Probability of radiation of twisted photons in the infrared domain

The infrared asymptotics of probability of radiation of twisted photons in an arbitrary scattering process of quantum electrodynamics (QED) in a vacuum is investigated. This asymptotics is universal and corresponds to the radiation produced by a classical current. Such a radiation is known as the edge radiation. We represent it in terms of the twisted photons: the exact analytical formulas for the average number of radiated twisted photons are derived. We find the average projection of the total angular momentum of the edge radiation and the angular momentum per photon. It is shown that the edge radiation can be used as a source of twisted photons with large angular momentum. Moreover, this radiation can be employed as a superradiant coherent source of twisted photons in the infrared domain, in particular, in the THz part of the electromagnetic spectrum. Several general selection rules for the radiation and absorbtion of twisted photons are proved. These selection rules allow one, in particular, to modulate the one-particle radiation probability by means of scattering of charged particles on symmetrically arranged crystals.