Stars as Laboratories for Fundamental Physics - MPP Theory Group

Solar Neutrinos 391
The small- and large-angle MSW solutions suggested by Fig. 10.19
would cause a day/night variation of the solar neutrino signal as indicated
in Fig. 10.20. The expected signal is shown in bins for the angle
between the Sun and the nadir of the detector, i.e. in bins of the intersection
length of the neutrino flight path with the Earth. Also shown
are the current Kamiokande measurements, and the expected error bars
after 1 month and 1 year of Superkamiokande running time, respectively.
Shortly after Superkamiokande starts taking data one should be
able to decide whether the large-angle MSW solution applies!
The MSW solutions would also cause a spectral distortion of the recoil
electron spectrum from the primary boron neutrinos. The expected
spectral shape relative to the standard one, arbitrarily normalized at an
electron kinetic energy of T e = 10 MeV, is shown in Fig. 10.21 for several
values of the assumed mixing angle. After several years of running,
Superkamiokande should be able to identify clearly the small-angle solution
if it applies.
If Superkamiokande measures neither a spectral distortion nor a
day/night effect, the deficiency of the boron flux probably would have
Fig. 10.21. Spectral distortion of the recoil electrons from the primary boron
neutrinos in a water Cherenkov detector. The ratio relative to the standard
spectrum is arbitrarily normalized at an electron kinetic energy of T e =
10 MeV. For the large-angle example (solid line) the assumed mass-square
difference is ∆m 2 ν = 2×10 −5 eV 2 , for the small-angle examples (broken lines)
it is ∆m 2 ν = 0.6×10 −5 eV 2 . The anticipated error bars after 5 years of
running Superkamiokande are also indicated for two energies. (Adapted
from Krastev and Smirnov 1994.)