Etudes des proprietes des neutrinos dans les contextes ...

tel-00450051, version 1 - 25 Jan 2010
of a cylindrical stainless steel tank holding 50,000 tons of ultra-pure water. It is
overseen by about 11100 photomultipliers tubes able to detect Cherenkov light.
The neutrino-electron scattering reaction concerned to detect solar neutrinos is
νa + e − → νa + e − . (2.3)
This reaction has zero physical threshold, but one has to introduce energy cuts
to suppress the background. In the Kamiokande experiment solar neutrinos with
the energies E > 7.5 MeV were detected, whereas the threshold used by Super-
Kamiokande was 5.5 MeV. With these energy cuts, the Kamiokande and Super-
Kamiokande detection rates are only sensitive to the 8 B component of the solar
neutrino flux 3 . Compared to the radiochemical experiments, Cerenkov detectors
are able to detect the direction of the sources. Indeed, the electrons coming from
this reaction are confined to a forward cone. Hence detecting the Cerenkov radiation
from the final electron one can determine neutrino’s direction 4 . Moreover, for
neutrino energies E ≫ me, the angular distribution of the reaction (2.3) points
in the direction of the momentum of the incoming neutrino. The angular distributions
of neutrinos detected in the Kamiokande and Super-Kamiokande experiments
have a prominent peak at 180 ◦ from the direction to the sun. The ability
of the Kamiokande experiment to observe the direction of electrons produced in
solar neutrino interactions allowed experimentalists to directly demonstrate for
the first time that the Sun was the source of the neutrinos detected.
2.1.4 The solar neutrino problem
In all five solar neutrino experiments, fewer neutrinos than expected were detected,
the degree of deficiency being different in the experiments of different
types (fig. 2.3). The solar neutrino problem is not just the problem of the
deficit of the observed neutrino flux: results of different experiments seem to be
inconsistent with each other. Many explanations for this neutrino deficit were
proposed:
1. The most obvious explanation could be the presence of experimental errors,
such as miscalculated detection efficiency or cross section. The fact is that
all the solar neutrino experiments but one (Homestake 5 ) have been calibrated,
and their experimental responses were found to be in a very good
agreement with expectations.
3 The detection rates are also sensitive to hep fluxes but their contribution is small compared
to the 8 B component of the solar neutrino flux.
4 Nevertheless, in this type of reaction it is very difficult to determine the energy of the
neutrino from the measured energy of the final electron because of the kinematical broadening.
However the measured energy spectra of the recoil electrons can yield valuable information
about the neutrino energy spectrum.
5 The argon extraction efficiency of the Homestake detector was also checked by doping it
with a known small number of radioactive argon atoms, but no calibration has been carried
out since no artificial source of neutrinos with a suitable energy spectrum exists.
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