Ultrahigh-energy photonuclear cross sections

We present results of calculations of the total inelastic photon-air cross sections at ultrahigh energies (up to ${10}^8$ GeV in the laboratory) of relevance to on-going cosmic-ray experiments. The calculations take into account the high-energy QCD structure of the photon and are performed for a variety of photon and proton structure functions. The total inelastic photon-air cross section is obtained from the photon-proton jet cross section via an eikonalization procedure using a QCD-based diffractive model. The results are discussed in the context of the apparent excess muon content of air showers associated with point sources.