URA 2005 Annual Report

Pierre Auger Observatory Project

Cosmic rays are high-energy charged particles from space that constantly bombard the Earth from all directions. The majority of the particles are single protons— the nuclei of hydrogen atoms—but some are much heavier, ranging up to the nuclei of lead atoms. A small fraction of cosmic rays are the most energetic particles ever observed in nature. Direct measurement requires locating detectors above most of the Earth's atmosphere, using high-altitude balloons and orbiting satellites, which necessarily imposes limits on the weight of instrumentation required for the highest energy cosmic rays. About 60 years ago, French physicist Pierre Auger discovered that cosmic rays can also be detected indirectly on the surface of the Earth by observing the showers of secondary particles they produce when colliding with atmospheric molecules. Named in his honor, the Pierre Auger Observatory is a broad-based international effort to solve the mystery of the origins of the extremely rare, ultra-high energy cosmic rays showering the earth at energies above 10¹⁹ electron volts (eV), or about 10 million times greater than the energy of the protons accelerated by Fermilab's Tevatron. These highest-energy cosmic rays can be considered messengers from the extreme universe, and thus represent great opportunity for discoveries. Tracking these particles back to their source will help explain how nature creates the conditions to accelerate them to such high energies.

The especially interesting cosmic rays, which have energies over 10²⁰eV (equivalent to the kinetic energy of a tennis ball traveling at 340 miles per hour, but packed into a single proton), have an estimated arrival rate of just 1 per square kilometer per century! To record a large number of these ultra-high energy cosmic rays events requires a very large observing area, roughly the size of the state of Rhode Island. The Pierre Auger Observatory is constructing an array of 1,600 surface detectors spaced about 1.5 kilometers apart and spread over 3,000 square kilometers in Argentina's Mendoza Province, just east of the Andes Mountains. For more accurate detection of cosmic ray events, the Observatory is the first to use a hybrid approach: a surface detector array to record the showers of particles produced when cosmic rays strike the earth's surface; and fluorescence detectors to record the atmospheric flares produced by particle showers, visible on dark clear nights. Each surface detector, consisting of a tank filled with 3,000 gallons of ultra-pure de-ionized water and associated electronic equipment, can record the electromagnetic shock waves produced by cosmic ray shower particles traveling through the water. Surrounding the surface array is a set of 24 fluorescence telescopes, which on clear moonless nights observe the ultraviolet fluorescence produced as cosmic ray shower particles travel through the atmosphere.

The Auger Project was initiated in 1995 by James W. Cronin, Professor of Physics and Nobel Laureate at the University of Chicago. Currently, the Pierre Auger collaboration is led by Alan A. Watson, Professor of Physics at the University of Leeds in the United Kingdom, and by Giorgio Matthiae, Professor of Physics at the University of Rome. Thus far, the Auger collaboration includes over 370 scientists and engineers from Argentina, Australia, Brazil, Bolivia, Czech Republic, France, Germany, Italy, Mexico, Netherlands, Poland, Slovenia, Spain, United Kingdom, and the USA. The U.S. collaboration comprises scientists from 16 universities plus Fermilab, home of Auger Project Manager Paul Mantsch and Deputy Project Manager Carlos Hojvat.

In March 1999, an international agreement was signed in Argentina for the organization, management and funding of the Pierre Auger Observatory, and the southern hemisphere site was inaugurated at Malargue in Mendoza Province. In November 2000, construction was completed for the Detector Assembly Building at the Central Campus in Malargue and for the first fluorescence detector building. In December 2000, the project received a gift of $1 million from the University of Chicago for the construction of the Central Campus Office Building. An “Engineering Array,” consisting of 32 surface detector stations and two fluorescence telescope units, was deployed in 2001. With data collected in 2002-2003, the Engineering Array demonstrated that the Observatory will perform better, and will have greater discovery potential, than originally expected. With the experience gained from the Engineering Array, construction of the entire southern hemisphere array is now underway, and is scheduled for completion in 2007. The construction cost for the southern hemisphere Auger Observatory is about $50 million.

In November 2005, Auger scientists held a celebration in Malargue to mark the progress on installation of the Observatory's detectors and presentation of first physics results. About 60% of the Auger surface array was operational, three fluorescence detector buildings, each with six telescopes, were operating routinely, and construction was initiated on the fourth fluorescence detector building. During 2005 the first Auger results were presented at the International Cosmic Ray Conference in India and at other scientific conferences. These presentations included: a new cosmic ray spectrum at the highest energies; the results of anisotropy and point source searches; and new limits on the photon content of the primaries, the cosmic ray particles that initially strike the earth's atmosphere. Within the scientific community, there is great interest in the mysterious origin of the ultra-high energy cosmic rays that the Observatory is analyzing. Theorists have developed a number of exotic theories for such origin, including the collapse of hypothetical objects, called “topological defects,” left over from the Big Bang.

In order to get a complete view of the heavens as seen from the earth, the Auger collaboration is working to establish a northern hemisphere partner of the southern hemisphere Observatory, likely to be based in southeastern Colorado. With observatories in both hemispheres, the Auger collaboration will have the opportunity to view cosmic rays across the entire sky visible from earth.

In a grant to URA on behalf of the U.S. Project participants under Dr. Cronin's continuing leadership, the National Science Foundation and the Department of Energy are providing some $10 million toward the U.S. share for the construction of the southern hemisphere Observatory. URA serves as the sponsoring organization for the U.S. participants, and as such oversees those activities currently funded by the U.S., such as component R&D and production. In addition, DOE and NSF have designated URA to be the agent on behalf of the U.S. on the Project's international oversight board, currently chaired by URA President Fred Bernthal.

For further information about the Pierre Auger Observatory, visit the it's website at http://www.auger.org.