Particle Astrophysics with Milagro

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Very high energy gamma rays provide an important tool to investigate relativistic energetic astrophysical sources of the ubiquitous cosmic rays which are accelerated in our galaxy and in the universe to very high energies. When high energy cosmic rays interact with matter, magnetic fields and ambient photons they generate high energy gamma rays and/or neutrinos of high energy. Some of these neutral particles travel directly to the earth and, when observed by gamma ray telescopes, can both provide essential information about not only the origin and distribution of cosmic rays and their sources, but also can serve as probes for gamma rays expected from the annihilation of dark matter. Thus they allow the study fundamental high energy astrophysics as well as particle physics. As the flux of these high energy gamma rays is small, large acceptance detectors with good pointing and adequate cosmic ray rejection capabilities are essential for this study. The Milagro detector uses a new technique to detect high energy gamma rays. It allows the simultaneous and continuous viewing of the entire overhead sky in the TeV energy domain. It detects the bundles of energetic particles produced by high energy gamma ray interacting in the atmosphere, which acts like an amplifier of each high energy photon. Detection occurs by sampling light photons produced by these relativistic particles in a 6 million gallon water tank instrumented with 737 photomultiplier tubes. The pulse height and arrival times of these photons permit reconstruction of the celestial direction of the original particle, as well as rejection of most of the unwanted cosmic ray showers. Its continuous operation allows observation of both steady and transient sources of TeV gamma rays. Milagro has detected and measured the energy spectrum of gamma rays from the Crab neubla; from active galactic nuclei (AGNS) blazars Mrk 501 and 421 in their active phases; from a gamma ray burster (GRB); and of diffuse TeV gamma ray emission from the inner galactic disc region from interaction of cosmic rays with matter in the galactic disc, and from TeV gamma ray sources in the Cygnus arm of the galaxy. These observations provide new and important data for the understanding of the origin, acceleration and propagation of cosmic rays. Under this NSF grant, work will continue on refining the observations of these and other possible objects, and on analyzing the data.The Milagro experiment and data have provided excellent training ground for undergraduate student research experience and for graduate student training in hardware and software. UCI also has several active programs, such as COSMOS and Quarknet , for outreach to K-12 teachers and students. We have been involved in communicating to school students at K-12 levels and to seniors (Academy of Continued Life Long learning).