The Pierre Auger Observatory Project

In the last 30 years, ground based detectors have observed just over a dozen events with energies equal to, or larger than, 100 EeV (1 EeV = $10^{18}$ eV). For brevity, we may call these ``ultra high energy cosmic rays" (UHECR). There is a common agreement that no known conventional astrophysical mechanism is able to accelerate particles to energies exceeding 100 EeV. Moreover, we know that the UHECR must come from ``nearby" sources (within 100 Mpc) as interactions with the 2.7 K microwave background radiation - the so-called GZK cutoff - limit the distance from which they can reach us. We also expect that the incident direction of the UHECR should point to within a few degrees of their sources, but no observation has been made confirming the existence of any astrophysical object fulfilling the above constraints. The only alternative ways of producing the UHECR are exciting but highly speculative theories such as that of collapsing cosmic strings or other topological defects followed by the disintegration of grand-unification gauge bosons having masses as large as $10^{15}$ GeV. In short, all possible (future) explanations will open new windows in the fields of astrophysics, cosmology and/or fundamental interactions. The ``Pierre Auger Observatory" project is a dedicated experiment which mainly addresses the puzzle of the existence of the UHECR, although it gives access to many other fields of investigation (neutrino astronomy in the highest energy range, exploration of extragalactic magnetic fields, tests of interaction models well above the energies accessible to present or future accelerators...) and can be used by other communities as a facility. \\ \\The present status of the project is that of a hybrid detector which combines the statistical power of a giant array of particle detectors with the observational quality of a fluorescence telescope based on the Fly's Eye technique. The array will consist of a 6000 km$^2$ detector installed over two sites (an area needed because of the extremely low fluxes of the UHECR: about 10 per km$^2$ per \emph{century} above the GZK cutoff). 3200 water Cerenkov tanks will sample the particles produced at ground level by the giant air showers generated by the interaction of the UHECR with the atmosphere. The optical telescopes (three at each site) will observe the fluorescence light produced by the shower. The detector will use unconventional technologies -~in particle physics~- such as solar power, data transfer and acquisition with cellular telephone techniques, synchronisation using GPS satellites etc... It is designed in such a way that the identification of the incident particles should be possible, at least to some extent (distinction between photons, protons and heavy nuclei and detection of neutrinos). It is hoped that in a very few years, the Auger Observatory will help to disentangle the threads of this astrophysical enigma. \\ \\The first phase of the observatory will be built at Pampa Amarilla in Malarg\"{u}e, Mendoza, Argentina. Construction will start during 1999 with the deployment of an engineering array of 40 tanks over 55 km$^{2}$ and a 1/6 th size fluorescence detector. Full operation of 3000 km$^{2}$ will happen 3-4 years later.