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.
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