Neutrino Astrophysics: A Research Briefing (1995)

telescope is projected to have an effective area of several times 10,000 m² for TeV neutrinos and to be operating in 1996.

AMANDA [41], a collaboration of U.S. and Swedish universities, uses hot water drilling to position detectors in deep polar ice (the South Pole) with a geometry similar to that of DUMAND. Ice is a sterile medium (no significant amounts of radioactive elements are present), which acts as a mechanical support for the detector. During the Antarctic summer of 1993-1994, four strings were deployed. Based on the first data from this deployment, the detector architecture has been redesigned: (1) spacings of strings and photomultipliers have been increased to take advantage of an absorption length of light that is more than two times larger than estimated from laboratory measurements; and (2) new strings will be positioned deeper in 1995-1996 to avoid residual air bubbles in the ice that were observed at 1 km.

The NESTOR collaboration [42], consisting of groups from Greece, Russia, Italy, and France, has proposed an underwater array similar to DUMAND, but employing a string of 32-m-diameter umbrella-like structures to place a stack of hexagonal clusters of photomultiplier tubes attached to a common vertical stalk. The array would be placed off the southwest coast of Greece, near Pylos. The experimenters are constructing an infrastructure, making tests, and ordering long-lead-time components, with an actual array several years away. The higher density of detectors and the 3.5-km depth will yield a relatively low energy threshold.

It is generally agreed that significant high-energy neutrino astronomy cannot be done without constructing a telescope with an effective muon detecting area of at least 1 km by 1 km. Two of the principal science goals for a kilometer-scale neutrino telescope are the search for neutrinos from the annihilation in the sun of weakly interacting dark matter particles (with masses in the GeV to TeV range) and the observation of individual active galactic nuclei with good statistics. With such a device, it might be possible to extend the study to neutrino oscillations involving tau-type neutrinos as well [43]. The instruments currently under construction have about 50 times larger areas than previous mine-based detectors; another increase of 50 is needed to reach the desired scale. In the last several years an increasing number of scientists have recognized the importance and the possible feasibility of such a construction [44]. A group of about 40 scientists has formed to act as an ad hoc steering group for a potential international effort. A substantial number of scientists in the high-energy physics community are ready to commit time and effort to an endeavor in the size class of colliding-beam accelerator detectors. A final decision concerning such a project should await the experience gained with the instruments now under construction, such as AMANDA and DUMAND.

Solar neutrino experiments carried out over the past 30 years have closed an important chapter in the history of science that began in the early part of this century. We now have conclusive, direct evidence for nuclear fusion reactions that occur among light elements in the center of the sun. However, further progress in understanding the details of the solar nuclear reactions requires a knowledge of neutrino physics that is at the frontier of particle physics.

In order to disentangle the astronomical observations of solar neutrinos, we must be able to answer the following basic physics question: What happens to MeV neutrinos that pass through 10¹¹ g cm⁻² (~ <(solar density) × (solar radius) >) of matter or travel 1013 cm (earth-sun separation) between production and detection? Quite remarkably, the answer to this question may involve neutrino masses in the range from 10⁻² eV to 10⁻⁵ eV, a mass range that is suggested as being especially interesting according to some particle theories that unify the strong and the electroweak forces. Solar neutrino experiments therefore tell us both about elementary-particle physics and about the details of nuclear fusion within the innermost region of the nearest star.

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 [45] 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.