Stars as Laboratories for Fundamental Physics - MPP Theory Group

Solar Neutrinos 345
There exist vast amounts of data concerning p-modes (periods between
2 min and 1 h) which can be measured from the Doppler shifts
of spectral lines on the solar surface. The main point is that one can
establish a relationship between the multipole order of the oscillation
pattern and the frequency. Because different vibration modes probe the
sound speed at different depths one can invert the results to derive an
empirical profile for the sound speed in the solar interior. The agreement
with theoretical expectations for the square of the sound speed
is better than about 0.3% except in the inner 0.2 R ⊙ which are not
probed well by p-modes (Christensen-Dalsgaard, Proffitt, and Thompson
1993; Dziembowski et al. 1994). Such results make it very difficult
to contemplate the possibility that the Sun is radically different from
a standard structure. On the other hand, these results are not precise
enough to reduce the uncertainty of solar neutrino predictions which
arise from the uncertainty of the opacity coefficients.
The differences between the solar neutrino predictions of different
authors are minimal when identical input parameters are used. Therefore,
the expected error resulting from solar modelling is very small,
except that some key input parameters remain uncertain. The dominant
source of uncertainty for the boron neutrino flux is the cross
section for the reaction p + 7 Be → 8 B + γ which appears as a multiplicative
factor for the solar flux prediction and thus is unrelated to
solar modelling.
A more astrophysical uncertainty are the opacity coefficients. Although
they are thought to be well known for the conditions in the
deep solar interior, even a relatively small error translates into a nonnegligible
uncertainty of the boron flux prediction because of its steep
temperature dependence. Crudely, a 10% error of the opacity coefficients
translates into a 1% error of the central solar temperature and
then into a 20% uncertainty of the boron flux. Indeed, a measurement
of the solar neutrino flux was originally envisaged as a method to
measure precisely the inner temperature of the Sun.
The Sun as a neutrino source naturally has received much attention
in the literature because of its outstanding potential to finally confirm
the existence of neutrino oscillations in nature. Because this book is
not primarily on solar neutrinos I will limit my discussion to what I
consider the most important features of this unique neutrino source,
and the role it plays for particle physics. A lot of key material can be
found in Bahcall’s (1989) book on solar neutrinos, in the more recent
Physics Report on the solar interior by Turck-Chièze et al. (1993), and
on the Sun in general in the book by Stix (1989).