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Astronomy and Astrophysics in the New Millennium: Panel Reports (2001)

Chapter: 3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics

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Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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3
Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

SUMMARY

Particle and nuclear astrophysics and gravitational-wave astronomy offer tremendous discovery potential in the next decade and beyond. The direct measurement of gravitational waves from astrophysical sources will open new investigations in both astrophysics and the physics of strong gravitational fields. High-energy charged particles and gamma rays as well as neutrinos carry unique information about the high-energy universe that is complementary to information obtained by more traditional astronomical approaches. The quest to identify the dark matter is of the utmost importance for astrophysics and cosmology as well as for elementary particle physics.

The Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics of the Astronomy and Astrophysics Survey Committee recommends that highest priority be given to the Laser Interferometer Space Antenna (LISA) because of the fundamental and novel exploration of the gravitational-wave universe it can accomplish, including the observation of massive black holes coalescing in colliding galaxies and the study of white dwarf binaries in our own galaxy. The panel’s highest recommendation among ground-based projects (and second overall) is the Very Energetic Radiation Imaging Telescope Array System (VERITAS), which together with the Gamma-ray Large Area Space Telescope (GLAST) will study many rapidly variable energetic sources, including nuclei of active galaxies, and will map the gamma-ray sky with unprecedented precision. An attractive small-scale opportunity is the Advanced Cosmic-ray Composition Explorer for the Space Station (ACCESS), which will be able to measure directly the spectrum of particles to 1000 TeV and for the first time to distinguish the spectrum produced by the cosmic accelerators from energy-dependent effects of propagation in the Galaxy.

In setting priorities, the panel used three criteria: scientific importance, technological readiness, and budgetary reality. In some cases, however, where the path forward depends on results of investigations just now starting, it is not yet possible to evaluate a project even though it addresses an extremely important problem and is likely to be ready within the coming decade. The panel therefore recommends a broad program of particle astrophysics building on the important new initiatives of the past decade, including solar neutrino observatories, giant air shower detectors, neutrino telescopes, and searches for dark matter.

The scientific interest and importance of all the projects of this panel

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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are strengthened by their multidisciplinary character. The panel therefore recommends policies to nurture such research.

SCIENCE OPPORTUNITIES

A unifying theme of many of the projects considered by this panel is the desire to study energetic processes in the cosmos not only in all wavelength ranges but with a variety of signal carriers. The idea would be to detect gravitational waves, neutrinos, hadrons, and photons from the same source and so take advantage of the complementary information they carry:

  • Gravitational waves provide information on the bulk motions of matter in the most energetic events in nature, such as the coalescence of black holes.

  • High-energy (nonthermal) photons trace populations of accelerated particles.

  • Cosmic-ray protons and nuclei carry information about the cosmic accelerators that produced them.

  • Neutrinos emerge directly from deep inside regions that are opaque to photons.

Cosmological gamma-ray bursts (GRBs) offer a good example of the potential benefits of complementary observations with more than one probe as well as at different wavelengths. The distribution of bursts is isotropic over the sky, but until 2 years ago it was not known if they were in the halo of our galaxy or at cosmological distances. Now, following a coordinated series of x-ray, optical, infrared, and radio observations, it is known that many bursts are cosmological.

Although detailed mechanisms of gamma-ray bursts are not understood and there may be substantial beaming by the sources, there is now little question that bursts represent the conversion of a significant fraction of a stellar rest mass into energy. To achieve this level of power output will probably involve ultrarelativistic motions of stellar masses drawing energy from the gravitational potential. Plausible concepts include the formation of a black hole from the coalescence of orbiting compact objects or a new class of stellar collapse resulting in a black hole. In view of the energies involved, it can be expected that models for the burst

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

mechanism will be constrained by the measurement of (or useful upper limits on) coincident high-energy gamma rays, neutrinos, elementary particles, and gravitational waves, as well as wide-field optical and radio observations.

In what follows, the major accomplishments of the past decade and the future opportunities are grouped into four broad categories. Gravitational waves offer the potential to revolutionize our understanding of the role of massive black holes in the dynamics and evolution of galaxies. Cosmic particle acceleration (as manifested in gamma-ray, charged-particle, and high-energy neutrino astrophysics) is an essential feature of energetic processes on all scales. Neutrino astrophysics is the study of low- and medium-energy neutrinos from the Sun and energetic sources such as supernovae. Identifying the dark matter is a key goal for understanding the large-scale structure of the universe, because until this is done researchers cannot know what most of the mass of the universe is made of.

GRAVITATIONAL-WAVE ASTROPHYSICS

The role that gravitational radiation plays in the energy loss of massive, rapidly moving astrophysical systems has been established by means of the orbital period change of the binary neutron star system discovered by Hulse and Taylor. The direct measurement of gravitational waves from astrophysical sources will open up new opportunities for investigations in both astrophysics and the physics of strong gravitational fields. The gravitational waves will convey information about the large-scale motions in the dense inner regions of astrophysical systems normally not open to view in electromagnetic observations. Observation of the final inspiral of two black holes would serve as a unique probe of the strong-field limit of general relativity. The sources of gravitational waves are changing-mass quadrupole moments. Astrophysical processes can result in impulsive, periodic, and stochastic gravitational waves. Impulsive sources include the cores of supernova explosions, the metric perturbations in the formation and dynamics of black holes, and the coalescence of compact binary systems. Periodic gravitational waves may originate in the coalescence of massive black holes, in the accretion-driven excitation of normal modes in neutron stars, or in the rotation of pulsars. A stochastic background of gravitational waves would result from a collection of spectrally unresolved binary stellar systems and possibly from the metric fluctuations in the primeval universe.

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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The search for gravitational radiation from astrophysical sources has so far been executed primarily with acoustic bar detectors that set upper limits for gravitational-wave strains of 10−18 in bands several hertz wide in the spectral region around 1 kHz. To set the scale, the supernova 1987A, which gave the first evidence for neutrinos from a stellar collapse, would have produced a strain of 10−18 at Earth if as much as 1 percent of the rest energy of the imploding star had been converted into gravitational radiation in 10−2 seconds. None of the sensitive acoustic detectors was operating at the moment when the prompt neutrino signals from SN1987A arrived at Earth.

Laser interferometers currently under construction with arm lengths of 4 km will initially operate at frequencies from 40 Hz to several kilohertz with a strain sensitivity of 10−21. Improvements are planned that will enhance strain sensitivity by a factor of 10 to 30 and extend the observing band to lower frequencies. Potential sources include chirps resulting from the coalescence of binary neutron star systems similar to the Hulse-Taylor system, supernovae, and formation or collisions of 1- to 1000-solar-mass black holes. By extending the search to cosmological distances, the improvements to the long-baseline detectors on Earth will make it likely that coalescing binary neutron stars will be detected.

Lowering the frequency sensitivity of gravitational-wave detectors would open the window on an important new class of sources involving the formation and interaction of ~106-solar-mass black holes, which are thought to lie at the centers of many galaxies. A detector with sufficient sensitivity in the frequency band between 10−4 and 10−1 Hz could expect to witness the last year in the merger of two supermassive black holes in colliding galaxies as they spiral in toward a final cataclysmic event. Observation of the characteristic orbital period as it decreases from hours to minutes would enable the gravitational-wave detector to predict the time and general location so that the final event could be observed by a variety of telescopes and detectors on the ground and in space. In addition, such a detector could observe the gravitational radiation patterns of a large number of white dwarf binaries in our galaxy.

Detecting low-frequency gravitational radiation requires going into space to escape the effects of density fluctuations in the ground and the atmosphere. Such density fluctuations cause Newtonian gravitational forces on the mirrors. The mirrors cannot be shielded nor can the forces be eliminated by vibration isolation systems. These backgrounds limit the sensitivity of terrestrial detectors to frequencies above a few hertz. A long-baseline detector in space will open up the low-frequency range

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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and provide insight into the processes in the centers of galaxies involving dynamics in strong gravitational fields.

COSMIC PARTICLE ACCELERATION

High-energy, nonthermal particles are a prominent feature of energetic astrophysical sources ranging from supernovae and flare stars in the galaxy to accreting massive black holes in the centers of distant active galaxies, to mention just two. In this section, the study of high-energy gamma rays produced in interactions of electrons or ions in the sources is considered first. Then, the status of cosmic-ray protons and nuclei in the Galaxy is considered (their relation to specific sources and acceleration processes is still not fully understood). Next, the very highest energy cosmic ray particles, whose origin is even more puzzling, are discussed. Finally, the possibility is discussed of opening a new window on particle acceleration by detecting high-energy neutrinos produced deep inside energetic astrophysical sources.

GAMMA-RAY ASTROPHYSICS

The study of very-high-energy gamma rays is a powerful tool for understanding particle acceleration in energetic astrophysical sources in distant galaxies as well as in the Milky Way. The development of the imaging technique for Cherenkov telescopes over the past decade has revolutionized ground-based gamma-ray astronomy by dramatically lowering the diffuse background of cosmic-ray showers; this is done by rejecting events with the irregular shape characteristic of hadronic rather than electromagnetic cascades. This achievement led to the discovery of very-high-energy (VHE) gamma radiation from a variety of sources, including pulsar nebulae, shell-type supernova remnants (SNRs), and jets of active galaxies, by atmospheric Cherenkov telescopes on four continents. The Crab Nebula, the first unambiguous VHE gamma-ray source, was originally detected by the Whipple Observatory (and is now detected by a number of instruments at very high significance); it confirmed the prediction of inverse Compton radiation from synchrotron-emitting relativistic electrons. Two other pulsar nebulae, PSR1706–44 and Vela, have also been detected by the CANGAROO telescope in the Southern Hemisphere. The detection of very-high-energy emission from the shell-type SN1006 by CANGAROO (along with nonthermal x rays detected by

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

the Japanese x-ray satellite ASCA) also indicates the presence of relativistic electrons with energies up to 100 TeV. Such source detections at high significance can now be routinely made, so that detailed studies of various emission mechanisms are possible.

