Progress and Prospects in Neutrino Astrophysics

Progress and Prosp ects in Neutrino Astrophysics x y z xx John N Bahcall Kenneth Lande Rob ert E Lanou Jr John G Learned yy RG Hamish Rob ertson Lincoln Wolfenstein x Institute for Advanced Study Princeton NJ y Department of Physics University of Pennsylvania Philadelphi a PA z Department of Physics Brown University Providence RI xx Department of Physics Astronomy University of Hawaii Honolulu HI Physics Division Los Alamos National Lab oratory Los Alamos NM and Department of Physics University of Washington Seattle WA yy Department of Physics Carnegie Mellon University Pittsburgh PA The neutrino decits observed in four solar neutrino exp eriments relative to the theoretical predictions have led to fresh insights into neutrino and solar physics Neutrino emission from distant energetic astronomical systems may form the basis for a new eld of astronomy Additional exp eriments are needed to test explanations of the solar neu trino decits and to allow the detection of more distant sources astro-ph/9503047 10 Mar 95 Why neutrino astrophysics Neutrino astrophysics has developed from theoretical calculations to an exp erimental discipline over the last three decades As we shall see in later sections of this pap er solar neutrinos have b een observed in four underground exp eriments and a burst of neutrinos was detected from a sup ernova in the Large Magellanic Cloud The Accepted for publication in Nature Adapted with p ermission from Neutrino Astro physics A Research Brieng Copyright by the National Academy of Sciences Cour tesy of the National Academy Press Washington DC observed solar neutrino uxes all agree with the theoretical exp ectations to within factors of two or three successfully concluding a halfcentury struggle to understand how the sun shines Nevertheless p ersistent decits of electrontype neutrinos exist in all four solar neutrino exp eriments which has led to suggestions that neutrinos may have small masses and may oscillate b etween dierent types New neutrino exp eriments are required to resolve the longstanding solar neutrino problem We shall also describ e eorts that are underway to develop exp eriments that can detect astronomical neutrinos at high energies from sources that are well b eyond the Galaxy First however we shall sketch briey the development and overall state of the eld of neutrino astrophysics In the s and s scientists developed the theoretical basis for understand ing how the sun shines They prop osed that the nuclear fusion reactions among light elements o ccur near the the center of the sun and provide the energy emitted by the sun for four and a half billion years While most of this energy ends up as electromagnetic radiation from the surface approximately three p ercent is b elieved to b e emitted directly from the center of the sun in the form of neutrinos In a pioneering exp eriment that has b een going on for years Raymond Davis Jr and his collab orators rst detected solar neutrinos using radio chemical techniques and a cleaning uid p erchloroethylene as a target Over the last several years there has b een striking conrmation of the astronomical predictions of solar neutrinos The Kamiokande water Cerenkov detector lo cated in the Japanese alps has identied solar neutrinos by their directionality measured by observing the direction of recoil electrons from neutrino interactions Recently two radio chemical exp eriments using gallium the GALLEX exp eriment in Italy and the SAGE exp eriment in Russia have detected the copious low energy b elow keV neutrinos that are the primary comp onent of the solar neutrino ux A sup ernova represents one of the most awesome events in astronomy Type II sup ernovae are b elieved to result from the sudden gravitational collapse of a star that has completed cycles of nuclear burning The energy calculated to b e pro duced from this collapse is almost one thousand times larger than that observed as light Theory indicates that more than p ercent of the energy is emitted in the form of neutrinos In February neutrinos from SNa were detected as a series of pulses in two water Cerenkov detectors Kamiokande in Japan and IMB in the United States the neutrinos arriving as exp ected a few hours b efore the light from the explosion The denitive detection of solar neutrinos and the detection of neutrinos from SNa are two of the most remarkable scientic discoveries of the last decade They provide dramatic conrmation of fundamental theories concerning stellar interiors The detection of solar neutrinos demonstrates that fusion energy is the basic source of energy received from the sun The observations of solar and sup ernova neutrinos op en up a new area of science neutrino astrophysics The observation of astrophysical neutrinos