Discovering neutrinos
The existence of the neutrino was postulated by Wolfgang Pauli(1) and Enrico Fermi(2) in the early 1930s in order to explain the beta decay. The first observation of the new particle is dued by Clyde Cowan, Frederick Reines, F. B. Harrison, H. W. Kruse, and A. D. McGuire(3):
A signal rate produced by reaction 1 \[\nu_{-} + p^{+} \rightarrow \beta^{+} + n^0\] must be a linear function of the number of protons provided as targets for the neutrinos.They observed two signals:
The second pulse of the delayed pair signal observed was identified as being due to the capture of a neutron by cadmium in the water target.The discovery was rewarded with the Nobel Prize in Physics in 1995.
One of the most curious fact about neutrinos' Nobel Prizes is that the prize for the discoveries of the neutrinos' flavors was assigned in 1988 to Leon Lederman, Melvin Schwartz and Jack Steinberger, before the prize for the discovery!
Oscillations
There are three different types of based neutrinos, 1, 2 and 3. A superposition of these type of neutrinos generate the three neutrinos detected in our experiments: electron, muon or tau neutrino. Thanks to the based neutrinos, every neutrino of the family can change is flavor in one other: the dress does not make the neutrino!The first clue about neutrino oscillations was the detection of a deficit in the flux of solar neutrinos with respect to the theoretical predicions. This observation was performed at the Homestake Experiment, headed by Raymond Davis, Jr. and John Bahcall(4).
There are a lot of experiments that confirmed these results but the deficit was conclusively identified as an effect of the neutrinos' oscillation only after the Sudbury Neutrino Observatory that provide the definitive results in 2001(5, 6): the experiment was leaded by Arthur McDonald.
The first atmospheric neutrino oscillation was observed at the Super Kamiokande, a neutrino observatory located under Mount Kamioka in Japan: the spokesperson of the collaboration was Takaaki Kajita.
Kajita and McDonald awarded the Nobel Prize in Physics in 2015
for the discovery of neutrino oscillations, which shows that neutrinos have massSo this year the prize put together two of the best known physics' branches: astrophysics and particle physics.
Post scriptum
There are also two other different type of observations about neutrino's oscillation: in the reactor and in particle beam detectors. In this last case I must remember the most recent observation of the tau oscillations dued by OPERA experiment.
(1) W. Pauli in Rapp. Septieme Conseil Phys. Solvay, Brussels 1933
(2) Fermi, E. (1934). Radioattività prodotta da bombardamento di neutroni Il Nuovo Cimento, 11 (7), 429-441 DOI: 10.1007/BF02959915 (pdf)
(3) Cowan, C., Reines, F., Harrison, F., Kruse, H., & McGuire, A. (1956). Detection of the Free Neutrino: a Confirmation Science, 124 (3212), 103-104 DOI: 10.1126/science.124.3212.103 (pdf)
(4) Davis, R., Harmer, D., & Hoffman, K. (1968). Search for Neutrinos from the Sun Physical Review Letters, 20 (21), 1205-1209 DOI: 10.1103/PhysRevLett.20.1205
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