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An Auger surface detector in front of the Andes Mountains in the Pampa Amarilla of Argentina.

My quest is to understand the properties and the origins of the highest energy particles in the universe. These cosmic rays are fully ionized atomic nuclei with energies in excess of 10 Joules per particle. How and where they are produced in the cosmos is a long-standing mystery. Approximately 400 physicists from 18 countries have joined the Pierre Auger Collaboration to solve this puzzle. At Penn State, I am the associate director of the Institute for Gravitation and the Cosmos ( and a member of its Center for Particle Astrophysics.

The Pierre Auger Cosmic Ray Observatory

The Auger Observatory measures UHE cosmic rays landing anywhere within a region of Argentina whose area is equal to that of Rhode Island. The observatory works by measuring the particle cascade that is produced by the UHE cosmic ray. An energetic cosmic ray collides with an atomic nucleus high in Earth's atmosphere, producing hundreds of secondary energetic particles. Those particles also spawn new particles by their collisions, and the resulting cascade is called an "extensive air shower." There are billions of energetic charged particles in an extensive air shower. Two methods are used at the Auger Observatory to measure the air showers. The first is an array of 1600 "water Cherenkov detectors" separated from each other by 1.5 km. They record cascade particles as they arrive at ground level. The second method works only at night. "Air fluorescence telescopes" measure the cascade as it descends through the atmosphere at the speed of light. The air shower particles cause nitrogen in the air to fluoresce as they pass. There are 24 telescopes that measure this ultra-violet light. They are located in four buildings on the periphery of the surface array, six telescopes per building.

The Auger Collaboration has made important discoveries based on data collected at the observatory site in Argentina. Features in the UHE cosmic ray energy spectrum, anisotropy of the arrival directions, and properties of the cascade developments have led to important new understandings and new puzzles. (Extensive information about the Auger Observatory can be found at, and links to journal articles and conference presentations are at

This research area provides opportunities for undergraduate research.

P. Sommers : Research