makes it p ossible to prob e the inner most regions of stars dense regions from which light cannot escap e Because of their very small interaction probabilities neutrinos can escap e from these inner regions for the same reason of course it is dicult to detect these elusive particles The motivation for prop osing the rst practical solar neutrino exp eriment was astrophysical to test directly the hypothesis that stars shine and evolve b ecause of nuclear fusion reactions in their interiors However it has b ecome apparent in recent years that solar neutrinos provide a b eam of elementary particles that can b e used to investigate fundamental physics in particular to study intrinsic neutrino prop erties Of particular imp ortance is the question of whether neutrinos have mass and whether neutrinos transform oscillate from one type to another There are b elieved to b e three types of neutrinos electrontype muontype tautype e Only are created in the sun but all types are b elieved to b e emitted from e sup ernovae The question of neutrino mass is of fundamental imp ortance for particle physics and for cosmology In the case of particle physics neutrino mass may provide a clue to the problem of the origin of the masses of all particles An interesting class of theories called grand unied theories suggests that all interactions strong electromagnetic and weak are unied at a large energy scale and that the neutrino masses are inversely prop ortional to this scale Two related ways to prob e this very high energy scale are the search for proton decay and the search for very small neutrino masses Our basic ideas ab out cosmology that explain the microwave background radiation predict a similar background of relic neutrinos If the heaviest of these neutrinos usually taken to b e has a mass ab ove a few electron volts these relic neutrinos could b e an imp ortant comp onent of dark matter In this case with a much smaller mass could play a decisive role for solar neutrinos The only available way to study very small neutrino masses is through the p ossi bility of oscillations that transform one type of neutrino into another For neutrino masses greater than eV such oscillations have b een lo oked for in exp eriments not discussed in this pap er using neutrinos pro duced in the earths atmosphere by cosmic rays or neutrinos pro duced in the lab oratory by reactors or accelerators For smaller neutrino masses or with mass in the range to the only available p ossibility is to use solar neutrinos with oscillations depleting the ux of by converting some of them into or Of particular interest is the p ossibility e that the oscillations may b e enhanced by the coherent interaction of the neutrinos with the solar material This coherent interaction is called the MSW eect The observations made with the four exp eriments p erformed so far indicate uxes of electronneutrinos that are consistently b elow the results of astrophysical calcula tions In fact comparisons b etween the rates of dierent exp eriments suggest inde p endent of astrophysical uncertainties that some previously unaccounted for physical pro cess p erhaps involving nonzero neutrino mass may b e o ccurring The exp eri ments carried out so far are the rst pioneering exp eriments and nal conclusions require additional exp eriments that are currently b eing developed In this rep ort we also consider exp eriments designed to observe neutrinos of much higher energies six to nine orders of magnitude larger than those of solar neutrinos The p otential sources of higher energy neutrinos the techniques for their detection and the scientic justications for the research are all much dierent from the solar case Unlike the sun p ossible astrophysical sources of highenergy neutrinos are remote and not well understo o d estimates of their neutrino uxes are necessarily sp eculative Detectors of highenergy neutrinos are complementary to detectors of solar neutrinos b eing optimized for large areas rather than for low thresholds Candidates for sources of highenergy neutrinos include such exotic but relatively nearby ob jects as neutron stars and black holes in the Galaxy as well as the nuclei of distant but highly luminous galaxies Since we observe cosmic rays to o energetic to b e conned within the Galaxy we know that there exist enormously energetic pro cesses that might well b e sources of high energy neutrinos Will any of these sources pro duce detectable numbers of high energy neutrinos We will never know until we lo ok Ongoing solarneutrino exp eriments There is no single b est type of detector that can b e used to observe solar neutrinos The energies of the neutrinos vary over a range of oneandhalf orders of magnitude and the uxes from the most imp

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