Closing in on the origin of cosmic raysRead more...
Cosmic ray and accelerator based particle physics share common roots, and in fact many of the key discoveries early in the history of particle physics came from the study of cosmic rays. After a period of divergence between the two fields, both in methodology and in the key areas of interest, a confluence is now underway, driven in no small part by the mystery of the highest energy cosmic rays.
The Pierre Auger Observatory is studying ultra-high energy cosmic rays, the most energetic and rarest particles in the universe. When one of these particles reaches Earth, it collides with an air 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.
Auger scientists face a challenge, however, because cosmic rays with energies above 10^19 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 10^20 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 (see picture below). 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. Employing these two complementary observation methods provides the Auger Observatory with high quality information about the types of particles in the primary cosmic rays. Comparing results from the different types of detectors also helps scientists reconcile the two sets of data and produce the most accurate results about the energy of primary cosmic rays. UWM is one of the 18 US institutions participating in this (17-country) collaboration.
At present, there is no scientific consensus on how or where cosmic rays with ultra-high energies originate. With unprecedented collecting power and experimental controls, the Auger Observatory has been gathering the data needed to solve those puzzles.
The Auger Collaboration at UWM is currently supported by a National Science Fundation Award.
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.