Probability of radiation of twisted photons in the infrared domain

Highlights: •Infrared asymptotics of twisted photon radiation in QED is studied. •General selection rules for the twisted photon radiation are proved. •Exact formulas for the twisted photon edge radiation are obtained. •Edge radiation is a superradiant source of twisted photons in the infrared domain. -- Abstract: The infrared asymptotics of the probability of radiation of twisted photons in an arbitrary scattering process of quantum electrodynamics in vacuum is investigated. This asymptotics is universal and corresponds to the radiation produced by a classical current. Such radiation is known as edge radiation. We represent it in terms of twisted photons, and 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 edge radiation can be used as a source of twisted photons with a large angular momentum. Moreover, this radiation can be employed as a superradiant coherent source of twisted photons in the infrared domain, in particular, the THz part of the electromagnetic spectrum. Several general selection rules for the radiation and absorption of twisted photons are proved. In particular, these selection rules allow one to modulate the one-particle radiation probability by means of scattering of charged particles on symmetrically arranged crystals.