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Prof. G. Farrar is a member 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.

Auger data addresses fundamental questions of UHECR physics: 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. NYU researchers have used 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 focused on extracting information from arrival direction correlations and energy correlations about the UHECR sources, and on new models of the Galactic magnetic field.