On the vast plain known as the Pampa Amarilla (yellow prairie) in western Argentina, the Pierre Auger Cosmic Ray Observatory is studying the universe's highest energy particles, which shower down on Earth in the form of cosmic rays. While cosmic rays with low to moderate energies are well understood, those with extremely high energies remain mysterious. By detecting and studying these rare particles, the Auger Observatory is tackling the enigmas of their origin and existence.
Cosmic rays are charged particles (usually a proton or a heavy nucleus) that constantly rain down on us from space. When a cosmic ray particle reaches Earth, it collides with a nucleus high in the atmosphere, producing many secondary particles, which share the original primary particle's energy. The secondary particles subsequently collide with other nuclei in the atmosphere, creating a new generation of energetic particles that continue the process, multiplying the total number of particles. The resulting particle cascade, called an "extensive air shower,'' arrives at ground level with billions of energetic particles extending over an area as large as 10 square miles.
The Auger Observatory is a "hybrid detector," employing two independent methods to detect and study these high-energy cosmic rays. One technique detects high energy particles through their interaction with water placed in surface detector tanks. The other technique tracks the development of air showers by observing ultraviolet light emitted high in the Earth's atmosphere with fluorescence telescopes. The hybrid nature of the Pierre Auger Observatory provides for two independent ways to see cosmic rays.