The Telescope Array and its Low Energy Extention

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The Telescope Array (TA) Project is a joint Japan-U.S.-China-Taiwan effort in the field of ultra high energy cosmic ray (UHECR) physics. This experiment is designed to detect and measure the most powerful particles in the Universe, which can carry as much energy in a single atomic nucleus as a fast pitch baseball. Such particles are very rare. TA draws on the technical expertise of two previous smaller experiments, the Akeno Giant Air Shower Array experiment (AGASA), which operated between 1993-2003 in Japan and the High Resolution Fly's Eye (HiRes) experiment that ran between 1997 and 2006 in Utah. When an UHECR enters the upper atmosphere of the Earth, it interacts with an atmospheric nucleus to produce a spray of secondary particles, which subsequently collide with more nuclei deeper in the atmosphere, and so on. This avalanche of particles is known as an Extensive Air Shower (EAS). AGASA measured the footprint of EASs at ground level using an array of 111 (2-m2) detector units each of which counted the number of particles passing through it. HiRes observed the longitudinal development of EASs by imaging the fluorescence light emitted by the charged particles as they passed through air. TA combines these two techniques in a single location, which then gives a more complete picture of each cosmic ray event than could either technique alone. The TA Project is being jointly funded by the Ministry of Education, Culture, Sports, Science and Technology in Japan, and the National Science Foundation in the United States, and is hosted by the University of Utah. It is located near the town of Delta, Utah. The location combines the advantages of more than 400 square miles of undeveloped land needed by the ground array with infrastructure support provided by Delta. The desert environment is essential for the operation of fluorescence detectors, which run on clear, moonless nights and require clear, dry air for maximum transparency. After its completion in the spring of 2007, TA will be the most sensitive cosmic ray detector in the Northern Hemisphere. The collaborating institutions on TA include 15 universities and laboratories from Japan, led by the Institute for Cosmic Ray Research (ICRR) at the University of Tokyo. The U.S. collaborators include six universities and Los Alamos National Laboratory. Also participating are scientists from the Institute for High Energy Physics (IHEP) in Beijing, China, and the National United University of Taiwan. One of the goals of TA will be to find the sources of UHECRs, suspected to be active galactic nuclei (AGN). AGN's, which comprise about 10% of all known galaxies. These are among the most energetic objects known in the Universe. AGNs are thought to be powered by accretion of matter and energy in a central black-hole. Correlations between cosmic ray sources and AGNs have been observed by both the AGASA and HiRes experiments, with a subclass of these galaxies, called BL-Lacerate (BL-Lac) objects, which have their jets pointed toward Earth. Most of the known AGN's and BL-Lac objects are found in the Northern sky, away from the obscuration of our own Milky Way Galaxy that is prominent in the Southern Hemisphere. In addition, TA will study the energy region where the Greisen-Zatsepin-K'uzmin (GZK) suppression of the UHECRs is expected to occur from interactions between UHECRs and the Cosmic Microwave Background Radiation. This interaction becomes large when sufficient energy exists in the collisions to produce a short-lived particle called the pi-meson. HiRes has recently reported strong evidence for this suppression, but AGASA's results show no such abatement. A possible explanation for this discrepancy is an energy scale difference between the experiments; this will be resolved by simultaneous observation with both type of detectors. Another possibility, fluctuations due to limited statistics, will be addressed by TA with a ground array eight times the size of AGASA. The HiRes project has had a longstanding broader impacts project in ASPIRE, which has involved teachers and students at many levels in the science of UHECRs. In addition, TA will build a visitors center in the town of Delta, which will welcome visitors to its site.