Stars as Laboratories for Fundamental Physics - MPP Theory Group

342 Chapter 10
neutrino oscillations. At the present time, however, this interpretation
is not established “beyond reasonable doubt”—a final verdict can be
expected from the new experiments currently in preparation. They may
be able to discover a characteristic distortion of the neutrino spectrum,
or they may actually measure the “wrong-flavored” neutrinos that were
produced by oscillations from the ν e originating in the Sun.
Contrary to the ν e flux from a power reactor, the solar neutrino
spectrum arises from a small number of specific reactions (Fig. 10.1).
The main contribution (91%) is from the reaction p + p → d + e + + ν e
with a maximum neutrino energy 0.420 MeV (“pp neutrinos”). Second
at about 7% of the flux are the “beryllium neutrinos” from the electroncapture
reaction e − + 7 Be → 7 Li + ν e with a fixed energy 0.862 MeV.
Finally, very small fraction (≈ 10 −4 ) are the “boron neutrinos” from
8 B → 8 Be + e + + ν e . Still, they are of major importance because
their large energies of up to 15 MeV allow for a less difficult detection
procedure than is required for the soft part of the spectrum.
The first solar neutrino experiment is based on the nuclear reaction
ν e + 37 Cl → 37 Ar + e − . With a threshold of 0.814 MeV it picks up
both beryllium and boron neutrinos, although the argon production
rate is dominated by the latter. The target consists of about 4×10 5
liters (615 tons) of perchloroethylene (C 2 Cl 4 ) in a huge tank which
is located in the Homestake Mine in South Dakota (U.S.A.), about
1.5 km underground for protection against the cosmic-ray background.
After an exposure of a few months to the solar neutrino flux a few
argon atoms have been produced (about 0.4 atoms per day). They
are chemically extracted and counted by their subsequent decays (halflife
35.0 days).
When this pioneering experiment first produced data (Davis, Harmer,
and Hoffman 1968) there appeared a deficit relative to the theoretically
expected flux, a discrepancy which has persisted ever since—the
experiment is still taking data today! On the theoretical side, the ever
refined predictions of Bahcall and his collaborators (e.g. Bahcall 1989)
were instrumental at establishing the notion that this discrepancy—a
factor of around 3—was to be taken seriously. However, in spite of the
acknowledged experimental care of Davis and his collaborators, doubt
has always lingered about the reliability of the data because this detector
has never been subject to an on-off test as the Sun is the only
available neutrino source powerful enough to cause a detectable signal.
On the solar side, the boron neutrino flux depends crucially on the
reaction rate p + 7 Be → 8 B + γ with a cross section that is relatively
poorly known.