Stars as Laboratories for Fundamental Physics - MPP Theory Group

Chapter 10
Solar Neutrinos
The current theoretical and experimental status of the Sun as a neutrino
source is reviewed. Particle-physics interpretations of the apparent
deficit of measured solar neutrinos are discussed, with an emphasis
on an explanation in terms of neutrino oscillations.
10.1 Introduction
The Sun, like other hydrogen-burning stars, liberates nuclear binding
energy by the fusion reaction
4p + 2e − → 4 He + 2ν e + 26.73 MeV (10.1)
which proceeds through a number of different reaction chains and cycles
(Fig. 10.2). With a total luminosity of L ⊙ = 3.85×10 33 erg s −1 =
2.4×10 39 MeV s −1 , the Sun produces about 1.8×10 38 s −1 neutrinos, or
at Earth (distance 1.50×10 13 cm) a flux of 6.6×10 10 cm −2 s −1 . While
this is about a hundred times less than the ν e flux near a large nuclear
power reactor it is still a measurable flux which can be used for
experimentation just like the flux from any man-made source.
The most straightforward application of the solar neutrino flux is
a search for radiative decays by measurements of x- and γ-rays from
the quiet Sun. Because of the long decay path relative to laboratory
experiments one obtains a limit which is about 9 orders of magnitude
more restrictive (Sect. 12.3.1).
A more exciting application is a search for neutrino oscillations.
In fact, the current measurements of the solar neutrino flux are neither
compatible with theoretical predictions nor with each other (“solar
neutrino problem”); all discrepancies disappear with the assumption of
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