Electron-positron Photoproduction Research Articles

Charge asymmetry in the spectra of bremsstrahlung and pair production

We calculate the first Coulomb correction to the spectra of two processes: the electron bremsstrahlung and electron-positron photoproduction in the Coulomb field. We show that, in contrast to the results obtained in the Born approximation and in the high-energy limit, the obtained corrections for these two process are not related by the crossing symmetry substitutions. The found corrections determine the leading contribution to the charge asymmetry in these processes. We use modern multiloop methods based on the IBP reduction and on the differential equations for master integrals. The results are presented in terms of classical polylogarithms. We provide both the threshold and the high-energy asymptotics of the obtained expressions and compare them with available results.

Semiclassical picture for electron-positron photoproduction in strong laser fields

The nonlinear Breit-Wheeler process is studied in the presence of strong and short laser pulses. We show that for a relativistically intense plane-wave laser field many features of the momentum distribution of the produced electron-positron pair like its extension, region of highest probability and carrier-envelope phase effects can be explained from the classical evolution of the created particles in the background field. To this end an intuitive semiclassical picture based on the local constant-crossed field approximation applied on the probability-amplitude level is established and compared with the standard approach used in QED-PIC codes. The main difference is the substructure of the spectrum, which results from interference effects between macroscopically separated formation regions. In order to compare the predictions of the semiclassical approach with exact calculations, a very fast numerical scheme is introduced. It renders the calculation of the fully differential spectrum on a grid which resolves all interference fringes feasible. Finally, the difference between classical and quantum absorption of laser four-momentum in the process is pointed out and the dominance of the former is proven. As a self-consistent treatment of the quantum absorption is not feasible within existing QED-PIC approaches, our results provide reliable error estimates relevant for regimes where the laser depletion due to a developing QED cascade becomes significant.

Photon emission and electron-positron photoproduction processes in the planar field of a bent single crystal

The process of photon emission by a high-energy electron or positron moving in a bent single crystal with a constant curvature is considered. The relations for differential energy losses of particles and polarization of emitted photons are obtained. Corrections due to multiphoton production are found. The comparison of calculations with existing experimental data is carried out. The process of photoproduction of electron-positron pairs in a bent single crystal is also studied. The differential probabilities of the process taking into account the photon polarization are presented. Equations are obtained which determine the variation of the Stokes parameters of $\ensuremath{\gamma}$ quanta, and their solutions are given.

Quasiclassical approach to high-energy QED processes in strong laser and atomic fields

An approach, based on the use of the quasiclassical Greenʼs function, is developed for investigating high-energy quantum electrodynamical processes in combined strong laser and atomic fields. Employing an operator technique, we derive the Greenʼs function of the Dirac equation in an arbitrary plane wave and a localized potential. Then, we calculate the total cross section of high-energy electron–positron photoproduction in an atomic field of arbitrary charge number (Bethe–Heitler process) in the presence of a strong laser field. It is shown that the laser field substantially modifies the cross section at already available incoming photon energies and laser parameters. This makes it feasible in principle to observe with present technology the analogous effect in a laser field of the Landau–Pomeranchuk–Migdal effect for the Bethe–Heitler process.

Charge asymmetry in the differential cross section of high-energye+e−photoproduction in the field of a heavy atom

First quasiclassical correction to the differential cross section of high-energy electron-positron photoproduction in the electric field of a heavy atom is obtained with the exact account of the field. This correction is responsible for the charge asymmetry ${\cal A}$ in this process. When the transverse momentum of at least one of the produced particles is much larger than the electron mass $m$, the charge asymmetry can be as large as tens percent. We also estimate the contribution $\tilde{\cal A}$ to the charge asymmetry coming from the Compton-type diagram. For heavy nuclei, this contribution is negligible. For light nuclei, $\tilde{\cal A}$ is noticeable only when the angle between the momenta of electron and positron is of order of $m/\omega$ ($\omega$ is the photon energy) while the transverse momenta of both particles are much larger than $m$.

Effect of a strong laser field on electron-positron photoproduction by relativistic nuclei

We study the influence of a strong laser field on the Bethe-Heitler photoproduction process by a relativistic nucleus. The laser field propagates in the same direction as the incoming high-energy photon and it is taken into account exactly in the calculations. Two cases are considered in detail. In the first case, the energy of the incoming photon in the nucleus rest frame is much larger than the electron's rest energy. The presence of the laser field may significantly suppress the photoproduction rate at soon available values of laser parameters. In the second case, the energy of the incoming photon in the rest frame of the nucleus is less than and close to the electron-positron pair production threshold. The presence of the laser field allows for the pair production process and the obtained electron-positron rate is much larger than in the presence of only the laser and the nuclear field. In both cases we have observed a strong dependence of the rate on the mutual polarization of the laser field and of the high-energy photon and the most favorable configuration is with laser field and high-energy photon linearly polarized in the same direction. The effects discussed are in principle measurable with presently available proton accelerators and laser systems.

Radiative effects in the standard model extension

The possibility of radiative effects induced by the Lorentz and CPT non-invariant interaction term for fermions in the Standard Model Extension is investigated. In particular, electron-positron photo-production and photon emission by electrons and positrons are studied. The rates of these processes are calculated in the Furry picture. It is demonstrated that the rates obtained in the framework of the model adopted strongly depend on the polarization states of the particles involved. As a result, ultra-relativistic particles produced should occupy states with a preferred spin orientation, i.e., photons have the sign of polarization opposite to the sign of the effective potential, while charged particle are preferably in the state with the helicity coinciding with the sign of the effective potential. This leads to evident spatial asymmetries which may have certain consequences observable at high energy accelerators, and in astrophysical and cosmological studies.

Enhanced type-B coherent effect in collimated electron–positron photoproduction from Si crystal

The relative cross section of pair creation from a silicon single crystal has been measured with a bremsstrahlung beam from 350 MeV electrons. A clear enhancement was observed above 230 MeV as expected from the theory based on the type-B coherent effect.