original by Bruce Scott
updated 5-JUN-1994 by SIC

The Solar Neutrino Problem

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 which 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 which may 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 the 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:

        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 the 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 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 Myr, 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 per cent would entirely explain GALLEX; indeed, Bahcall has recently published an article arguing that there may be no solar neutrino problem at all. However, the community of solar seismologists, who observe small oscillations in spectral line strengths due to pressure waves traversing through the Sun, argue 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 periodically change flavor 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 further data with better statistics are needed to settle the matter.

References:

The main-sequence Sun: D. D. Clayton, Principles of Stellar Evolution and Nucleosynthesis, McGraw-Hill, 1968. Still the best text.
Solar neutrino reviews: J. N. Bahcall and M. Pinsonneault, Reviews of Modern Physics, vol 64, p 885, 1992; S. Turck-Chieze and I. Lopes, Astrophysical Journal, vol 408, p 347, 1993. See also J. N. Bahcall, Neutrino Astrophysics (Cambridge, 1989).
Experiments by R. Davis et al: See October 1990 Physics Today, p 17.
The GALLEX team: two articles in Physics Letters B, vol 285, p 376 and p 390. See August 1992 Physics Today, p 17. Note that 80 "units" correspond to the production of 9 atoms of Ge(71) in 30 tons of solution containing 12 tons Ga(71), after three weeks of run time!
Bahcall arguing for new physics: J. N. Bahcall and H. A. Bethe, Physical Review D, vol 47, p 1298, 1993; against new physics: J. N. Bahcall et al, "Has a Standard Model Solution to the Solar Neutrino Problem Been Found?", preprint IASSNS-94/13 received at the National Radio Astronomy Observatory, 1994.
The MSW mechanism, after Mikheyev, Smirnov, and Wolfenstein: See the second GALLEX paper.
Solar seismology and standard solar models: J. Christensen-Dalsgaard and W. Dappen, Astronomy and Astrophysics Reviews, vol 4, p 267, 1992; K. G. Librecht and M. F. Woodard, Science, vol 253, p 152, 1992. See also the second GALLEX paper.