Updated 1998 by John Bahcall.
Original by Bruce Scott.Fusion reactions in the core of the Sun produce a huge flux of neutrinos. Theseneutrinos can be detected on Earth using large underground detectors, and the fluxmeasured to see if it agrees with theoretical calculations based upon our understanding ofthe workings of the Sun and the details of the Standard Model (SM) of particlephysics. The measured flux is roughly one half of the flux expected fromtheory. The cause of the deficit is a mystery. Is our particle physicswrong? Is our model of the Solar interior wrong? Are the experiments inerror? 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. Thestudy of this problem may yield important new insights to help us go beyond the StandardModel. There are many experiments in progress, so stay tuned.
A middle aged main sequence star like the Sun is in a slowly evolving equilibrium, inwhich 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, fusionreactions occurring in the core heating the gas and replacing the energy lost byradiation, and slow structural adjustment to compensate for changes in entropy andcomposition.
We cannot directly observe the center, because the mean free path of a photon againstabsorption or scattering is very short, so short that the radiation-diffusion time scaleis of order 10 million years. But the main proton-proton reaction (PP1) in the Suninvolves emission of a neutrino:
PP1: p + p --> D + positron + neutrino + 0.26 MeVwhich is directly observable, since the cross section for interaction with ordinary matteris 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 makesit difficult to detect the neutrinos. The first experiments by Davis and collaborators,involving large tanks of chloride fluid placed underground, could only detecthigher 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 MeVRecently, however, the GALLEX experiment, using a gallium-solution detector system, hasobserved the PP1 neutrinos to provide the first unambiguous confirmation of proton-protonfusion in the Sun.
There is a "neutrino problem", however, and that is the fact that every experiment hasmeasured a shortfall of neutrinos. About one- to two thirds of the neutrinosexpected are observed, depending on experimental error. In the case of GALLEX, thedata read 80 units where 120 are expected, and the discrepancy is about two standarddeviations. To explain the shortfall, one of two things must be the case: (1) eitherthe temperature at the Sun's center is slightly less than we think it is, or (2) somethinghappens to the neutrinos during their flight over the 150 million km journey toEarth. A third possibility is that the Sun undergoes relaxation oscillations incentral temperature on a time scale shorter than 10 million years, but since no one has acredible mechanism this alternative is not seriously entertained.
(1) The fusion reaction rate is a very strong function of the temperature, becauseparticles much faster than the thermal average account for most of it. Reducing thetemperature 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 neutrinoproblem at all. But the community of solar seismologists, who observe smalloscillations 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 theneutrinos self-interact to change flavor periodically between electron, muon, and tauneutrino types. Here, we would only expect to observe a fraction of the total, sinceonly electron neutrinos are detected in the experiments. (The fraction is notexactly 1/3 due to the details of the theory.) Efforts continue to verify thistheory in the laboratory. The MSW phenomenon, also called "neutrino oscillation",requires that the three neutrinos have finite and differing mass, which is also stillunverified.
To use explanation (1) with the Sun in thermal equilibrium generally requiresstretching several independent observations to the limits of their errors, and inparticular the earlier chloride results must be explained away as unreliable (there wassignificant scatter in the earliest ones, casting doubt in some minds on the reliabilityof the others). Further data over longer times will yield better statistics so thatwe will better know to what extent there is a problem. Explanation (2) depends ofcourse on a proposal whose veracity has not been determined. Until the MSWphenomenon 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 neutrinoemission. Fewer neutrinos are observed than expected, by two standard deviations inthe best result to date. This can be explained either by a slightly cooler centerthan expected or by a particle physics mechanism by which neutrinos oscillate betweenflavors. The problem is not as severe as the earliest experiments indicated, andmore data with better statistics are needed to settle the matter.
The one missing element in this 1994 article is the new and extraordinarily preciseagreement between the predictions of the standard solar model for sound speeds in the Sunand the recent accurate measurements of those sound speeds over nearly the entire volumeof the Sun. The root-mean-squared agreement is 0.1%! The agreement is soprecise that it has changed our view of the problem and physicists are now much moreconfident than before that the problem must be explained by new physics.
For more info visit John Bahcall's web site, which has considerable information aboutsolar neutrinos:
http://www.sns.ias.edu/~jnb