ries of fundamental particle physics. There are three complementary approaches to attacking the dark-matter problem: direct detection in the laboratory, indirect detection by way of astronomical observations, and searches for candidate particles in human-made high-energy particle accelerators. The panel’s recommendations concern only indirect detection by astrophysics, although all three approaches will be important in ultimately resolving this mystery.

The indirect detection of dark matter involves searching not for the darkmatter particles themselves, but rather for products of the annihilation or decay of dark-matter particles. These may be gamma rays, cosmic rays, or neutrinos. The sources will be places in the cosmos where scientists believe that dark matter concentrates, such as in the gravitational potential wells of galaxies. Therefore, the panel recommends a program of gamma-ray and particle searches for dark matter.

The field of high-energy and very-high-energy particle astrophysics has blossomed in the past decade, with a wealth of results from spaceborne and ground-based gamma-ray telescopes and cosmic-ray detectors, and it is hoped that similar exciting results will come soon from neutrino telescopes. These instruments provide unique views of astronomical sources, exploring the extreme environments that give rise to particle acceleration near, for example, supermassive black holes and compact binary systems. The panel recommends continued involvement in high-energy particle astrophysics, with particular investment in new gamma-ray telescopes that will provide a much deeper and clearer view of the high-energy universe, as well as a better understanding of the astrophysical environment necessary to disentangle the dark-matter signatures from natural backgrounds. The panel’s highest-priority recommendation for ground-based instrumentation is significant U.S. involvement in a large international telescope array that will exploit the expertise gained in the past decade in atmospheric Čerenkov detection of gamma rays. Such a telescope array is expected to be an order-of-magnitude more sensitive than existing telescopes, and it would for the first time have the sensitivity to detect, in other galaxies, dark-matter features predicted by plausible models.

The panel also recommends a broad program for particle detectors to be flown above the atmosphere, making use of the cost-effective platforms provided by balloons and small satellites. Major developments in large ground-based detectors for neutrinos are in progress already. These programs are an important component of dark-matter and astrophysical particle characterization and should be continued, along with the research and development that will improve the sensitivity of neutrino detectors in decades to come.

The above recommendations are possible only because there is now available a suite of new instruments that have recently achieved technical readiness. In the program areas that the panel considered, a significant component of the technology development has been done outside the United States. To maintain the nation’s ability to participate in research in astrophysics in the future, the panel recommends



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