these relic neutrinos could be an important component of dark matter. In this case, νμ with a much smaller mass could play a decisive role for solar neutrinos.
The only available way to study very small neutrino masses (νμ or ντ with mass in the range of 10−2 to 10−5 eV) is by using solar neutrinos. (Larger neutrino masses may be revealed by experiments, not discussed in this report, using neutrinos produced in the earth's atmosphere by cosmic rays or neutrinos produced in the laboratory by reactors or accelerators.) Neutrino oscillations, which may occur in vacuum [14] and which may be amplified in solar material (the MSW effect [15,16]), can deplete the flux of νe by converting some of them into νμ or ντ. The observations made with the four experiments performed so far indicate fluxes of electron-type neutrinos that are consistently below the results of astrophysical calculations. In fact, comparisons between the rates of different experiments suggest (independent of astrophysical uncertainties) that some previously unaccounted for physical process, perhaps involving neutrino non-zero neutrino mass, may be occurring. This deficit of “electron-type neutrinos” provides preliminary evidence for a non-zero neutrino mass. The experiments carried out so far are the first pioneering experiments, and final conclusions require additional experiments that are currently being developed.
In this report, the panel also considers experiments designed to observe neutrinos of much higher energies, six to nine orders of magnitude greater than those of solar neutrinos. The potential sources of higher-energy neutrinos, the techniques for their detection, and the scientific justifications for the research are all much different from the solar case.
Unlike the sun, which is the astronomical object about which we have the most information and the most detailed understanding, astrophysical sources of high-energy neutrinos are remote and not well understood (which is partly why they are interesting), and estimates of their neutrino fluxes are speculative. Detectors of high-energy neutrinos are complementary to detectors of solar neutrinos, being optimized for large areas rather than for low thresholds. It is hoped that the large detectors will extend neutrino astronomy far beyond the Galaxy. Candidates for sources of high-energy neutrinos include such exotic but relatively nearby objects as neutron stars and black holes in the Galaxy as well as the nuclei of distant but highly luminous galaxies. We know that particles of extremely high energies are produced somewhere outside the Galaxy because we observe cosmic rays too energetic to be confined within the Galaxy [17].
Will any of the previously discovered exotic sources of photons also be sources of detectable numbers of high-energy neutrinos? We will never know until we look. What we do know is that astrophysical theory has not traditionally been a good guide to new phenomena. For example, only a few years ago nearly every theoretical astrophysicist who had expressed an opinion on the subject believed that the energetic gamma-ray bursts that contain so much photon energy in short flashes in the MeV energy range are associated with neutron stars in the Galaxy. Recent satellite measurements have shown instead that all, or at least most, of the gamma-ray bursters must be located far outside the Galaxy and must be much more luminous than previously believed. High-energy neutrino astronomy, like much of observational astronomy, is an exploratory science.
Ongoing Solar Neutrino Experiments
There is no single, best type of detector that can be used to observe solar neutrinos. The average energies of the neutrinos from different nuclear sources range from 0.2 MeV to about 7 MeV, and the fluxes from the most important neutrino sources vary by four orders of magnitude. According to standard solar model calculations [2], the numerous neutrinos from the initiating proton-proton reaction with energies less than 0.4 MeV have a flux of about 6 × 106 cm−2 s−1; the rare, higher-energy neutrinos from 8B decay with energies up to 14 MeV have a flux of about 6 × 106 cm−2 s−1. Eleven recently published solar models, each constructed using a different computer code and different input physics, give fluxes of neutrinos that are less energetic than 1 MeV and that are the same to better than 10 percent [18].
Figure 1 shows the theoretically calculated solar neutrino spectrum from different neutrino sources. Because of the wide range in energies
and in fluxes, a set of detectors with sufficient complementarity and some redundancy is required to untangle the different aspects of neutrino physics and of solar physics.
The Chlorine Experiment
In this remarkable experiment [3], neutrinos are detected using a large tank containing 615 tons of perchloroethylene, C2Cl4, which is a commercially available cleaning fluid. The target isotope, 37Cl, can capture a neutrino, producing a radioactive isotope 37Ar by the following process:
.ve + 37Cl → 37Ar + e−. (1)
This reaction can occur for electron-type neutrinos with energy greater than 0.8 MeV. The neutrino reactions are registered by counting the radioactive nuclei, 37Ar. In order to avoid extraneous (background) interactions caused by cosmic rays incident on the surface of the earth, the experiment is conducted 1500 meters underground in the Homestake gold mine in Lead, South Dakota. After about two months in the solar neutrino “sunshine,” the buildup of 37Ar atoms from neutrino capture by 37Cl is approximately balanced by the loss due to decay. At this point, the standard solar model [19] predicts that only 54 37Ar atoms are present in the 615 tons of C2Cl4. It is an experimental challenge to extract and count this minuscule number of atoms in such a large target mass. Over the past two decades, more than 100 extractions of 37Ar have been performed. After correction for detection efficiency the number of 37Ar atoms in the detector at extraction is observed to be only 17, corresponding to a solar-neutrino-induced production rate of 0.48 atoms per day, far fewer than the 1.5 atoms per day expected on the basis of the standard model. In terms of the solar neutrino unit, SNU (pronounced “snew,” 1 SNU = 10−36 interactions per target atom per second), the observations yield 2.55 ± 0.25 SNU, about one third of the 8 ± 1 SNU predicted by the standard model. For two decades, this discrepancy between the measured chlorine event rate and the event rate predicted by the standard model constituted the well-known solar neutrino problem.
The Kamiokande Experiment
Like a supersonic aircraft creating a sonic boom, a charged particle moving through matter faster than the speed of light gives off a conical shock wave of light called Cerenkov radiation. Sensitive photomultiplier tubes can record the flashes of light that betray the presence and direction of these high-speed particles. With the aid of special large photomultiplier tubes, experimenters working in a mine in the Japanese alps and using a 3000-ton detector of ultrapure water (680-ton fiducial volume), called Kamiokande, were able to detect electrons that had been struck by neutrinos from the sun [4,5]. The reaction that was observed is
ν + e− → ν + e−. (2)
At the energies of interest for solar neutrino detection, reaction (2) is about six times more likely for νe than for νμ or ντ. Despite a significant background from the radioactivity of the rocks in the mine, from radioactivity in the water, and from cosmic-ray-induced events, the neutrinos were unambiguously detected because the electrons struck by neutrinos recoiled preferentially in the direction from the sun to the earth. Kamiokande, the first “active” (electronic rather than radiochemical) solar neutrino experiment, showed two very important things: first, the detected neutrinos do indeed come from the sun, and, second, the observed flux above an electron-detection threshold of about 8 MeV is about half as large as predicted by the standard solar model. The discrepancy between prediction and observation is less severe at energies above 8 MeV (the Kamiokande experiment) than it is for all energies above 0.8 MeV (the chlorine experiment).
The predicted counting rate for any particular solar neutrino experiment depends on calculations that are usually based on a standard solar model. Solar models are constrained by measurements of the solar luminosity, age, chemical composition, and thousands of seismological frequencies. Nevertheless, the predicted neutrino emission rates cannot be tested independent of solar neutrino experiments. Changes in the input data for the standard solar model could in principle be made just so as to fit any one of the existing experiments. However, it appears essentially
