The Pierre Auger Observatory (Auger) measures the ultra high energy cosmic ray flux and arrival directions indirectly by detecting the showers of charged particles that cosmic rays generate in the earth and apos;s atmosphere. These air showers are detected using two different techniques: air fluorescence detectors, like the High Resolution Fly and apos;s Eye experiment (HiRes), observe the light from excited nitrogen molecules in the path of the air shower; huge arrays of surface detectors, similar to the A GASA experiment, detect the shower and apos;s particle profile at ground level. The Auger experiment can combine both detection methods in hybrid detection mode. This is an important advantage over previous experiments, especially in light of the ongoing d iscussion about the existence or non-existence of the GZK flux suppression: The cosmic ray flux is expected to decline sharply at an energy around 60 EeV due to photopion production in the cosmic microwave background radiation. The HiRes experiment has obs erved this effect, whereas AGASA sees an unchanged continuation of the cosmic ray flux. This discrepancy poses a serious problem, since, in the absence of nearby sources for the and quot;super GZK and quot; cosmic rays, the non-existence of the GZK effect would require the introduction of new physics models.We intend to develop the necessary reconstruction and Monte Carlo simulation programs to study the cosmic ray data observed by Auger in hybrid mode. We will directly compare the reconstructed shower geo metries and energies that the two different methods yield for these cosmic rays and search for systematic differences between the air fluorescence and the surface detector technique. A systematic shift in reconstructed energy between the two methods for in stance could explain the discrepancies in the HiRes and AGASA spectra. In this study, sophisticated air shower and detector simulation programs are needed to test the reconstruction of events and our understanding of the underlying physics in general.
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