1. Where do ultra-high energy cosmic rays come from? No one knows the origin of ultra-high energy cosmic rays. The majority of low-energy cosmic ray particles which hit the Earth come from someplace in our galaxy, the Milky Way. Many come from the explosions of stars called supernovas. However, the majority of ultra-high energy cosmic rays probably come from outside the Milky Way, but from where? There is no known process in the cosmos which can produce particles with such energies, not even the most violent explosions of stars. Understanding the origin of ultra-high energy cosmic rays may reveal secrets about the evolution and possibly the origin of the universe, due to the mystery of their enormous energies. How do we study cosmic rays? Cosmic rays may be detected indirectly at the Earth’s surface by observing the showers, or cascades, of particles they produce in the atmosphere. A cascade is created when a very high-energy particle collides with an air molecule. Fragments from the primary collision may collide with other air molecules in the same fashion, making additional fragments. The shower formation continues until the energy of the original particle is distributed among millions of particles raining down on the Earth. By studying these air showers, the properties of the original cosmic ray particle (energy, incident direction, particle type) may be determined by scientists. Three photomultiplier tubes Polyethylene tank 12,000 liters of purified water Solar panel Communications antenna GPS Receiver Surface Detector Battery box Possible origin of ultra-high energy cosmic rays Something in the cosmos is hurling extremely high- energy particles at the universe. Where are these particles coming from? From a very powerful cosmic explosion? From an enormous black hole which absorbs stars in a violent death? From collisions between galaxies? From the disintegration of matter left over from the Big Bang which created the universe? The Pierre Auger Observatory is looking for answers to these questions to make progress in our understanding of the universe What are cosmic rays? Cosmic rays are particles which come from outer space and constantly bombard the Earth from all directions. The majority of these particles are atomic nuclei or electrons. Some of them are more energetic than any other particle observed in nature. Ultra-high energy cosmic rays travel very close to the speed of light and are hundreds of millions of times more energetic than particles produced in the world’s largest particle accelerator laboratories. Fluorescence Detector Building Panoramic View

2. The Pierre Auger Observatory To solve the mystery of ultra-high energy cosmic rays, the Pierre Auger Observatory will measure the air showers created when cosmic rays collide with molecules in the upper atmosphere. The energy, incident direction, and original particle type will be determined for the very highest energy cosmic rays. The Auger Observatory will be located at two sites: a northern hemipshere site in southeast Colorado, USA, and in the region of Malargüe in the Province of Mendoza, Argentina. As the earth rotates, the two sites will allow scientists to “view” the entire universe. The southern hemisphere site is being constructed first, in the years 2000-05. Each site will consist of an array of 1600 detectors, separated by 1.5 kilometers and covering a surface area of 3000 km 2. The array of surface detectors will be complemented by a set of highly sensitive telescopes which, on clear nights with no moon, observe the faint ultraviolet light which the air showers produce as the shower particles travel through the atmosphere. Approximately 250 scientists from more than 30 institutions in 15 countries participate in this experiment: Argentina, Australia, Brasil, Bolivia, Czech Republic, England, France, Germany, Italy, Mexico, Poland, Slovenia, Spain, United States, Vietman. Simulation of the measurement of a cosmic ray air shower with the surface and fluorescence detectors Technical Data Objective: Determine the particle type, energy, and incoming direction of primary cosmic ray particles with energy greater than 10 19 electron volts (eV). Type of observatory: “Hybrid” observatory, consisting of a surface array of water Čerenkov detectors and a system of atmospheric fluorescence telescopes to observe cosmic ray air showers. Statistics: 100 events per year with primary cosmic ray energy greater than 10 20 eV. Observatory sites: Malargüe, Mendoza, Argentina in the southern hemisphere and southeast Colorado, USA, in the northern hemisphere. Surface detectors: Coverage area: 3000 km 2 per site. Number of detectors: 1600 per site. Type of detector: Čerenkov radiation detector containing 12,000 liters of purified water, each detector viewed by 3 photomultiplier tubes. Distance between detectors: 1.5 km. Fluorescence telescopes: Number of telescopes: 4 per site, with a total of 24 mirrors. Range: 20 km for cascades of 10 20 eV. Mirrors: 3.6 m  3.6 m with 30º x 30º aperture, each employing 440 photomultiplier tubes Pierre Auger Observatory For more information, see our web site http://www.auger.org Auger Observatory Headquarters in Argentina Proposed Northern Hemisphere Headquarters is Lamar Community College