The most exciting new discovery in ground-based gamma-ray astronomy in the past decade was the detection of VHE emission from active galactic nuclei (AGN), such as Mrk 421 and Mrk 501, both of which belong to the blazar class, in which the observer is in the beam of a jet of the AGN. In 1997, flares from Mrk 501 showing variability on timescales as short as 1 h were monitored by four different experiments in the Northern Hemisphere. Time variations as short as 30 min have been seen in the gamma-ray fluxes from individual AGN. These discoveries initiated large multiwavelength campaigns, using instruments at radio, optical, x-ray, and gamma-ray energies to study variability in blazars and to constrain their emission models.

These studies will be expanded to develop a detailed picture of particle acceleration in SNRs and the jets of AGN. Absorption of blazar spectra at gamma-ray energies can be used in conjunction with measurements made at infrared wavelengths to understand the radiation fields near active galaxies and the cosmic IR background. The potential for future discoveries by ground-based gamma-ray telescopes is good. So far, only a small portion of the sky has been studied at very high energies, and a major band at energies between 20 and 250 GeV has yet to be explored by any instrument. The most important instrumental innovation for the coming decade will be stereoscopic imaging Cherenkov telescopes with greatly improved sensitivity and angular resolution.

In addition to the new directions outlined above, the next-generation Cherenkov telescopes have the potential to address other exciting topics, including (1) the discovery of sources that are bright at very high energies but faint at other wavelengths, (2) the detection of evidence for proton acceleration in SNRs through measurements of energy spectra with good angular resolution and in conjunction with measurements at other wavelengths to identify the πº component, (3) the detailed study of the high-energy spectrum of gamma-ray bursts, (4) the detection of attenuation in the spectrum of extragalactic sources at high energy, indicating absorption by pair production on the cosmic infrared background radiation, and (5) the search for cold dark matter in the galactic center by means of gamma-ray line emission.

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×
PARTICLE ACCELERATION IN THE GALAXY

It is generally believed that cosmic rays are accelerated by sources distributed in the Galactic disk. They subsequently diffuse through the disk and halo before they escape from the Galaxy or are lost by nuclear interactions with the interstellar medium. The only sources that seem to be capable of providing the ~3×1040 erg/s required to maintain this balance between acceleration and escape are supernova explosions. While there is evidence to support this idea, two key questions remain about the supernova origin of cosmic rays:

  • What is the mechanism by which cosmic rays gain their enormous energies? It is widely suspected that the bulk of cosmic rays are accelerated by diffusive shock acceleration, but the evidence for this hypothesis is somewhat indirect because cosmic-ray energy spectra measured in the Galaxy (∝E−2.7) are apparently modified from the accelerated spectra (expected to be ∝E−2.1) by energy-dependent diffusion through and leakage from the Galaxy. To correct for such propagation effects and obtain the source spectrum requires precise measurements of both primary accelerated species (such as H, He, C, O, Si, and Fe) and secondary species (such as Li, Be, and B) that are produced by nuclear interactions with the interstellar medium. Extending measurements of secondary/primary nuclei from the present limit of ~100 GeV per nucleon by at least an order of magnitude would for the first time permit an unambiguous determination of the source spectrum. Measurements of the primary nuclei are needed up to an energy approaching 106 GeV, where shock acceleration by supernova blast waves is expected to reach its limit, perhaps causing the spectra to steepen. Such measurements require extended exposure of a large detector in space, outside the atmosphere.

  • What are the nature and source of the matter that is injected for acceleration to cosmic-ray energies? The well-known fact that cosmic rays are depleted in elements with first ionization potential more than ~10 eV suggests that they originate in the coronas of stars like the Sun. It was suggested recently, however, that volatility may be the relevant atomic parameter and that cosmic rays may be grain-destruction products mixed with some interstellar gas. The panel endorses the development of instruments to resolve this key issue.

At energies below a few GeV per nucleon, cosmic-ray spectra at

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

Earth are attenuated from those in interstellar space by an (unknown) factor of 100 or more because low-energy cosmic rays are largely excluded from the heliosphere by the solar wind. NASA’s proposed Interstellar Probe mission, which would send a spacecraft beyond 200 AU, would make a broad range of measurements of matter and fields in interstellar space. As an in situ investigation, it is outside the purview of the Astronomy and Astrophysics Survey Committee, but it is mentioned here because it would include measurements of the spectra and composition of cosmic rays in the local interstellar medium (ISM). It could observe shock acceleration in situ and assess the contribution of cosmic rays to radio and gamma-ray observations and to galactic dynamics. Interstellar Probe would also study acceleration processes at the solar-wind termination shock and measure the composition of the interstellar gas.

HIGHEST-ENERGY COSMIC RAYS

As indicated above, the SNR acceleration mechanism becomes inadequate above 1014 to 1015 eV, yet the cosmic-ray spectrum is known to continue for at least five more decades in energy. New acceleration sites and mechanisms are needed. AGN may be able to accelerate particles to 1020 eV, and gamma-ray burst sources have also been suggested. Achieving such high energy with these sources, however, requires optimistic assumptions about the conditions and parameters of the acceleration mechanisms. Moreover, there is no clear evidence yet that singles out one particular class of astrophysical objects as the most likely source of the highest-energy events. To observe a more exotic class of sources would require that the events be produced by the decay of massive relics from the early universe, possibly topological defects in space. Detailed studies of spectral shape, particle composition, and anisotropy are required to elucidate what is going on in this energy region.

The first report of an event with energy of approximately 1020 eV came from the Volcano Ranch experiment in 1965. A few other such large events were gradually accumulated with large ground arrays at Haverah Park, United Kingdom; Yakutsk, Russia; and Sydney, Australia. The Fly’s Eye detector has measured the profile of a shower with 3×1020 eV, and data from the ground array at Akeno (currently the largest) now confirm that the cosmic-ray flux continues past the predicted Greisen-Zatsepin-Kuz’min (GZK) cutoff. This cutoff in the cosmic-ray

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

spectrum is expected to be due to the onset of inelastic interactions between 1020 eV protons and the 2.7 K universal blackbody radiation. If sources of such protons are at cosmological distances, the cosmic-ray spectrum should cut off near 6×1019 eV. A similar effect will occur for nuclei, but it will be due to photospallation. Thus, protons and nuclei with energies beyond the GZK cutoff must originate in the local supercluster of galaxies. At these energies, charged particles should propagate nearly rectilinearly over such distances, but no obvious candidate sources such as AGN have been found in the error boxes of the seven events so far discovered. The mechanism that accelerates particles to these energies is thus completely unknown and represents the most significant departure from thermal equilibrium found in the universe. The desire to solve this mystery motivates current and planned efforts to build giant air-shower detectors with unprecedented acceptance (area× solid angle).

HIGH-ENERGY NEUTRINOS

An entirely new window into the deep interior of energetic sources could be provided by high-energy neutrinos produced in interactions of accelerated protons with gas or photons. Because neutrinos interact weakly with matter, they can escape from environments so dense that high-energy photons are absorbed or degraded in energy. For the same reason, however, neutrinos are difficult to detect, and very large detectors are needed. Moreover, neutrino detectors must be deeply buried to reduce the abundant cosmic-ray backgrounds that are present near the surface. A current example of a tracking neutrino detector is the Super-Kamiokande detector in a deep mine in Japan. At 50 kilotons the detector is big enough to detect copious solar and atmospheric neutrinos but not the high-energy neutrinos that might be tracers of acceleration processes in distant astrophysical sources. To achieve this goal, it is believed that detector volumes on a scale of at least a cubic kilometer (1000 megatons of water) will be needed. The effective volume for µv-induced muons is projected detector area×muon range (>2 km for Ev>1 TeV).

The essential characteristics of a high-energy neutrino telescope have been known for more than 20 years. All current architectures bury a sparse array of optical sensors within deep ice, deep seas, or deep lakes. The optical sensors respond to the UV-dominated Cherenkov radiation emitted by neutrino-induced muons or neutrino-induced hadronic or

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

electromagnetic cascades. Astronomy is possible because the muon direction is aligned with the incident neutrino to within 1 deg if the energy of the neutrinos exceeds 1 TeV. The muon is detected by distributing the photon sensors (large-diameter photomultiplier tubes) over the largest possible volume of transparent medium and recording the arrival times and intensity of the Cherenkov wavefront. The detectors are deployed in string or tiered arrangements.

The path toward very large neutrino telescopes has been advanced steadily during this last decade by the commissioning of detectors at the South Pole and in Lake Baikal, in Siberia. The experience with these detectors indicates that a kilometer-scale, high-energy-neutrino telescope could be built within the decade 2001 to 2010. Such a large size is needed to have a high probability of detecting neutrinos from astrophysical sources.

NEUTRINO AND NUCLEAR ASTROPHYSICS

SOLAR NEUTRINOS

Stars emit neutrinos directly from the fusion processes that power them. Attempts to measure neutrinos from the nearest star, the Sun, began over 30 years ago. The program in solar neutrino research aims to measure the entire spectrum of solar neutrinos, but it is incomplete as the 21st century begins.

