Stars as Laboratories for Fundamental Physics - MPP Theory Group

344 Chapter 10
tional sensitivity it is a true “neutrino telescope” and for the first time
established that indeed neutrinos are coming from the direction of the
Sun. It is perplexing, however, that the measured flux is less suppressed
relative to solar-model predictions than that found in the Homestake
experiment. This is the reverse from what would be expected on the
grounds that 37 Cl is sensitive to boron and beryllium neutrinos.
The situation changed yet again when the experiments SAGE (Soviet-American
Gallium Experiment) and GALLEX began to produce
data in 1990 and 1991, respectively. They are radiochemical experiments
using the reaction ν e + 71 Ga → 71 Ge + e − which has a threshold
of 233 keV. Therefore, these experiments pick up the dominant pp neutrino
flux which can be calculated from solar models with a precision of
a few percent unless something is radically wrong with our understanding
of the Sun. Therefore, a substantial deficit of measured pp neutrinos
would have been a “smoking gun” for the occurrence of neutrino oscillations.
While the first few exposures of SAGE seemed to indicate
a low flux, the good statistical significance of the data that have since
been accumulated by both experiments indicate a flux which is high
enough so that no pp neutrinos are reported missing, but low enough
to confirm the existence of a significant problem with the high-energy
part of the spectrum (beryllium and boron neutrinos).
With four experiments reporting data, which represent three different
spectral responses to the solar neutrino flux, the current attention
has largely shifted from a comparison between experiments and theoretical
flux predictions to a “model-independent analysis” which is based
on the small number of possible source reactions each of which produces
neutrinos of a well-defined spectral shape. This sort of analysis currently
indicates a lack of consistency among the experiments which can
be brought to perfect agreement if neutrinos are assumed to oscillate.
Even though the attention has currently shifted away from theoretical
solar neutrino flux predictions it should be noted that in recent
years there has been much progress in a quantitative theoretical treatment
of the Sun. Independently of the interest in the Sun as a neutrino
source it serves as a laboratory to test the theory of stellar structure
and evolution. Particularly striking advances have been made in the
field of helioseismology. There are two basic vibration patterns for the
Sun, one where gravity represents the restoring force (g-modes), and
normal “sound” or pressure (p) modes. The former are evanescent in
the solar convection zone (depth about 0.3 R ⊙ from the surface) and
have never been unambiguously observed. The oscillation period of the
highest-frequency g-modes would be about 1 h.