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URA 2006-2007 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 protons—the nuclei of hydrogen atoms—but some are much heavier, ranging up to the nuclei of uranium 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. Nearly 70 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 ultrahigh- 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 they 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 of over 1020 eV (equivalent to the kinetic energy of a tennis ball traveling at 60 miles per hour, but packed into a single proton), have an estimated arrival rate of less than 1 per square kilometer per century! They are important because they are expected to be only slightly deflected by galactic and intergalactic magnetic fields. 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 completing construction of an array of 1,600 surface detectors spaced about 1.5 kilometers apart and spread over 3000 square kilometers in Argentina’s Mendoza Province, just east of the Andes Mountains. For more accurate measurement 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, supplemented with 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 3000 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 array are four fluorescent detector buildings, each of which houses a set of 6 telescopes. On clear moonless nights the telescopes observe the ultraviolet fluorescence produced as cosmic ray shower particles travel through the atmosphere.
In order to get a complete view of the heavens as seen from the earth, one array would be located in the northern hemisphere and one in the southern hemisphere
The Auger Project was initiated in 1995 by JamesW. Cronin, Professor of Physics and Nobel Laureate at the University of Chicago and Alan A Watson, Professor of Physics at the University of Leeds, in the United Kingdom. Currently, the Pierre Auger collaboration is led by Dr. Watson and by Giorgio Matthiae, Professor of Physics at the University of Rome. At present, the Auger collaboration includes over 300 scientists from about 70 institutions in Argentina, Australia, Bolivia, Brazil, Czech Republic, France, Germany, Italy, Mexico, Netherlands, Poland, Portugal, Slovenia, Spain, United Kingdom, United States, and Vietnam. The U.S. collaboration comprises scientists from 17 universities plus Fermilab and Argonne National Laboratory. Fermilab is the home of Auger Project Manager, Paul Mantsch.
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 inMalargue 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 the data collected in 2002-2003, the Engineering Array demonstrated that the Observatory will perform better, and will have greater discovery potential, than originally expected. The southern hemisphere Auger Observatory is expected to be completed by the end of 2007 at a cost of about $50 million.
During construction, with data from the Observatory’s operating detectors, the Auger collaboration presented results at the 2005 and 2007 International Cosmic Ray Conferences, and at other scientific conferences. These presentations included: results for the cosmic ray spectrum at the highest energies; measurements that suggest that the mass of the most energetic particles is higher than expected; results of searches for emission from the galactic center region, clustering 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. As of mid-2007, the southern hemisphere Observatorywas about 80%completed,with all of the fluorescence telescopes and 1300 surface detectors in operation. The Auger data collected since January 2004 are about three times that of the AGASA surface array in Japan and about twice that of the HiRes fluorescence detectors in the U.S.
In 2007, the Auger collaboration began a public release of one percent of the cosmic-ray events recorded by the southern hemisphere Observatory. New cosmic-ray data—about 70 events per day—are posted on the Auger website on a daily basis. The one-percent release is part of the worldwide Pierre Auger education and outreach program. It will allow teachers to expose students to real scientific data and the breathtaking processes that take place in the cosmos, where charged particles are hurled toward Earth. Data are provided both as graphical displays and in tabular form. For each cosmic-ray air shower, the Auger displays show the energy and direction of the incoming cosmic-ray particle.
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.
To get a complete view of the heavens as seen from the earth, the Auger collaboration has established a design for a northern hemisphere partner of the southern hemisphere Observatory, 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. If cosmic rays are found to arrive from specific directions, the Auger Observatories will be able to identify and study possible cosmic ray sources all over the sky with equal sensitivity. If discrete sources are not found, the full-sky coverage provided by the two sites will be essential for determining whether cosmic ray arrival directions are characterized by subtle large-scale patterns in the universe, or whether they are completely arbitrary.
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, which has been chaired by URA President Fred Bernthal from2004 to 2007.