Development of a Liquid Scintillator and of Data ... - Borexino - Infn

Neutrino Source Ä (m/MeV) ¡Ñ (eV )
Reactor 1 - -
Accelerator -10 -
Atmosphere - -
Sun - -
Supernova - -
1.3 The Solar Neutrino Puzzle
Table 1.2: Sensitivity level of different types of neutrino oscillation experiments depending on
the neutrino energy and the distance source - detector [Bah89].
This inequality allows to estimate the sensitivity to the parameter ¡Ñ of different types
of neutrino experiments (assuming large mixing angles). These estimates are presented in
table 1.2.
Neutrino Oscillations in Matter (MSW-Effect)
Neutrino oscillations in matter were first investigated by Wolfenstein [Wol78]. Mikheyev and
Smirnov found a resonance effect in matter that strongly enhances the oscillation probability
[Mik85] for electron neutrinos when traveling through a region with variable electron density.
During the propagation of the neutrinos through the sun they coherently scatter forward on
the particles of the solar plasma. Unlike - and -neutrinos, which only interact via NCreactions,
electron-neutrinos can in addition couple via W ¦ -bosons to the electrons. Therefore,
the scattering cross section of is enhanced compared to the one of the other two neutrino
flavours. The difference in the forward scattering amplitudes for electron and muon neutrinos
is
Ô
Ô
where denotes the Fermi constant and Ô the momentum of the neutrino. The forward
scattering amplitude is related to the refractive index via the optical theorem,
Ò« Æ
«
Ô
where Æ is the particle density of electrons. Thus, the refractive index is different for than
for
Ò
Ò Æ ¢
Ô
£
Ô
The oscillation length for electron neutrinos in matter then becomes
ÄÑØ
Ô Æ
¢ Ñ
Ô Æ
where is the electron number density in units of Avogadro’s number. The oscillation length
in matter is independent from the neutrino energy. It is km in the sun’s core and
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