMB, HPW and Kamioka detectors .
1980s The IMB, the first massive underground nucleon decay search instrument and neutrino detector is built in a 2000' deep Morton Salt mine near Cleveland, Ohio. The Kamioka experiment is built in a zinc mine in Japan.
1985 The "atmospheric neutrino anomaly" is observed by IMB and Kamiokande.
1986 Kamiokande group makes first directional counting observation solar of solar neutrinos and confirms deficit.
1987 The Kamiokande and IMB experiments detect burst of neutrinos from Supernova 1987A, heralding the birth of neutrino astronomy, and setting many limits on neutrino properties, such as mass.
1988 Lederman, Schwartz and Steinberger awarded the Nobel Prize for the discovery of the muon neutrino.
1989 The LEP accelerator experiments in Switzerland and the SLC at SLAC determine that there are only 3 light neutrino species(electron, muon and tau).
1991-2 SAGE (in Russia) and GALLEX (in Italy) confirm the solar neutrino deficit in radiochemical experiments.
1995 Frederick Reines and Martin Perl get the Nobel Prize for discovery of electron neutrinos (and observation of supernove neutrinos) and the tau lepton, respectively.
1996 Super-Kamiokande, the largest ever detector, begins searching for neutrino interactions on 1 April at the site of the Kamioka experiment, with Japan-US team(led by Yoji Totsuka).
1998 After analyzing more than 500 days of data, the Super-Kamiokande team reports finding oscillations and, thus, mass in muon neutrinos. After several years these results are widely accepted and the paper becomes the top cited experimental particle physics paper ever.
2000 The DONUT Collaboration working at Fermilab announces observation of tau particles produced by tau neutrinos, making the first direct observation of the tau neutrino.
2000 SuperK announces that the oscillating partner to the muon neutrino is not a sterile neutrino, but the tau neutrino.
2001 and 2002 SNO announces observation of neutral currents from solar neutrinos, along with charged currents and elastic scatters, providing convincing evidence that neutrino oscillations are the cause of the solar neutrino problem.
2002 Masatoshi Koshiba and Raymond Davis win Nobel Prize for measuring solar neutrinos(as well as supernova neutrinos).
2002 KamLAND begins operations in January and in November announces detection of a deficit of electron anti-neutrinos from reactors at a mean distance of 175 km in Japan. The results combined with all the earlier solar neutrino results establish the correct parameters for the solar neutrino deficit.
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).
Neutrino ne nm nt
Charged Partner electron (e) muon
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).
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.