On the vast plain known as the Pampa Amarilla (yellow prairie) in western Argentina, the Pierre Auger Observatory is studying the highest-energy particles in the Universe, which hit the Earth from all directions, so-called cosmic rays. Cosmic rays with low to moderate energies are well understood, while those with extremely high energies remain highly mysterious. By detecting and studying these rare particles, the Pierre 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 acceleration of most low energy cosmic rays is related to various types of magnetic fields in space. These magnetic fields are known to exist on the sun, in the solar wind, and in the remnants of supernova explosions in our Milky Way Galaxy. Interactions of charged particles with these fields can account for cosmic rays with energies ranging from 1 billion (109) electron volts to 10 thousand trillion (1016) electron volts. Occasionally, however, a cosmic ray with an energy above 10 million trillion (1019) electron volts is detected. It would take 10 million Tevatrons, the world's largest particle accelerator, to achieve energies as high as these remarkable cosmic rays! There is no scientific consensus on how or where cosmic rays with these ultra-high energies originate. With unprecedented collecting power and experimental controls, the Auger Observatory has been gathering the data needed to solve those puzzles.
Auger scientists face a challenge, however, because the highest energy cosmic rays are extremely rare. Cosmic rays with energies above 1019 eV arrive on Earth at a rate of only 1 particle per square kilometer per year. The especially interesting cosmic rays, which have energies of over 1020 eV (equivalent to the kinetic energy of a tennis ball traveling at 53 miles per hour, but packed into a single proton!), have an estimated arrival rate of just 1 per square kilometer per century! In order to record a large number of these remarkable events, the Auger Observatory has created a detection area in western Argentina's Mendoza Province that is the size of the state of Rhode Island (USA), or a bit larger than the country of Luxembourg.
The Auger Observatory is a "hybrid detector", employing two independent methods to detect and study 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. The hybrid nature of the Pierre Auger Observatory provides for two independent ways to see cosmic rays.