Stars as Laboratories for Fundamental Physics - MPP Theory Group

Solar Neutrinos 367
Fig. 10.10. Differential scattering cross section for ν e e → eν e according to
Eq. (10.18), normalized to unity for θ = 0, where θ is the angle between the
incident neutrino and final-state electron.
device and thus a true “neutrino telescope.” Thus, for the first time
the Kamiokande detector actually proved that neutrinos are coming
from the direction of the Sun. The first main publication of these
results was by Hirata et al. (1991) from a data sample of 1040 live detector
days, taken between 1987 and 1990; an update including data
until July 1993 (a total of 1670 live detector days) was given by Suzuki
(1995). The angular distribution of the registered electrons relative to
the direction of the Sun is shown in Fig. 10.11. Even though there
remains an isotropic background, probably from radioactive impurities
in the water, the solar neutrino signal beautifully shows up in these
measurements.
In elastic neutrino-electron scattering, the energy distribution of the
kicked electrons is nearly flat so that the spectral shape of the incident
neutrino flux is only indirectly represented by the measured electron
spectrum. In Fig. 10.12 the electron recoil spectrum is shown for an
incident spectrum of 8 B neutrinos. A water Cherenkov detector can
resolve the energy of the charged particles from the intensity of the
measured light which for low-energy electrons is roughly proportional
to the energy. Therefore, the electron recoil spectrum from the interaction
with solar neutrinos can be resolved at Kamiokande. The measured
shape relative to the theoretically expected one is shown in Fig. 10.13,
arbitrarily normalized at E ν = 9.5 MeV (Suzuki 1995). Within statistical
fluctuations the agreement is perfect.