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Neutrino Astrophysics: A Research Briefing
Experiments that are currently under way (cf. section entitled “Ongoing Solar Neutrino Experiments”) have the potential to establish– independent of theoretical calculations carried out with solar models –whether new physics is required to explain the solar neutrino observations. If the number of neutrinos that are observed by the Sudbury Neutrino Observatory (SNO) in reaction (4b), which can be induced by any of the known types of neutrinos, exceeds the number observed in reaction (4a), which can be initiated only by electron-type neutrinos, this will be direct evidence for neutrino oscillations. The first measurements of a neutrino energy spectrum by the SNO and the Superkamiokande experiments will test whether the spectrum of the higher-energy ( 8B) solar neutrinos that arrive at earth from the sun is the same as the energy spectrum of neutrinos from the same radioactive isotope observed in the laboratory. The shape of the energy spectrum is, for all practical purposes, independent of any solar influence  and must be the same as the shape inferred from measurements made on laboratory sources–unless new physics is occurring. If, as indicated by theoretical calculations of the MSW effect, the deficit of electron-type neutrinos is energy dependent, the number of lower-energy 7Be neutrinos observed in BOREXINO will be much smaller than predicted by the standard model.
Solar neutrino experiments are fundamental both for physics and for astronomy. It is essential that we not rely on just a few experiments, since the history of science has shown that systematic uncertainties can sometimes lead to mistaken conclusions unless results are checked by performing measurements in different ways. This is particularly important when the measurements are as intrinsically difficult as they appear to be for solar neutrino experiments.
No experiments are currently funded that can measure the energies of individual low-energy neutrinos (energies < 1 MeV) from the basic p-p reaction or from 7Be electron capture, although these are–according to theory–the most abundant neutrinos produced in the sun. The panel recommends that the highest priority be accorded to the development of detectors that can measure the energies of individual low-energy neutrinos, leading to the initiation of one or more new solar neutrino experiments within the next several years. It would also be of great importance to develop other detectors that can measure the neutrino type or the energy spectrum of the high-energy 8B neutrinos.
The National Science Foundation has recognized the synergism between different fields that is currently occurring in solar neutrino research and has recently provided funds for U.S. participation in the development of both the BOREXINO and the iodine detectors. The NSF has stressed the importance of the technological implications of these new activities and has encouraged physicists, chemists, and engineers from academic life and from industry to work together to help develop new detectors and new technologies. The Department of Energy supports similar synergistic collaborations, the GALLEX, SAGE, SNO and Superkamiokande experiments.
Within the $1 billion combined nuclear and particle physics research budgets of the U.S. Department of Energy, the primary operating support for solar neutrino physics is currently about $3 million per year (all in nuclear physics) and, ending in Fiscal Year 1994, a comparable amount of support for capital expenses. The operating base does not yet include any support for the development of new experiments beyond those that will begin operating in 1996. In view of the extraordinary scientific potential of the field of solar neutrinos–both for nuclear physics and for particle physics–it seems to the panel that increasing DOE support for this research would yield strong dividends for science. It is especially important that continuity be maintained in the development and construction of new experiments with advanced capabilities.
The timing of SN1987A was extremely fortunate since the water Cerenkov detectors had been operating for only a few years. An important lesson of that supernova neutrino detection is that neutrinos could reveal supernovae in our Galaxy even if the light is obscured by the Galaxy itself. It would be very desirable if a number of detectors were prepared to detect neutrinos from the next supernova in our galaxy. In particular it is important that there be a coordination of the experiments capable of detecting supernova neutrinos to be sure that they do not go off-line at the same time.