Stars as Laboratories for Fundamental Physics - MPP Theory Group

Solar Neutrinos 365
10.3.4 Water Cherenkov Detector (Kamiokande)
A water Cherenkov detector like the one located in the Kamioka metal
mine (Gifu prefecture, Japan) is a large body of water, surrounded
by photomultipliers which register the Cherenkov light emitted by relativistic
charged particles. Solar neutrinos are detected by virtue of
their elastic scattering on the electrons bound in the water molecules,
ν + e → e + ν. (10.14)
This process has no significant threshold, although in practice the detection
of the electrons is background-limited to relatively large energies.
The Kamiokande detector began its measurement of solar neutrinos
in January 1987 with an effective analysis threshold of about 9 MeV
which was reduced to about 7 MeV in mid 1988. The much larger
Superkamiokande detector, which is scheduled to begin data-taking in
April 1996, will have a threshold of about 5 MeV.
In contrast with the radiochemical detectors which are effectively
based on the charged-current reaction ν e + n → p + e, the elastic
scattering on electrons is sensitive to all (left-handed) neutrinos and
antineutrinos. The cross section is given by the well-known formula
(e.g. Commins and Bucksbaum 1983)
[
dσ νe
dy
= G2 Fm e E ν
A + B (1 − y) 2 − C y m ]
e
, (10.15)
2π
E ν
where G F is the Fermi constant and the coefficients A, B, and C are
tabulated in Tab. 10.7. Further,
y ≡ E ν − E ν
′ = T e
2E ν
and 0 < y < , (10.16)
E ν E ν 2E ν + m e
where E ν and E ν ′ are the initial- and final-state neutrino energies while
T e is the kinetic energy of the final-state electron.
Table 10.7. Coefficients in Eq. (10.15) for elastic neutrino electron scattering.
Flavor A B C
ν e (C V + C A + 2) 2 (C V − C A ) 2 (C V + 1) 2 − (C A + 1) 2
ν e (C V − C A ) 2 (C V + C A + 2) 2 (C V + 1) 2 − (C A + 1) 2
ν µ,τ (C V + C A ) 2 (C V − C A ) 2
ν µ,τ (C V − C A ) 2 (C V + C A ) 2 CV 2 − CA
2
CV 2 − CA
2
C V = − 1 2 + sin2 Θ W ≈ −0.04, C A = − 1 2 .