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

2.1 Physics Goals
- For the LOW solution there is also an incomplete conversion of in the sun, but there is
a resonant regeneration of inside the earth which would lead to a significantly higher
count rate during the night hours than during the day, the so-called “day-night effect”.
This effect is maximal in the energy region below 1 MeV(see fig. 2.3). BOREXINO,
which is the only real-time experiment measuring neutrinos at these low energies, would
see a day-night difference of 10 - 20 %.
- The eccentricity of the earth’s orbit causes an annual variation of the solar neutrino flux,
with a difference of 7 % between perihelion and aphelion. For ¡Ñ ¡ Î the
vacuum oscillation length for Be neutrinos is about 3 % of the sun-earth distance.
This would lead to significant seasonal variations of the counting rate (see fig. 2.4), as
the oscillation probability changes with the distance from the sun.
- In the case of oscillations into sterile neutrinos, the conversion of the electron
neutrinos is almost complete. As the sterile neutrinos don’t interact in any way in the
detector, the measured count rate in BOREXINO would range from 0 to 25 % of the SSM
flux.
Figure 2.3: Energy dependent survival probabilities of electron neutrinos for the different MSW
scenarios. The time averaged probability is shown in addition to the separate probabilities for
day and night hours. Only for the LOW solution there is a significant day-night effect for the Be
neutrinos.
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