Coherent ρ0 Photoproduction in Bulk Matter at High Energies

The momentum transfer Δk required for a photon to scatter from a target and emerge as a ρ0 decreases as the photon energy k rises. For k>3x1014 eV, Δk is small enough that the interaction cannot be localized to a single nucleus. At still higher energies, photons may coherently scatter elastically from bulk matter and emerge as a ρ0, in a manner akin to kaon regeneration. Constructive interference from the different nuclei coherently raises the cross section and the interaction probability rises linearly with energy. At energies above 1023 eV, coherent conversion is the dominant process; photons interact predominantly as ρ0. We compute the coherent scattering probabilities in slabs of lead, water, and rock, and discuss the implications of the increased hadronic interaction probabilities for photons on ultrahigh energy shower development.