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



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 3 Report of the Panel on Particle, Nuclear, and Gravitational-Wave Astrophysics

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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.

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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.

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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.

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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.

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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.

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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.

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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.

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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)

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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)

OCR for page 123
Astronomy and Astrophysics in the New Millennium: Panel Reports 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