4.6.7. The Solar Neutrino Experiment

Neutrinos are by products of nuclear reactions which occur in the center of the Sun. The core neutrino flux is directly proportional to the nuclear reaction rate which depends sensitively on the core temperature. Neutrinos created at the center of the sun pass directly through it. Detection of these neutrinos on the earth is difficult, but not impossible. By the early 90s 4 solar neutrino telescopes were in operation. Three of these 4 experiments are relatively new and employ a more advanced detector design than the original solar neutrino telescope, essentially a vat of cleaning fluid, in the Homestake Mine in South Dakota. These three newer experiments give fairly consistent results and indicate a total neutrino deficit of about 35%. Specifically, the combined SAGE and Gallex results indicate a measured neutrino flux of 77 ± 10 SNUs (Solar Neutrino Units). Two respective standard solar models give predictions of 132 ± 7 and 123 ± 7 SNUs. Each experiment also has a possible ± 6 SNU systematic error in the calibration. The level of statistical significance is in the range 3.3 - 5.1. The neutrinos which are detected from the sun are not the multitude that are generated by the standard proton-proton chain because they have insufficient energy. Rather, the detected neutrinos come from the rare process of Beryllium and Boron production but the production rate is quite sensitive to the core temperature.

The significant deficiency of observed solar neutrinos has three possible resolutions:

1. The standard model of the sun is wrong and the temperature is cooler than we think. This reduces the overall neutrino flux and the rate of Beryllium and Boron production. A cooler sun seriously impacts the the theoretical age of the Sun and by extrapolation the ages of globular cluster stars. This is regarded as an unlikely solution because stellar evolutionary theory has been so successful at predicting the observed properties of stars in old stellar clusters. Furthermore, Vignaud (1995) strongly concludes that even non-standard solar models can not be tweaked enough to explain the observed deficit.

2. The detector efficiencies are not well understood and the observed fluxes are actually consistent with theory. This is extremely unlikely to be the resolution. Unlike the Homestake Mine experiment, where understanding detector efficiency is crucial, the SAGE and Gallex experiments use a different detection scheme in which the instrumental calibration is fairly well understood. The low systematic error (10% of the observed flux) bears this out.

3. The current experiments are only sensitive to the presence of electron neutrinos. Muon and tau neutrinos pass completely through the detector without registration. Thus, if some physical process operates to change the flavor of the electron neutrino in the 8.5 minutes of travel time between the sun and the detector, an observed deficit of electron neutrinos would result. However, since some electron neutrinos are detected, complete conversion can not occur. A process known as the Mikheyev-Smirnov-Wolfenstein (see Wolfenstein and Beier 1989) effect may come into play here. The MSW effect states that any coherent forward scattering of electron neutrinos in electronic matter results in a density-dependent effective mass. This means that electron neutrinos which travel through an inhomogeneous medium (like the density gradients in the sun or the earth) have some probability of changing their effective mass and hence their flavor. The MSW effect thus requires the neutrino to have some mass but the effect itself is only sensitive to the square of the mass difference and can not provide a good estimate of actual neutrino mass. The current observations suggest a mass difference between species of only m² 10^-5 eV².

The MSW resolution to the solar neutrino problem gives some confidence that the neutrino has a mass. The search for this mass is ongoing at several facilities but the history of claimed neutrino mass detections is not good. In particular, in early 1992 it was announced that several detectors had discovered evidence for a neutrino mass of 17 Kev. Such a large mass, of course, would have caused the Universe to collapse long ago and hence these massive neutrinos, if real, must have decayed into other products. One possible decay channel is the photon and there has been speculation that decay photons have sufficient energy to be relevant to the re-ionization of the Universe after recombination. This possibility if more fully discussed in Chapter 6. About a year later, a detector response problem was discovered which fully explained the 17 KeV neutrino mass as an artifact of the detector. This ended the flurry of theoretical papers on the decay of this kind of neutrino but the general idea that a massive particle could produce ionizing photons in its decay process has been retained as it is of some cosmological interest.

An ongoing experiment (LSND) at Los Alamos is attempting to measure the mixing between electron and muon neutrinos. Early experimental results suggest m² of order 1eV² which is completely inconsistent with that required to explain the solar neutrino deficit via the MSW effect. However, if one considers the effects of muon-tau neutrino mixing (see Wolfenstein 1995) it is possible to reconcile LSDN with SAGE/Gallex by endowing 2 of the 3 neutrino species with a degenerate mass of 2.5 eV. This mass range has very interesting cosmological consequences as it could contribute to the large scale features seen in the power spectrum of the galaxy distribution, without producing = 1. This possibility has also opened up another class of dark matter models which are hybrid CDM-HDM models. These are referred to as MDM for mixed dark matter and will be further discussed in Chapter 5.