The highlights of the past decade include the first real-time, directional detection of solar neutrinos in the Kamiokande and Super-Kamiokande light-water detectors; the confirmation by these detectors that the flux of neutrinos is much lower than stellar evolution calculations predict; the study of systematic errors in the pioneering 37Cl experiment, putting the experiment on a more stable foundation; and the remarkable suppression of the low-energy neutrinos observed in two calibrated 71Ga detectors, GALLEX and SAGE. In addition, a series of helioseismological measurements confirms that the temperature and density profile of the Sun are essentially as predicted by stellar-structure calculations. The indications are, therefore, that the solar neutrino deficit reflects novel physical properties of neutrinos rather than some poorly understood feature of the solar model.

The discovery by Super-Kamiokande of neutrino flavor oscillations involving muon neutrinos strongly reinforces the idea that the solar neutrino problem is indeed another manifestation of the pattern of

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

mixing and mass differences of different types of neutrinos. Claims, still unconfirmed, of yet another manifestation of neutrino oscillations by the Liquid Scintillation Neutrino Detector experiment would require the existence of a new, sterile neutrino. Complete understanding of solar neutrino observations will be an integral part of understanding fully the properties of neutrinos. This is important for astrophysics as well as for particle physics, because neutrinos play a fundamental role in many processes in the cosmos, from the Big Bang to supernovae.

Various nuclear fusion processes in the Sun’s interior produce electron neutrinos. Low-energy pp neutrinos, which have a continuous spectrum up to 0.42 MeV, dominate neutrino production. The much rarer 8B neutrinos have a continuous spectrum extending to 15 MeV, making direct detection somewhat simpler. Intermediate in flux and energy are the line sources from 7Be neutrinos at 0.86 and 0.38 MeV and from the pep neutrinos at 1.44 MeV. A small flux from the carbon-nitrogen-oxygen processes in the Sun also contributes at energies below 2 MeV. The hep neutrinos have a continuous spectrum extending to 18 MeV; their abundance is difficult to predict but is thought to be small. The first-generation experiments mentioned above are sensitive to different components of the solar neutrino flux, but all observe fewer neutrinos than predicted by solar models. The hypothesis of neutrino mixing, or oscillation, can account for the results of all experiments so far. In this process, electron neutrinos produced in the Sun become transformed into another neutrino type to which the first-generation detectors are not fully sensitive. However, more may be going on, because the Super-Kamiokande experiment observes a high-energy spectral distortion that may be accounted for by a great abundance of hep neutrinos.

The second generation of solar neutrino experiments currently getting under way is designed to investigate in more detail the neutrino oscillation hypothesis. The results of these experiments will determine the future direction of solar neutrino research. Completion of the solar neutrino program will require a further series of experiments that can map the solar neutrino energy spectrum, including the low-energy pp neutrinos, separately in electron neutrinos and in all types of neutrinos. Comparison of the two series of experiments will permit the mixing of neutrinos to be measured as a function of energy. Only then will it be possible to use solar neutrinos as a precision probe of processes in the solar interior.

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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SUPERNOVA WATCH

SN1987A was unique among supernovae because of the information that came from detection of neutrinos emitted during the stellar collapse and from measurement of the light curve resulting from the explosion. The new generation of neutrino detectors would produce much statistical information on neutrino yields and spectra from a supernova that occurs in our galaxy. Galactic supernovae are rare, with perhaps three occurring every century. Since the neutrino signal from a supernova precedes the optical signal by hours, it could be useful to predict the onset of such a supernova to allow optical instruments to point and thereby see the early rise of the light curve. For example, the large Super-Kamiokande detector could point with an accuracy of about 5 deg within an hour to a supernova at 10 kpc. The corresponding pointing accuracy for the smaller SNO detector is about 20 deg. These detectors, supplemented by future underwater (under-ice) experiments, will form a fast alarm network to identify and point to any galactic supernova, serving to alert other detectors before the onset of the optical outburst.

As the gravitational interferometers come on line, they will join the network of neutrino detectors, adding complementary early information, possibly for supernovae as far away as the Virgo cluster if the collapse is sufficiently nonaxisymmetric.

A goal for the future is to build sufficiently sensitive detectors so that the neutrino burst from a supernova in a distant galaxy could be detected. Concepts for large detectors to see out to 10 Mpc have been discussed, but the technology is not yet in place. If this goal could be realized, researchers could hope to detect several galactic supernovae each year.

NUCLEAR ASTROPHYSICS

The cosmos is powered by gravity and by nuclear reactions. Much of what is understood about processes in the universe is learned through the study of nuclear physics and nuclear astrophysics.

It is believed that in the early moments of the Big Bang, before nuclear matter could form, the universe was a plasma of quarks and gluons. As the universe expanded and cooled, the quarks and gluons condensed into protons, neutrons, and other particles. One current thrust of research in nuclear physics is the attempt at the Relativistic

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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Heavy Ion Collider (RHIC) to create and study tiny volumes of space in which the quark-gluon plasma is produced by colliding relativistic heavy ions. In this way, it should be possible to recreate briefly conditions similar to those in the early universe. In addition, producing a quarkgluon plasma many times in the laboratory may make it possible to create new forms of matter. It is even possible that such new forms of matter exist in the universe but have not yet been detected.

After the quark-gluon plasma condenses into neutrons and protons, these particles in turn begin the synthesis of the light elements. Understanding this primordial nucleosynthesis requires knowledge of the neutron lifetime and the reaction rates of the light nuclei. Our present understanding of primordial nucleosynthesis explains the observed abundances of the elements through lithium and is a triumph of nuclear physics and cosmology.

As galaxies and stars evolved, the nuclear reactions responsible for stellar evolution produced elements heavier than lithium. For the most massive stars, nuclear burning continues until the nuclear fusion processes in the stars produce iron. The nuclear fuel is then quickly exhausted and the gravitational collapse of the star produces a supernova explosion. Heavy-element nucleosynthesis comes about in nuclear reactions following the explosion. The debris from the explosions feeds the interstellar medium and determines the chemical evolution of the galaxy. Much active research in nuclear physics is being carried out in this important area. Especially relevant to rapid nucleosynthesis in supernova explosions is research with radioactive beams to investigate processes far from the valley of nuclear stability. In the United States, the radioactive-beam facilities at Argonne National Laboratory, Oak Ridge National Laboratory, and Michigan State University are powerful instruments. A proposed rare-isotope accelerator would extend the capabilities of these facilities and put the United States in the forefront.

The details of the supernova mechanism are determined in part by the nuclear equation of state for the very dense matter formed in the gravitational collapse of a massive star. Further, when the remnant neutron star cools, the equation of state of the nuclear matter determines the internal structure of the neutron star. Glitches in pulsar timing may be caused by transfers of angular momentum between regions of the neutron star interior. Thus we may learn about the equation of state of cold, dense nuclear matter from these astrophysical processes in a way that is complementary to the way we learn from collisions of relativistic heavy ions.

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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SEARCH FOR DARK MATTER

The nature of the dark matter in the universe remains one of the central problems in astrophysics and cosmology. A number of cosmological observations favor a nonbaryonic nature. For one thing, measurements of the average density of the universe consistently yield at least four times the value of the baryon density deduced from nucleosynthesis. For another, observations of clustering of galaxies at large scales and COBE measurements of temperature fluctuations of the cosmic microwave background also favor nonbaryonic dark matter. Moreover, nonbaryonic dark matter provides the most natural explanation for the large-scale structure in terms of collapse of the initial density fluctuations inferred from COBE. Finally, a general argument comes from the implausibility of hiding a large quantity of baryons in the form of compact baryonic objects.

Several nonbaryonic candidates have been proposed, including shadow universes, condensates formed at a quark-hadron phase transition, and very massive particles produced during inflation. By selecting candidates that also solve important questions in particle physics, researchers arrive at three particularly reasonable possibilities: massive neutrinos, axions, and weakly interactive massive particles (WIMPs). Interpretation of recent data on atmospheric and solar neutrinos in terms of oscillations suggests that neutrinos have a small mass, but it is unlikely that such an interpretation provides the full explanation for dark matter because it alone cannot explain structure formation. In addition, neutrinos, as fermions, are excluded by phase space considerations from providing an explanation for the dark matter in galactic halos. Axions have been postulated to prevent dynamically the violation of charge parity in strong interactions in the otherwise extremely successful theory of quantum chromodynamics. Present limits on axion parameters are such that if they exist, they would form a significant portion of cold dark matter. Current axion searches cover only a portion of the allowed mass range. WIMPs are particles that were in thermal equilibrium in the early universe and decoupled when they were nonrelativistic. For these particles to have critical density, their annihilation rate has to be roughly the value expected for weak interactions, i.e., determined by physics at the W and Z mass scale. Conversely, in order to stabilize the mass of the W and Z particles at the 100 GeV mass scale, particle physicists are led to predict the existence of new families of undiscovered particles. The leading candidate for the class of particles created by this convergence

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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between particle physics and cosmology is the neutralino of supersymmetry.

The most direct method to detect WIMPs is by elastic scattering on a suitable target in the laboratory. WIMPs interacting with the nuclei in the target would produce a roughly exponential distribution of the recoil energy, which could be detected by an ultralow-background detector. The challenge is to construct experiments of sufficient mass with very low radioactive backgrounds and instruments capable of recognizing WIMPs interactions. Current experiments are reaching a sensitivity that begins to explore the supersymmetric models for the WIMPs.

WIMPs can also be sought through the products of their annihilation. The most specific signature would be a gamma line at half the WIMP mass, detected from the center or halo of our galaxy. A detection possibility that covers a larger region of particle-physics parameter space and is, in addition, less model-dependent arises from the fact that WIMPs would be trapped in the center of the Sun or Earth. Their annihilation products could be detected by properly designed deep neutrino detectors. Finally, WIMP annihilation would also produce an anomalous flux of antiparticles that might be detected by particle detectors in space.

EXISTING PROGRAMS

GRAVITATIONAL WAVES

In the next 3 years, four long-baseline (0.6 to 4 km) laser interferometer detectors will start running: LIGO, with interferometers in Louisiana and Washington State; VIRGO, with an interferometer near Pisa, Italy; GEO, with an interferometer in Hannover, Germany; and TAMA, in Japan. There are also plans for an interferometer in Australia. The interferometers will operate as a network, providing multiple coincidences and correlations and thereby a means for determining the location of the sources and the polarization states of the waves. A second generation of interferometers with improved isolation from environmental perturbations, increased light power, and different optical configurations that trade bandwidth for sensitivity will be installed in 5 to 10 years as upgrades to the existing long-baseline facilities. These improvements, which were part of the initial research plan for LIGO and incorporated into the design of its long-baseline facilities, will require about $50 million over the rest of this decade.

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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VERY-HIGH-ENERGY GAMMA RAYS

There are numerous ground-based gamma-ray telescopes in operation around the world. The most sensitive instruments are those using the imaging atmospheric Cherenkov technique, in which large mirrors focus Cherenkov radiation created in air showers onto photodetector arrays. Imaging Cherenkov telescopes operate on clear nights at typical energies between 250 GeV and 25 TeV. The Whipple Observatory (Mt. Hopkins, Arizona) has been the leading telescope for the last decade. Other instruments include HEGRA (La Palma, Spain), CAT (Themis, France), and CANGAROO (Woomera, Australia). Outside the United States, there are three next-generation gamma-ray telescopes in development: CANGAROO-IV (Australia), HESS (Namibia), and MAGIC (La Palma, Spain). These projects have different scientific emphases and are geographically well separated in latitude and longitude, so they will complement a new telescope in the southwestern United States.

A second type of Cherenkov telescope uses large mirror arrays originally built for solar energy research to achieve energy thresholds as low as 40 GeV. These experiments, STACEE (Albuquerque, New Mexico) and CELESTE (Themis, France), observe in the largely unexplored energy region that is beyond the reach of the existing spaceborne Compton Gamma Ray Observatory and nearer than the reach of imaging Cherenkov telescopes.

Air-shower experiments detect the particles produced in the high-energy gamma-ray cascades. Operating with a wide field of view and with 100 percent duty cycle, these experiments are well suited for sky surveys and for studying transient phenomena such as gamma-ray bursts. MILAGRO (Los Alamos, New Mexico), a large, water Cherenkov detector soon to come into full-scale operation, will be the first such detector to work in the energy region between 500 GeV and 50 TeV, which is below the threshold for conventional air-shower arrays.

GALACTIC COSMIC RAYS

Direct measurements of the primary cosmic radiation made with detectors carried above the atmosphere by balloons and spacecraft now extend to energies of about 1012 to 1014 eV. Information at the top end of this range comes from balloon-borne emulsion chamber experiments and is limited to H, He, and groups of heavier nuclei rather than individual elements. Higher energies are explored only indirectly by ground-

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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based air-shower arrays that do not measure the primaries directly but only the secondary cascades. Direct measurements with magnetic spectrometers and transition radiation detectors have measured the spectra of individual elements up to about 100 GeV/nucleon.

The Alpha Magnetic Spectrometer (AMS) is a magnetic spectrometer experiment for the International Space Station (ISS) designed to search for primordial antimatter and signatures of dark matter in the galactic cosmic rays. It will also measure the spectra of some primary species up to the TeV energy range.

At much lower energies, NASA’s Advanced Composition Explorer (ACE), launched in 1997, is measuring the isotopic composition of cosmic-ray nuclei at energies below 0.5 GeV/nucleon. TREK, a joint U.S./Russian experiment to detect cosmic rays with Z≥75 that flew on Mir, has verified the existence of actinides (nuclei with Z≥90) in the cosmic radiation.

HIGHEST-ENERGY COSMIC RAYS

Japan’s Akeno Giant Air Shower Array (AGASA) uses measurements of ground-level particle densities in shower cascades to infer the primary-particle energy and arrival direction. AGASA has an aperture acceptance of 200 km2sr and will continue running until the newer experiments overtake it statistically.

The High-Resolution Fly’s Eye (HiRes), in Dugway, Utah, was completed in 1999. The experiment images nitrogen fluorescence from shower cascades to reconstruct the longitudinal shower profile and measure the primary energy calorimetrically. The profile is also used to measure the position of the extensive air shower in the atmosphere, which is sensitive to the primary-particle composition. HiRes will have a time-averaged aperture of 1000 km2sr at 1020 eV, implying that it will detect about 40 events above this energy in a 5-year period if the AGASA results are correct.

The Pierre Auger Observatory project is an international collaboration, which is currently building a 7000 km2sr detector in Argentina. The detector consists of a large ground array of water tanks on a 1.5-km grid and atmospheric fluorescence detectors. Coincidence data taken with both the ground and fluorescence detectors will be used to calibrate the energy and type of the primary particles. The Auger South detector is schedule to be completed in 2004; based on the AGASA spectrum, it will

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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accumulate 300 events above 1020 eV in a 5-year run, of which 10 percent would be in coincidence mode.

NEUTRINO ASTRONOMY

The existing underground detectors Super-Kamiokande, MACRO, and LVD currently form a Supernova Early Warning System (SNEWS), which will be joined by other neutrino detectors, including SNO and AMANDA. By comparing alerts via automated e-mail, false alarms of individual detectors can be virtually eliminated, allowing the possibility of automatic notification of the occurrence of a supernova in the Milky Way galaxy or its satellites hours before the optical outburst. It is essential to maintain and enhance this capability over the long term as old detectors are retired and new ones come on line.

There are currently two operating experiments for high-energy neutrino astronomy: Baikal (Siberia) and AMANDA (South Pole). Both experiments have succeeded in measuring upward-going muons produced by atmospheric neutrinos, demonstrating in principle that the technique works. The construction of AMANDA II, with a detection area of 30,000 m2, has been completed; it is a prototype for a larger detector. There could also be an expansion of the Baikal detector.

Three other groups (ANTARES, NESTOR, and NEMO) have concentrated on site testing and feasibility studies at a variety of Mediterranean sites. NESTOR and ANTARES are conceived as experiments using different deployment schemes, array designs, and signal-processing technologies. ANTARES has begun construction of a detector with an effective area of up to 0.1 km2.

SOLAR NEUTRINOS

The second generation of solar neutrino experiments includes the Super-Kamiokande light-water Cherenkov detector, the Sudbury Neutrino Observatory (SNO) heavy-water Cherenkov detector, and the Borexino and Kamland scintillation detectors. The first two detectors are sensitive to the 8B neutrinos, while the second two are designed to study the7Be neutrinos. SNO will compare the total rate of interactions of neutrinos of all types with the rate of electron neutrinos, which will check directly whether electron neutrinos produced at the Sun are changing identity en route to Earth. Super-Kamiokande has been operational

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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since 1996; SNO began operation in the summer of 1999; and Borexino and Kamland are under construction.

DARK MATTER SEARCHES

The second-generation microlensing experiments currently being started (OGLE II and Super MACHOs) should have the photometric and statistical accuracy to break the degeneracy among mass, distance, and velocity of the lensing objects. The Space Interferometry Mission (SIM) should also pin down the distance of the lenses.

The Axion experiment has published preliminary limits and will reach the required sensitivity for one generic type of axion between 10−6 and 10−5 eV/c2. By pushing the sensitivity with SQUID amplifiers and by operating at 100 mK, an upgraded Axion experiment would cover all the present axion models in this mass range, which is one-third of the 10−6 to 10−3 eV/c2 range allowed by astrophysical constraints.

The Cryogenic Dark Matter Search II promises to be the most sensitive WIMP experiment at the beginning of the 2000 decade. It should improve current sensitivities by two orders of magnitude and reach well into the supersymmetric region.

AMANDA II will increase the sensitivity of the search for neutrinos from WIMP annihilation and AMS will search for an excess of antiprotons and positrons in the cosmic rays.

RECOMMENDED NEW INITIATIVES

The panel was able to identify several key challenges that are ripe for progress at this time:

  • To detect gravitational radiation from interacting massive objects, including massive black holes;

  • To understand the origin of gamma rays of very high energy from sources such as AGN and SNRs;

  • To identify positively the sources of galactic cosmic rays and to measure the output of these cosmic accelerators;

  • To identify the nature and distribution of the bulk of the matter in the universe;

  • To understand the origin of the highest-energy particles in nature;

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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  • To detect neutrinos from energetic astrophysical objects and events; and

  • To measure the full energy spectrum and flavor content of neutrinos from the Sun.

GRAVITATIONAL-WAVE ASTRONOMY (LISA)

Because of the fundamental and novel new phenomena that can only be studied by a long-baseline gravitational-wave detector in space, and because of its strong and well-developed science and technology plans, the Laser Interferometer Space Antenna (LISA) is the highest priority project of the panel. The mission has generated considerable interest within NASA and has become a cornerstone mission of the ESA program. It is hoped that LISA will be put forward as a joint mission for launch before the end of the first decade of this new century. LISA should observe, for the first time, the coalescence of supermassive black holes as distant galaxies merge. Although merger rates are quite uncertain, event rates are estimated to range from 1 to 100 per year. Location of events in the sky depends on frequency, ranging from several degrees at 10−4 Hz to 30 arcmin at 10−2 Hz, with an angular resolution of 1 arcmin in the last few days before coalescence. This should be sufficient for gamma-ray detectors to point and observe the final explosion. LISA will also survey the gravitational radiation from galactic white-dwarf binaries and possibly study gravitational fluctuations from the early universe. The LISA team will carry out a major theory challenge by computing the expected gravitational waveforms from black-hole mergers. This will require developing three-dimensional general relativistic codes with adaptive mesh refinements.

LISA consists of three spacecraft maintained in an equilateral triangular configuration with sides 5×106 km long (Figure 3.1). The system is placed in solar orbit at 1 AU with the plane of the triangle at 60 deg to the ecliptic. The orbit requires little station keeping. The three spacecraft are launched by a single Delta rocket and then deployed into the triangular configuration. The triangle enables the operation of three almost independent interferometers along adjacent pairs of sides. The interferometry is done by heterodyne detection with a single optical pass. Both the frequency range and the science of LISA are complementary to those of the ground-based interferometers (Figure 3.2).

The LISA team has identified three technical areas that would benefit from a dedicated technology mission in space: the inertial reference

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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FIGURE 3.1 Artist’s conception of the Laser Interferometer Space Antenna, consisting of three spacecraft in a triangular configuration separated by 5 million km. LISA is aimed primarily at studying strong-field gravity, the coalescence of massive black holes in galactic nuclei, and compact binary star systems in the Milky Way. Courtesy of W.Folkner, Jet Propulsion Laboratory, California Institute of Technology.

mass, the precision thrusters, and the high-precision interferometry. The most critical area is the development of the sensor and control system to maintain the LISA spacecraft in pure inertial (drag-free) orbits. The technical challenge is to reduce the nongravitational disturbing forces (such as fluctuating electric fields and fluctuating radiation pressure from thermal gradients) on a reference mass used to guide the motion of the spacecraft containing the optical components. The concept of surrounding a reference mass by a spacecraft shell that follows its motion and shields it from nongravitational forces was tested in the TRIAD program almost two decades ago at acceleration levels approximately 107 times greater than needed for LISA. The Gravity Probe B cryogenic gyro

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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FIGURE 3.2 Comparison of the sensitivity of LISA with that of ground-based interferometers such as LIGO for various potential sources. Because of its lower frequency range, LISA is sensitive to the coalescence of massive black holes. It also has the potential to survey the Milky Way for binary systems involving white dwarfs, neutron stars, and stellar mass black holes. Courtesy of W.Folkner, Jet Propulsion Laboratory, California Institute of Technology.

mission, to be flown in 2002, will provide a test at an acceleration level approximately 105 times greater.

The controllers that will be used to make the spacecraft follow the reference mass also need demonstration. Several thruster designs exist that develop proportional ion thrust control at the micronewton force level. The key technical issues are lifetime and reliability.

The laser interferometry in LISA needs to operate at displacement sensitivities of 10−13 m, which is less sensitive by a factor of 105 than the initial terrestrial interferometers. The technical challenge is to achieve the performance at low frequencies (10−1 to 10−4 Hz).

The thruster and laser interferometry can be tested on the ground.

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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However, to establish the low perturbation levels on the reference mass will require an integrated test of all three elements. Technology development missions are being contemplated in both the United States and Europe to test an integrated system at acceleration levels low enough to allow final design of the LISA system. The panel strongly encourages a technology development mission to reduce the risk in the LISA mission. The total cost of the LISA mission (exclusive of technology development) is estimated at close to $400 million, of which $250 million would be borne by NASA.

GROUND-BASED GAMMA-RAY ASTROPHYSICS (VERITAS)

The VERITAS project would greatly expand astronomers’ understanding of the high-energy gamma-ray sky and has significant potential for achieving a breakthrough in astrophysics. It is therefore the panel’s top-ranked ground-based project and second overall. VERITAS is envisioned as an array of seven 10-m-diameter reflectors, each equipped with its own imaging camera of 500-pixel elements. The reflectors can be operated either as individual telescopes or together in a stereoscopic mode. As individual telescopes, VERITAS has the greatest discovery potential and can carry out effective sky surveys. In a stereoscopic configuration, where each air shower is viewed by multiple cameras, VERITAS achieves its best sensitivity (more than an order of magnitude improvement over existing instruments; see Figure 3.3) and its optimal energy and angular resolution (better than existing instruments by a factor of between 2 and 3). For individual photons, the angular resolution of VERITAS will be as good as 3 arcmin, and the point-source location will be better than 25 arcsec. The design of VERITAS is a natural outgrowth of the successful Whipple Observatory on Mt. Hopkins. Technological advances include the development of high-speed (500 MHz) digitizers for each pixel element, which will permit the telescope to operate with a low energy threshold (50 GeV) and improved energy resolution (less than 15 percent).

VERITAS will observe TeV gamma rays from the jets of AGN and possibly also from GRBs. These highly variable energetic signals reflect violent processes occurring in the active inner regions of their distant sources. VERITAS should detect an order of magnitude more AGN than existing ground-based detectors, approximately 30 or more x-ray-selected BL Lacs and 15 radio-selected quasars. Detailed studies of AGN during

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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FIGURE 3.3 Comparison of VERITAS sensitivity with that of present and proposed detectors. Together, GLAST and VERITAS will study the gamma-ray sky at energies from less than 1 GeV to greater than 10 TeV. Courtesy of T.Weekes, Harvard-Smithsonian Center for Astrophysics.

states of high activity will be possible (see Figure 3.4). For distant sources, spectral cutoffs that are correlated with redshift will provide important information about the cosmic IR background radiation. An important theory challenge posed for VERITAS is to understand the origin and characteristics of the energetic signals from AGN and GRBs, including acceleration mechanisms, the relative importance of electrons and ions, and spectral shapes and cutoffs. Multiwavelength coverage, including radio, optical, x-ray, and gamma-ray bands, will be crucial in understanding these sources.

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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FIGURE 3.4 Example of potential offered by the increased sensitivity of VERITAS. Left: Whipple observations of a rapid flare from Mrk 421. Dashed curve is a possible intrinsic flux variation. Right: Simulated response of VERITAS to such a flare showing the much better resolution that would be possible with its greater sensitivity. Courtesy of R.Ong, University of Chicago.

VERITAS will also study the emission from shell-type SNRs. TeV energies are well suited for detection of this emission, owing to the lower Galactic diffuse background and superior angular resolution of VERITAS relative to satellite-borne instruments. VERITAS should enlarge the sample of VHE pulsar nebulae by a factor of 3 or 4 and should map the emission from strong sources such as the Crab; it would, as well, provide a unique test of pulsar wind models.

VERITAS will perform the first sky survey at very high energies with good sensitivity. It should be possible to complete a survey of the Galactic plane within a year for sources as weak as 2 percent of the Crab

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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above 300 GeV. Spectral measurements made with GLAST and VERITAS at energies from 1 GeV to 1 TeV should be able to identify uniquely a significant fraction of the 170 EGRET unidentified sources, 40 or so of which are in the Galactic plane, where the good angular resolution of VERITAS is particularly advantageous. VERITAS will also be an effective survey tool for potential sources of VHE neutrinos and UHE cosmic rays.

VERITAS has significant potential for new discoveries in several areas. By operating at higher energies, it will complement GLAST in the search for dark matter in the form of supersymmetric particles near the Galactic center, whose annihilation products would include photons, or in the form of cold molecular clouds, which would be revealed by concentrations of cosmic-ray-produced photons. With its larger sky coverage relative to existing instruments, VERITAS will carry out better searches for VHE sources not seen at other wavelengths, such as primordial black holes.

The construction cost for VERITAS is estimated at $20 million, including instrumentation.1 It is expected to be fully operational by 2003 and will operate until the end of the decade, thus completely overlapping in time with GLAST. The performance of VERITAS could be significantly enhanced by technological improvements to the photodetectors (high quantum efficiency) or the optics (wider field of view). The appropriate design for an instrument beyond VERITAS is not obvious at the present time, but technology development for future telescopes should be encouraged.

PROGRAM IN PARTICLE ASTROPHYSICS

To take advantage of several important scientific opportunities, the panel recommends a balanced and coherent program in particle and nuclear astrophysics. Such a program will include, in addition to the gamma-ray observations carried out by GLAST, VERITAS, and other ground-based detectors, opening up the neutrino astronomy window and understanding the origins of the high-energy cosmic radiation. Further studies of solar neutrinos will be necessary to understand fully the properties of neutrinos and their role in astrophysical processes. More

1  

The estimated cost for VERITAS that appears in the survey committee report includes grants and operations in addition to instrumentation, as described in this report’s preface.

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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sensitive searches are needed to identify and study the nature of the dark matter. This section discusses the projects the panel expects to need funding in the decade 2001 to 2010. In several cases, the design of the new detectors will depend on the outcome of current experiments. For this reason, the panel did not assign a higher priority to one or another of these projects. In addition, the panel highlights one small space experiment (ACCESS) because of its readiness and potential scientific payoff.

A SMALL SPACE MISSION FOR GALACTIC COSMIC RADIATION

The Advanced Cosmic Composition Explorer on Space Station (ACCESS) is a small NASA experiment with an estimated cost of $100 million designed to make direct measurements of individual cosmic-ray nuclei to the highest possible energy allowed by extended exposure of a calorimeter on the ISS (Figure 3.5). The experiment is motivated by suggestions that the standard picture of cosmic-ray acceleration in supernova remnants followed by energy-dependent diffusion out of the galaxy has difficulties accounting for the relative fractions of various elements in the cosmic radiation. ACCESS addresses this problem in two ways. First, by measuring individual elements to energies approaching 1000 TeV, the experiment will show definitively whether different species have the same momentum spectra, as expected in the simplest super-nova-diffusive escape model. A possible outcome is that structure in the energy spectra may indicate that some acceleration sources have reached a maximum energy within the ACCESS energy range. Second, the lever arm in energy is large enough so that the contributions of diffusion and acceleration to the observed energy spectrum can be distinguished. This will be accomplished by measuring the ratio of secondary to primary cosmic-ray nuclei. Thus, ACCESS will for the first time allow a measurement of the energy spectrum of galactic cosmic particle accelerators rather than the convolution of acceleration and diffusion. A theory challenge posed by ACCESS is to identify signatures that discriminate among models of the origin of the most energetic galactic cosmic rays. In the supernova picture, for example, this would require relating a realistic and detailed distribution of various supernova types, including characteristic spectra of particles that they accelerate, to elemental composition at PeV energies as observed locally after propagation in the galaxy.

The ISS is a good platform for ACCESS because the detector does

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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FIGURE 3.5 Drawing of the International Space Station, proposed site of the Advanced Cosmic Composition Explorer on Space Station (ACCESS). The inset illustrates the process of particle acceleration at a blast wave driven by a supernova explosion. With the aid of turbulent magnetic fields, large-scale kinetic energy is believed to be transferred to some interstellar ions, accelerating them to relativistic energies. ACCESS is designed to probe the limits of this widely accepted but still hypothetical picture of the origin of Galactic cosmic rays. Courtesy of R.Mewaldt, California Institute of Technology.

not require accurate pointing. An accommodation study shows that the project is feasible. It is technologically ready and constitutes an obvious next step in the exploration of high-energy cosmic rays. Moreover, it has the potential to change the paradigm for the origin of Galactic cosmic rays if the source spectrum is significantly steeper than expected in the model of first-order diffusive shock acceleration by supernova blast waves.

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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QUEST FOR HIGHEST-ENERGY COSMIC PARTICLES

HiRes and the Auger South experiment will clearly be able to confirm the continuation of the UHE cosmic ray spectrum past the GZK cutoff (Figure 3.6). If the sources of these particles are AGN or other pointlike objects, then anisotropy or clumping on the scale of 5 deg is expected in the direction of the sources. A Northern Hemisphere experiment with

FIGURE 3.6 The spectrum of the highest-energy cosmic rays as reported by the AGASA experiment. The dotted line shows the spectrum that would be observed for a uniform distribution of sources in the universe. The few events above 1020 eV must have sources that are relatively nearby on a cosmological scale. The numbers beside the high-energy data points indicate the number of events on which each datum is based. The HiRes group Fly’s Eye Experiment has reported a similar number of events above 1020 eV, confirming by a different technique that the spectrum indeed extends beyond the expected cutoff from photopion production during propagation from cosmological distances. Source: M.Takeda et al., “Extension of the Cosmic-Ray Energy Spectrum Beyond the Predicted Greisen-Zatsepin-Kuz’min Cutoff,” Physical Review Letters 81(1998):1163.

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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sensitivity comparable to that of Auger South will then be necessary to study and catalog these sources over the whole sky and to establish correlations with visible, radio, x-ray, and gamma-ray observations. A site in Millard County, Utah, near the HiRes experiment has already been identified by the Auger collaboration, and a proposal for a northern site is expected around 2002.

A good understanding of the composition of cosmic rays is critical to unraveling the acceleration mechanism. For example, in a top-down process like the decay of topological defects, a large gamma/hadron ratio is expected, in contrast to the proton flux expected from AGN. Fluorescence detection allows measuring the longitudinal profile of each shower and hence makes possible the most direct calorimetric measurement of the primary energy. It also gives a measure of the identity of the primary particles different from and complementary to the Auger ground array. An international collaboration is proposing to build a chain of HiRes-type detector stations around the proposed Auger North site with a time-averaged aperture of near 8000 km2sr. In an enhanced northern detector, half the events will undergo determination of the atmospheric profile (by Telescope Array) and half will have ground array information gathered (from Auger North). This will allow a comparison of the spectrum and composition results at the highest energies based on two independent techniques with equal statistical strength. Ten percent of the data will be observed in coincidence and will be usable for intercalibrating the energy scales.

If the cosmic ray flux extends beyond 1021 eV, as predicted in many top-down models, its study requires detectors with apertures an order of magnitude larger. Since detectors with such a large area are impractical on the ground, the possibility of observing atmospheric fluorescence of giant showers from a detector in space is being considered. Several downward-looking detectors at 500- to 700-km orbits can achieve time-averaged apertures of 50,000 km2sr. Such a detector would look for the characteristic rebound in the UHE cosmic-ray spectrum near 1021 eV due to particles produced with energies well beyond the GZK cutoff. It would also be sensitive to UHE neutrino fluxes, predicted in some models.

There are many technical challenges to observing fluorescence from space, including the development of very-wide-angle optics, million-pixel photon detectors with high quantum efficiency, data acquisition electronics with very low power, and accurate determination of obscuring cloud cover and atmospheric transmission. Groups in the United States (the

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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Orbiting Wide-angle Light Collector (OWL)) and Italy (AIRWATCH) are cooperating on research and development to address these issues, and both NASA and the Italian Space Agency have provided preliminary funding. A balloon flight is planned to measure the ambient UV background looking down at Earth.

HIGH-ENERGY NEUTRINOS

Observation of high-energy neutrinos from astrophysical sources would open a new window on the cosmos because neutrinos can reach the observer over cosmological distances from deep inside the sources. The discovery potential is therefore great. Although predicted event rates vary substantially, detection of high-energy neutrinos would provide unambiguous proof of proton acceleration and would likely produce a better understanding of the dynamical role of hadrons in the astrophysical milieu. Plausible models of neutrino production in GRBs and active galaxies, which predict tens of events or more per year per square kilometer, can be tested with a kilometer-scale neutrino telescope. A few dramatic neutrino events correlated with GRB observations would provide strong evidence that these spectacular objects are the sources of the highest-energy cosmic rays.

The IceCube proposal for a kilometer-scale neutrino detector is designed to expand significantly the reach of high-energy neutrino telescopes. IceCube capitalizes on the success of AMANDA and on the laboratory and logistical infrastructure of the Amundsen-Scott South Pole Station, which is currently being expanded and modernized.

In addition to searching for high-energy neutrinos from distant sources, IceCube will detect about 5000 atmospheric neutrino events per year, which will be used for calibration. It may also be possible to see manifestations in the atmospheric neutrinos of the neutrino oscillation phenomena found in Super-Kamiokande. There will also be searches for neutrinos from Galactic compact objects and for emission from the Galactic center region. IceCube will search for bursts of low-energy (10 to 20 MeV) neutrinos coming from supernova collapse by monitoring the photomultiplier tube (PMT) dark counting rates for short-term increases. It would be sensitive to neutrinos from WIMP annihilation in the Sun, with good sensitivity for WIMP masses >200 GeV. If extragalactic sources are found, neutrino mass can be investigated over baselines of unprecedented length. Because hadronic processes in the source produce ve and vµ, the appearance of vτ from a potential cosmological

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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source can probe neutrino mass with a sensitivity of δm2≥10−17 eV2. Tau neutrinos with PeV energies can be identified by the characteristic pair of separated showers associated with the production of a tau lepton followed by its decay.

There has already been substantial development of the IceCube concept, including aspects of string installation, detector operation, and event reconstruction. Strings of sensors have been deployed in vertical holes to a depth of nearly 2500 m, sufficient for IceCube. The reference design (Figure 3.7), which is based on the transmission of analog signals over optical fiber, requires no new technology development. Scaling from the experience with AMANDA II, IceCube construction will take 6 years. The operational lifetime will be a minimum of 4 years. An initial proposal for IceCube was submitted in November 1999. AMANDA II, with an acceptance about an order of magnitude larger than the current AMANDA, was completed in early 2000 and can be considered as a prototype for the full IceCube.

SOLAR NEUTRINOS

A third generation of solar neutrino experiments will be required to complete the program for measuring the energy spectrum and flux of solar neutrinos. The goal is to make a complete set of precise measurements of neutrinos from an astrophysical source. The new experiments will be directed at measuring both the electron-neutrino component of the dominant low-energy pp flux and the total pp flux. It is highly likely that one or more of these experiments will be proposed for construction early in the decade 2001 to 2010.

It is important to compare the measured flux and spectrum of neutrinos with predictions based on stellar evolution calculations and laboratory data. As both the neutrino data and the laboratory data improve, the astrophysical calculations are put to more stringent tests.

Specific examples of detectors under development are HERON, utilizing a 10-ton superfluid 4He target; LENS, employing 176Yb or 160Gd dissolved in liquid scintillator; and HELLAZ, using high-pressure gaseous 4He in a time-projection chamber. The helium experiments, which are sensitive to the total flux and spectrum of low-energy neutrinos through elastic scattering of neutrinos by electrons, are complementary to the LENS experiment, which measures the flux and spectrum of low-energy electron neutrinos directly. Very preliminary cost estimates for these

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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FIGURE 3.7 Artist’s drawing of the proposed IceCube high-energy neutrino telescope at the South Pole. The composite photograph at the top shows the dome over the present Amundsen-Scott South Pole Station together with the Martin A.Pomerantz Observatory, which houses astrophysical facilities, including the data acquisition system for the present AMANDA experiment. Each dot in the diagram represents one of the approximately 5000 optical modules that will make up the detector at depths from 1.5 to 2.5 km in the clear Antarctic ice. The display here depicts a large electromagnetic cascade initiated by an electron neutrino interacting near the center of the detector A high-energy muon neutrino would appear as an elongated series of hits along the path of a high-energy muon produced when the neutrino interacts, either inside the detector or in the surrounding ice. Courtesy of S.Barwick, University of California, Irvine.

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

projects put them in the same range as moderate-cost, ground-based projects.

The scientific motivation for these experiments is to make the most precise measurements possible of the entire solar neutrino spectrum and of the physics parameters involved in neutrino mixing. The solar neutrino experiments will lead to an understanding of the emission spectrum of neutrinos by the Sun and the fundamental physics of neutrinos. Without this knowledge, it is not possible to make progress in other areas of astrophysics where neutrinos play an important role. For example, neutrino oscillations will affect the dynamics of core-collapse supernova explosions and nucleosynthesis through the n-process and the r-process. This in turn has implications for the chemical evolution of our galaxy.

DARK MATTER

Deciphering the nature of dark matter remains one of the most important goals of astrophysics. With the Axion and CDMS II experiments in the United States, the nation is currently engaged in searches that are probing cosmologically important regions. It is very likely that at the end of these experiments (around 2005), there will be a need to start at least one second-generation dark matter experiment. Although such an experiment could be motivated by the need for greater sensitivity, it will become compelling if a discovery is made in the current nonbaryonic dark matter searches or if a new feature of particle physics pertinent to the dark matter problem is uncovered at accelerators. If, for instance, supersymmetry is discovered, the next question will be, Is it responsible for the dark matter in the universe? If the new, direct searches now getting under way find a signal, this will determine the nature of detectors needed for more sensitive studies. If and when particle dark matter is discovered in direct searches, dark matter detectors would be able to map the local velocity distribution of the Galactic halo. This information would revolutionize the study of the distribution of dark matter in the Galaxy as well as theories of galaxy formation. It would allow us to overcome the current fundamental limit to galaxy-formation theories that arises from the fact that researchers do not know the spatial or velocity distribution of the dominant mass component.

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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TECHNOLOGY FOR THE FUTURE

Technology development is critical for the future of particle, nuclear, and gravitational astrophysics. The LISA technology program was described in the section on gravitational-wave astronomy. Dark matter searches and solar neutrino experiments both need the kinds of advances that will bring about low backgrounds and ultrasensitive detectors with low thresholds: advanced analysis methods to select the purest materials; hardware that reduces inert mass; and manufacture of critical parts underground. Particularly important for WIMP searches would be directional detection of nuclear recoil, using very large (104 m3), low-pressure-gas, time-projection chambers or detection of athermal phonons in isotopically pure crystals.

Sensors and amplifiers need to approach the quantum limit, especially for gravitational-wave detection and axion searches. For example, SQUID amplifiers in the gigahertz range that are being developed for axion searches are nearly quantum limited and may also have important applications in radio astronomy. Photolithography, micromachining, low-temperature techniques, and optimal filtering are essential elements of this development direction.

Affordable optical photon detectors with higher quantum efficiency are essential for future Cherenkov telescopes and shower detectors, including OWL, which also needs low-cost, large-aperture optics.

POLICY ISSUES

One theme of this panel report is the need for multimessenger as well as multiwavelength astronomy to unravel the mysteries of the cosmos. To understand fully the most violent events in the universe will require the detection of gravitational waves and neutrinos as well as observations throughout the electromagnetic spectrum and the information provided by the flux and composition of cosmic rays (Figure 3.8).

For the astrophysics community, the emerging field of particle and nuclear astrophysics provides new approaches, a new population of enthusiastic scientists, new scientific cultures, and new funding sources, all of which if properly assimilated will offer wonderful scientific opportunities. However, the cross-cutting nature of the tools employed and the problems addressed do not fit neatly into the traditional categories of wavelength or ground- versus space-based observations. For the field to

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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FIGURE 3.8 Time line for studies of gravitational waves, gamma rays, cosmic rays, high-energy neutrino astronomy, solar neutrinos, and dark matter. Currently running or approved projects are shown in yellow boxes; prioritized projects are in pink; and proposed projects, not yet prioritized, are in blue boxes. (GLAST was prioritized by the Panel on High-Energy Astrophysics from Space.) Courtesy of T.Gaisser, University of Delaware, and A.Harding, NASA’s Goddard Space Flight Center.

thrive, the astronomy and astrophysics and physics communities and the funding agencies must work to overcome these boundaries and focus on using multiple approaches to solve diverse but interconnected scientific problems. The frontiers of particle and nuclear astrophysics, whether in space, on the ground, or underground, should be viewed as essential tools for answering fundamental questions of physics as well as astrophysics.

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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FACILITIES

Several existing and proposed experiments are carried out at special sites or observatories that require substantial investments in infrastructure and, in some cases, development. Generally, the diversity of needs means that establishment of one or two central laboratories for the whole field is not feasible. Rather, sites appropriate for each experiment are identified. Some may be in existing facilities (as in the use of the Soudan underground lab for CDMSII); others need to be developed (as in the case of the site in Argentina for the Auger South Observatory). Other examples are Dugway, Utah, for the HiRes experiment and the proposed Telescope Array, the Whipple Observatory on Mt. Hopkins in Arizona for atmospheric Cherenkov telescopes, and the Amundsen-Scott South Pole Station for the proposed IceCube detector.

NASA is currently developing an Ultralong Duration Ballooning (ULDB) program that promises to provide round-the-world flights of up to 100 days, for payloads of up to 1 or 2 tons. The first 100-day demonstration flight in this program is currently planned for 2001. The ability to fly a large payload above the atmosphere for a fraction of a year, at a fraction of the cost of a satellite mission, offers a great opportunity for high-energy astrophysics investigations as well as for investigations using solar and infrared instruments. The panel strongly recommends that NASA support technology developments in balloon, telemetry, and fine-pointing systems so that the ULDB program can develop a reliable science platform of broad utility to the community.

The International Space Station (ISS) can provide a useful platform for certain classes of heavy and/or large-area payloads (up to ~5 ton) that do not require fine pointing. Examples include high-energy cosmic-ray instruments and all-sky gamma-ray or x-ray monitors. The AMS experiment serves as a pathfinder for future ISS payloads. The proposed ACCESS mission is well suited to ISS. The panel endorses the use of the ISS for appropriate astrophysical experiments.

RECOMMENDATIONS FOR THE FUNDING AGENCIES

Particle astrophysics has developed into a coherent field and should be recognized as one: the funding agencies should institute robust mechanisms to support exciting new projects that cut across traditional funding categories. This requires cooperative funding and project coordination both within and across agency borders.

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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  • The Department of Energy should officially recognize that particle and nuclear astrophysics fall within its charter. Investigations in these disciplines probe the fundamental forces and the nature of matter in ways that directly complement accelerator-based experiments. Indeed, much of the evidence for physics beyond the standard model comes from particle astrophysics and cosmology, including evidence for neutrino masses from solar and atmospheric neutrinos; evidence for baryon-number violation and CP violation from the baryon asymmetry of the universe; cosmological evidence for a nonzero cosmological constant; and indications for a new physics at an ultrahigh-energy scale associated with inflation in the very early universe. Investigations such as the search for dark matter and solar neutrino experiments address fundamental physics problems, from supersymmetry to the origin of mass. Giant-air-shower experiments investigate the role of high-energy particles and their interactions in astrophysical settings. The panel expects that the DOE-supported community of high-energy and nuclear physicists will continue to make important contributions in these areas, from advanced instrumentation and detection techniques to data acquisition and analysis. An example where laboratory experiments contribute directly to astrophysics is the study of quark-gluon plasma at the Relativistic Heavy Ion Collider (RHIC), which is relevant to early universe physics and to neutron star interiors. The national laboratories will play an important role in particle astrophysics experiments, which are increasing in size: They can provide technical support for the deployment of large numbers of highly sophisticated components, and they are uniquely equipped to manage such large projects. It is not surprising, therefore, that most of DOE’s laboratories—Brookhaven National Laboratory, Fermi National Accelerator Laboratory, Lawrence Berkeley National Laboratory, Los Alamos National Laboratory, Lawrence Livermore National Laboratory, and the Stanford Linear Accelerator Center—are deeply involved in such programs.

  • At the National Science Foundation, this field spans two divisions, Astronomical Sciences (AST) and Physics (PHY), with different cultures and customs. The Division of Mathematics and Physical Sciences (MPS) recently initiated a program activity in Nuclear and Particle Astrophysics within PHY. This initiative cuts across several units of NSF, including programs in high-energy physics, nuclear physics, gravitational physics, and theory, as well as AST and the Office of Polar Programs. The panel

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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strongly endorses this effort as an important step toward ensuring that this exciting interdisciplinary science is coherently supported by NSF.

  • The panel strongly supports the increasing collaboration between the various agencies in this field, such as the collaboration of DOE and NASA on AMS and GLAST. Interest in two NASA projects, GLAST and LISA, has been expressed by NSF-supported groups and by NSF officials. Such cooperation would, for example, allow groups with relevant expertise developed under the aegis of one agency to participate readily in a project supported primarily by another. The panel expects that as more projects are cross-funded, there will be efforts to rationalize the procedures and customs at the different agencies.

  • To help agencies in this endeavor and to serve the needs of this growing community, there is a clear need for an interdisciplinary advisory structure to review proposed projects and to help set long-term priorities. The panel strongly supports the continuation of the Scientific Assessment Group for Experiments in Non-Accelerator Physics (SAGENAP) by DOE and NSF, with NASA participation as an observer, to assess cross-cutting projects. It is important for all relevant divisions and agencies to participate fully in this coordinated process, so that projects are not required to pass through multiple review committees. The agencies should also regularly seek long-range, coordinated strategic advice on the main scientific priorities, leading to a strategic plan for projects involving astrophysics. This is particularly important as the proposed projects become larger.

  • The funding mechanisms should also be adapted to the field. The NASA concept of missions is probably well suited to the large experiments being considered in particle and nuclear astrophysics: experiments should be of fixed duration, and their costing should include operations and the extraction of science. Various approaches to an important science theme could be coordinated by an organization set up for a fixed time.

  • A strong theoretical effort is essential for the field. In addition to individual researchers, relatively large theory groups that maximize interactions between postdoctoral fellows have made important contributions by exploring the interface between particle and nuclear physics and astrophysics. Such theoretical work lays the foundation for experiments over the next decade. Funding for theoretical astrophysics has gradually eroded, and all three agencies are urged to strengthen their support for theory in particle astrophysics, including the support of large, multidisciplinary groups.

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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  • International collaboration will become the norm in this field, because of the growing scale of particle, nuclear, and gravitational-wave astrophysics experiments. This will ultimately reduce costs and take advantage of the global diversity of expertise; it should help the community avoid a proliferation of competing, often subcritical projects. On the other hand, long-range planning and coordination of large international projects presents a challenge, because of differences of culture, procedures, and budgetary timescales. In this vein, the panel supports the International Union of Physics and Applied Physics in its creation of the Particle and Nuclear Astrophysics and Gravitation International Committee (PANAGIC). Its mission is to increase the circulation of information in the field and promote convergence on large international projects. The panel urges the agencies to coordinate with the community in this endeavor.

ACRONYMS AND ABBREVIATIONS

ACCESS

—Advanced Cosmic-ray Composition Experiment for the Space Station, a cosmic-ray experiment on the ISS

ACE

—Advanced Composition Explorer (NASA)

AGASA

—Akeno Giant Air Shower Array (Japan)

AGN

—active galactic nuclei

AMANDA

—Antarctic Muon and Neutrino Detector Array

AMS

—Alpha Magnetic Spectrometer

ANL

—Argonne National Laboratory (DOE)

ANTARES

—Astronomy with a Neutrino Telescope and Abyss Environmental Research

ASCA

—Advanced Satellite for Cosmology and Astrophysics (Japan)

AST

—Division of Astronomical Sciences (National Science Foundation)

AU

—astronomical unit: basic unit of distance equal to the separation between Earth and the Sun, about 150 million km

Baikal

—an underwater neutrino telescope in Lake Baikal, Russian Federation

BL Lacs

—BL Lacertae objects; galaxies with an extremely bright active galactic nucleus, sometimes referred to as a blazar

BNL

—Brookhaven National Laboratory (DOE)

CANGAROO

—Collaboration of Australia and Nippon (Japan) for a Gamma Ray Observatory in the Outback

CAT

—Cherenkov Array in Themis, France

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

CDMS

—cryogenic dark matter search

CELESTE

—Cherenkov Low Energy Sampling and Timing Experiment at Themis, France

COBE

—Cosmic Background Explorer; a NASA mission launched in 1989 to study the cosmic background radiation from the Big Bang

CP violation

—charge-parity violation

DOE

—Department of Energy

EGRET

—The Energetic Gamma Ray Experiment Aboard the Compton Gamma Ray Observatory

ESA

—European Space Agency, the European equivalent of NASA

FNAL

—Fermi National Accelerator Laboratory (DOE)

GALLEX

—an international solar neutrino research project that measured the solar neutrino flux produced inside the Sun by proton-proton fusion

GEO

—a laser-interferometric gravitational-wave observatory (Hannover, Germany)

GLAST

—Gamma-ray Large Area Space Telescope, a NASA-DOE mission

GRBs

—gamma-ray bursts

GZK cutoff

—upper limit to the cosmic-ray energy spectrum of around 1019 eV as specified by the theory of Greisen, Zatsepin, and Kuz’min

HEGRA

—High-energy Gamma Ray Astronomy experiment, a project that features a gamma-ray telescope in La Palma, Spain

HELLAZ

—proposed French solar neutrino detector

HERON

—a solar neutrino detector using superfluid helium

HESS

—High-Energy Stereoscopic System; gamma-ray telescope in Namibia

HiRes

—High-Resolution Fly’s Eye

IR

—infrared

ISM

—interstellar medium

ISS

—International Space Station

LANL

—Los Alamos National Laboratory (DOE)

LBNL

—Lawrence Berkeley National Laboratory (DOE)

LENS

—international Laboratory for Nonlinear Spectroscopy; also known as Solar Neutrino Interactions through Real-time Excitation of Nuclei (SIREN)

LIGO

—Laser Interferometer Gravitational-Wave Observatory

LISA

—Laser Interferometer Space Antenna

LLNL

—Lawrence Livermore National Laboratory (DOE)

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

LSND

—Liquid Scintillation Neutrino Detector experiment; searches for neutrino oscillations and explores other aspects of particle and nuclear physics

LVD

—Large Volume Detector (Gran Sasso, Italy)

MACHO

—massive compact halo object; MACHOs are dark stars or planets that may make up the Milky Way’s dark halo

MACRO

—Monopole, Astrophysics, and Cosmic Ray Observatory (Gran Sasso, Italy), a detector for atmospheric neutrinos and magnetic monopoles

MAGIC

—gamma-ray telescope in La Palma, Spain

MILAGRO

—large, water Cherenkov detector at Los Alamos, New Mexico

Mir

—space station of the Russian Federation

MPS

—Division of Mathematics and Physical Sciences (National Science Foundation)

MSU

—Michigan State University

NASA

—National Aeronautics and Space Administration

NEMO

—Neutrino Mediterranean Observatory, an international collaboration to study double-beta decay without the emission of neutrinos

NESTOR

—Neutrino Experimental Submarine Telescope with Oceanographic Research; a deep-sea neutrino-detector in the Mediterranean

NSF

—National Science Foundation

OGLE

—Optical Gravitational Lensing Experiment, a program to search for dark, unseen matter using the microlensing phenomena

ORNL

—Oak Ridge National Laboratory (DOE)

OWL

—Orbiting Wide-angle Light collectors

PANAGIC

—Particle and Nuclear Astrophysics, and Gravitational International Committee; created by the International Union of Physics and Applied Physics

PHY

—Division of Physics (National Science Foundation)

PMT

—photomultiplier tube

RHIC

—Relativistic Heavy Ion Collider at Brookhaven National Laboratory (DOE)

SAGENAP

—Scientific Assessment Group for Experiments in Non-Accelerator Physics (DOE and NSF)

SAGE

—Soviet-American Gallium Experiment

SIM

—Space Interferometry Mission

SLAC

—Stanford Linear Accelerator Center (DOE)

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
×

SNEWS

—Supernova Early Warning System

SNO

—Sudbury Neutrino Observatory, a heavy-water Cherenkov detector

SNRs

—supernova remnants

SQUID

—Superconducting Quantum Interference Device

STACEE

—Solar Tower Atmospheric Cherenkov Effect Experiment, a Cherenkov telescope at Albuquerque, New Mexico

Super MACHO project

—Massive Compact Halo Object project (United States/Australia)

Super-Kamiokande

—large, water Cherenkov detector for cosmic particles based at the University of Tokyo (Japan/United States)

TAMA

—a 300-m laser-interferometer gravitational-wave antenna (Japan)

TREK

—detector aboard Mir that probes the composition of the galactic cosmic rays

TRIAD

—Tucson Revised Index of Asteroid Data

UHE

—ultrahigh-energy

ULDB

—ultralong-duration ballooning

VERITAS

—Very Energetic Radiation Imaging Telescope Array System

VHE

—very high energy

VIRGO

—French-Italian gravitational-wave interferometry project

WIMP

—weakly interactive massive particles

Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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Page 163
Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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Page 164
Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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Page 165
Suggested Citation:"3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics." National Research Council. 2001. Astronomy and Astrophysics in the New Millennium: Panel Reports. Washington, DC: The National Academies Press. doi: 10.17226/9840.
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Astronomy and Astrophysics in the New Millennium: Panel Reports Get This Book
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In preparing the report,

Astronomy and Astrophysics in the New Millenium

, the AASC made use of a series of panel reports that address various aspects of ground- and space-based astronomy and astrophysics. These reports provide in-depth technical detail.

Astronomy and Astrophysics in the New Millenium: An Overview summarizes the science goals and recommended initiatives in a short, richly illustrated, non-technical booklet.

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