[Physics FAQ] - [Copyright]

Updated 1998 by John Bahcall.

Original by Bruce Scott.

The Solar Neutrino Problem

2008: The Solar Neutrino Problem is now considered solved, but I have yet to get a new page on this subject written up.  Click here to read further. (Editor)

The Short Story

Fusion reactions in the core of the Sun produce a huge flux of neutrinos.  These neutrinos can be detected on Earth using large underground detectors, and the flux measured to see if it agrees with theoretical calculations based upon our understanding of the workings of the Sun and the details of the Standard Model (SM) of particle physics.  The measured flux is roughly one half of the flux expected from theory.  The cause of the deficit is a mystery.  Is our particle physics wrong?  Is our model of the Solar interior wrong?  Are the experiments in error?  This is the "Solar Neutrino Problem".

There are precious few experiments that seem to stand in disagreement with the SM, which can be studied in the hope of making breakthroughs in particle physics.  The study of this problem may yield important new insights to help us go beyond the Standard Model.  There are many experiments in progress, so stay tuned.

The Long Story

A middle aged main sequence star like the Sun is in a slowly evolving equilibrium, in which pressure exerted by the hot gas balances the self gravity of the gas mass.  Slow evolution results from the star radiating energy away in the form of light, fusion reactions occurring in the core heating the gas and replacing the energy lost by radiation, and slow structural adjustment to compensate for changes in entropy and composition. 

We cannot directly observe the center, because the mean free path of a photon against absorption or scattering is very short, so short that the radiation-diffusion time scale is of order 10 million years.  But the main proton-proton reaction (PP1) in the Sun involves emission of a neutrino:

        PP1:    p + p --> D + positron + neutrino + 0.26 MeV
which is directly observable, since the cross section for interaction with ordinary matter is so small (the 0.26 MeV is the average energy carried away by the neutrino).  Essentially all the neutrinos make it to Earth.  Of course, this property also makes it difficult to detect the neutrinos.  The first experiments by Davis and collaborators, involving large tanks of chloride fluid placed underground, could only detect higher energy neutrinos from small side chains in the solar fusion:
        PP2:    Be(7) + electron --> Li(7) + neutrino + 0.80 MeV
        PP3:    B(8) --> Be(8) + positron + neutrino + 7.2 MeV
Recently, however, the GALLEX experiment, using a gallium-solution detector system, has observed the PP1 neutrinos to provide the first unambiguous confirmation of proton-proton fusion in the Sun.

There is a "neutrino problem", however, and that is the fact that every experiment has measured a shortfall of neutrinos.  About one- to two thirds of the neutrinos expected are observed, depending on experimental error.  In the case of GALLEX, the data read 80 units where 120 are expected, and the discrepancy is about two standard deviations.  To explain the shortfall, one of two things must be the case: (1) either the temperature at the Sun's center is slightly less than we think it is, or (2) something happens to the neutrinos during their flight over the 150 million km journey to Earth.  A third possibility is that the Sun undergoes relaxation oscillations in central temperature on a time scale shorter than 10 million years, but since no one has a credible mechanism this alternative is not seriously entertained.

(1) The fusion reaction rate is a very strong function of the temperature, because particles much faster than the thermal average account for most of it.  Reducing the temperature of the standard solar model by 6% would entirely explain GALLEX; indeed, Bahcall has recently published an article arguing that there may be no solar neutrino problem at all.  But the community of solar seismologists, who observe small oscillations in spectral line strengths due to pressure waves traversing through the Sun, argues that such a change is not permitted by their results.

(2) A mechanism (called MSW, after its authors) has been proposed, by which the neutrinos self-interact to change flavor periodically between electron, muon, and tau neutrino types.  Here, we would only expect to observe a fraction of the total, since only electron neutrinos are detected in the experiments.  (The fraction is not exactly 1/3 due to the details of the theory.)  Efforts continue to verify this theory in the laboratory.  The MSW phenomenon, also called "neutrino oscillation", requires that the three neutrinos have finite and differing mass, which is also still unverified.

To use explanation (1) with the Sun in thermal equilibrium generally requires stretching several independent observations to the limits of their errors, and in particular the earlier chloride results must be explained away as unreliable (there was significant scatter in the earliest ones, casting doubt in some minds on the reliability of the others).  Further data over longer times will yield better statistics so that we will better know to what extent there is a problem.  Explanation (2) depends of course on a proposal whose veracity has not been determined.  Until the MSW phenomenon is observed or ruled out in the laboratory, the matter will remain open.

In summary, fusion reactions in the Sun can only be observed through their neutrino emission.  Fewer neutrinos are observed than expected, by two standard deviations in the best result to date.  This can be explained either by a slightly cooler center than expected or by a particle physics mechanism by which neutrinos oscillate between flavors.  The problem is not as severe as the earliest experiments indicated, and more data with better statistics are needed to settle the matter.

1998 Update

The one missing element in this 1994 article is the new and extraordinarily precise agreement between the predictions of the standard solar model for sound speeds in the Sun and the recent accurate measurements of those sound speeds over nearly the entire volume of the Sun.  The root-mean-squared agreement is 0.1%!  The agreement is so precise that it has changed our view of the problem and physicists are now much more confident than before that the problem must be explained by new physics.

For more info visit John Bahcall's web site, which has considerable information about solar neutrinos: