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).
 | 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. |
