Hadron structure of high-energy photons

We identify a new class of potentially large inclusive cross sections for the production of hadrons when high-energy photons interact with protons. Their origin is the abundant gluon content of very-high-energy photons. They can enhance the hadron (and also muon) content of high-energy electromagnetic showers as indicated by cosmic-ray observations and most dramatically illustrated by our failure to understand the muon content of photon showers from point sources. Experiments at the DESY ep collider HERA are in an ideal position to identify these contributions to photoproduction