Etudes des proprietes des neutrinos dans les contextes ...

tel-00450051, version 1 - 25 Jan 2010
Neutrinos are extraordinary particles, as they play a pivotal role in the modern
physics, from nuclear physics to physics beyond the Standard Model, and
from astrophysics to cosmology. Let us briefly remind the milestones in the discovery
of neutrinos of different flavours as well as of the oscillation phenomenon.
Neutrinos were first born theoretically when Pauli proposed the existence of
a light (but massive) electrically neutral particle of spin half to solve the problem
of the observed continuous spectra of electrons produced in nuclear β-decay.
In 1933, Fermi writing his theory of weak interaction, named this particle the
neutrino (little neutron) since the neutron was discovered by Chadwick one year
before. In 1942 Wang first proposed to use neutrino capture to detect neutrinos
experimentally. Its experimental discovery was made by Cowan and Reines in
1956 who used the Savannah River nuclear reactor as a source of neutrinos shot
into protons producing neutrons and positrons both of which could be detected.
It finally turned out that both the proposed and the observed particles were actually
antineutrinos. In 1962 Lederman, Schwartz and Steinberger brought the
indication of the doublet structure of the leptons through the discovery of the
muon neutrino. The first detection of tau neutrinos was announced in the summer
of 2000 by the DONUT collaboration at Fermilab, making it the latest particle
of the Standard Model to have been directly observed. The existence of a family
of three neutrinos had already been inferred by both theoretical consistency and
experimental data from LEP from Z 0 decay. The Standard Model of particles
predict that the neutrino is massless and consequently cannot change its flavour.
In the same time, other crucial properties of neutrinos were being proposed
and investigated like the phenomenon of neutrino oscillation. It was first proposed
by Pontecorvo in 1957, using an analogy with the neutral kaon system,
who predicted an oscillation of neutrinos to antineutrinos. Only afterwards, developing
his theory, he finally thought of oscillations between flavours.
The theories of thermonuclear reactions made in the 20’s and 30’s, turned
out to explain the production of energy by the stars like our Sun. Gamow and
Schoenberg in the 40’s made the hypothesis that core-collapse supernovae could
produce a huge emission of neutrinos. After the observation of neutrinos it became
clear that stars were powerful neutrino sources and could give precious
information on neutrino properties and on star evolution as well.
Starting in the late 1960s, R. Davis pioneering experiment measured solar
neutrinos for the first time and found that the number of electron neutrinos arriving
from the sun was between one third and one half the number predicted
by the Standard Solar Model, a discrepancy which became known as the solar
neutrino problem. Another neutrino problem showed up when people decided
to measure the atmospheric neutrinos created by reactions of cosmic rays on the
atmosphere, since it was an important background in the proton decay search.
It turned out that the underground neutrino observatories measured an anomaly
in the atmospheric fluxes. One of the proposals to solve simultaneously the solar
neutrino problem and the atmospheric anomaly was to consider that neutrinos
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