LPM effect and pair production in the geomagnetic field: a signature of ultra-high energy photons in the Pierre Auger Observatory

The presence of photons in the cosmic rays of highest energy (above 10^20 eV) would be a strong indication of a "top down" production mechanism, e.g. the decay of topological defects. The Pierre Auger Observatory is aimed lo detect cosmic rays in this energy range with a large statistics. The pair production in the magnetic field of the Earth, followed by hard synchrotron radiation, generates early cascades containing mainly photons with energies below the LPM threshold (a few 10^19 eV) while unconverted photons above this energy produce in the atmosphere slowly developing showers. We describe some observables related lo the fluorescence profile and lo the ground signals (lateral distribution and front curvature), which can provide a clear signature of abnormally delayed showers, even if they represent a small fraction of the flux. Moreover, the magnetic conversion exhibit a characteristic anisotropy with respect to the local direction of the field, allowing an unambiguous confirmation of photons, against the hypothesis of weakly interacting objects.