This contribution describes two recent measurements of isolated photons appearing in photoproduction (measured by ZEUS) and deep inelastic scattering (measured by H1) at the ep collider HERA. 1. Introduction Isolated photons with high transverse momentum in the final state are a sensitive probe of the dynamics of the hard sub-process, since they are largely insensitive to hadronisation and frag...

The process of electron-positron pair production by linearly polarized photons is used as a polarimeter to perform mobile measurement of linear photon polarization. In the limit of high photon energies, ω, the distributions of the recoil-electron momentum and azimuthal angle do not depend on the photon energy in the laboratory frame. We calculate the power corrections of order m/ω to the above ...

The description of longitudinal photons is far from trivial, and their phenomenological importance is largely unknown. While the cross section for direct interactions is calculable, an even more important contribution could come from resolved states. In the development of our model for the interactions of (real and) virtual photons, we have modeled resolved longitudinal effects by simple multip...

After a brief review of the production dynamics of prompt single photons in hadron collisions, I summarize a new QCD calculation of the transverse momentum distribution of continuum prompt photon pairs produced by QCD subprocesses, including all-orders soft-gluon resummation valid at next-to-next-to-leading logarithmic accuracy. Resummation is necessary to obtain reliable predictions, as well a...

The purpose of this Letter is to prove that, contrary to a widespread belief, photons can be localized in space with an exponential falloff of the photon energy density and of the photodetection rates. In that respect, photons are not much different from massive particles. The only constraint on the photon localizability comes from the fundamental Paley-Wiener theorem [1]. The mathematical mech...

This Letter applies the concept of "jets," as constructed from calorimeter cell four-vectors, to jets composed (primarily) of photons (or leptons). Thus jets become a superset of both traditional objects such as QCD jets, photons, and electrons, and more unconventional objects such as photon jets and electron jets, defined as collinear photons and electrons, respectively. Since standard objects...

The physics of kinetic mixing between ordinary and mirror photons is elucidated. An important role is played by four linear combinations we dub the physical photon, the sterile photon, the physical mirror photon, and the sterile mirror photon. Because of the mass degeneracy between the two gauge bosons, quantum coherence effects are important. The physical photon becomes a certain coherent supe...