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Prof. G. Farrar and grad student Jeff Allen are members of the Pierre Auger Collaboration, made up of over 200 physicists from 55 institutions around the world; the 15 participating countries shared the $50 million construction cost. The Pierre Auger Observatory’s Southern Hemisphere detector in Malargue, Argentina, consists of a surface detector (SD) — 1600 instrumented water Cerenkov detectors with a 1.5 km spacing, to sample the lateral distribution of the shower — plus 4 air-fluorescence (AF) telescopes which operate at night allowing simultaneous measurement of the longitudinal development of about ten percent of the showers. By studying hybrid events, those that are simultaneously detected in the surface and air florescence detectors, the Auger Observatory is able to cross calibrate the energy measurement of the SD and AF detection methods. A Northern site in Colorado is being proposed; having full-sky coverage will be indispensable for addressing many important questions.
As of this writing in early 2007, Auger has recorded more events than all previous detectors combined. When deployment of the tanks is complete in Fall 2007, Auger will accumulate ultrahigh energy cosmic ray (UHECR) events at a rapid rate. This great increase in statistical power should allow the fundamental questions of UHECR physics to be resolved within a few years: What is the source or sources of UHECRs? Is there a cutoff in the spectrum at high energy as predicted by Greisen, Zatsepin and Kusmin, due to interaction of UHECRs with relic CMB photons? Can cosmic ray air showers be understood with the particle interaction models developed to describe accelerator-energy collisions, or is there evidence for new physics in UHE collisions ?
Understanding UHECR observations demands knowledge of fundamental physics at untested energies and extension of theoretical models of astrophysical objects into an unprecedented domain. Current work at NYU within Auger involves using the SENECA air shower simulation to test the validity of the underlying particle physics interaction models as well as the approximations made in air shower simulations. The speed of SENECA allows the effects of the atmosphere on shower development to be explored and permits detailed simulation of hybrid events, to allow direct comparison between simulations and real data for individual events. Other UHECR-related projects at NYU involve postdoc A. Berlind and grad student R. Jansson and focus on extracting information from arrival direction correlations and energy correlations about the UHECR sources, and on determining the Galactic magnetic field. If the GMF were known, observed arrival directions could be corrected at least approximately to have a better estimation of true source directions. Conversely, observed correlations may help constrain the large scale GMF.