Photoproduction Of Electron-positron Pairs Research Articles

Strong-field Breit-Wheeler pair production with bremsstrahlung γ rays in the perturbative-to-nonperturbative-transition regime

Photoproduction of electron-positron pairs by bremsstrahlung gamma-rays propagating through a high-intensity optical laser wave is studied. By calculating total pair production yields as well as energy and angular distributions of the produced particles for a range of laser intensities, we analyze the influence that the broad frequency spectrum of bremsstrahlung exerts on the properties of the process in the transition region between the perturbative and non-perturbative interaction regimes. In particular, we demonstrate a continuous shifting of the mainly contributing interval of bremsstrahlung frequencies during the course of the transition and indicate characteristic signatures of the laser-dressed particle mass.

Resonant photoproduction of ultrarelativistic electron-positron pairs on a nucleus in moderate and strong monochromatic light fields

For complete development of quantum electrodynamics in the presence of a strong external field, the proper understanding of resonant processes and all their peculiarities is essential. We present our attempt to analytically investigate the resonant case of laser-assisted electron-positron pair photoproduction on a nucleus. Due to the presence of external field, the intermediate virtual particle may become real, herewith the second order process in the fine structure constant effectively reduces into the two successive first order processes. All inherent kinematics features were discussed in details and the resonant differential cross section was obtained. We established that the resonant energies of produced particles ambiguously depend on the positron (channel A) or electron (channel B) outgoing angle, and the certain minimal amount of absorbed wave photons are required for resonance to happen. Furthermore, the resonant cross section significantly exceeds the corresponding one in the absence of the external field within the particular kinematic regions and consequently, the considered process can be used qua a marker for probing theoretical predictions of quantum electrodynamics with strong background field.

Collisional strong-field QED kinetic equations from first principles

Starting from nonequilibrium quantum field theory on a closed time path, we derive kinetic equations for the strong-field regime of quantum electrodynamics (QED) using a systematic expansion in the gauge coupling $e$. The strong field regime is characterized by a large photon field of order $\mathcal{O}(1/e)$, which is relevant for the description of, e.g., intense laser fields, the initial stages of off-central heavy ion collisions, and condensed matter systems with net fermion number. The strong field enters the dynamical equations via both quantum Vlasov and collision terms, which we derive to order $\mathcal{O}(e^2)$. The kinetic equations feature generalized scattering amplitudes that have their own equation of motion in terms of the fermion spectral function. The description includes single photon emission, electron-positron pair photoproduction, vacuum (Schwinger) pair production, their inverse processes, medium effects and contributions from the field, which are not restricted to the so-called locally-constant crossed field approximation. This extends known kinetic equations commonly used in strong-field QED of intense laser fields. In particular, we derive an expression for the asymptotic fermion pair number that includes leading-order collisions and remains valid for strongly inhomogeneous fields. For the purpose of analytically highlighting limiting cases, we also consider plane-wave fields for which it is shown how to recover Furry-picture scattering amplitudes by further assuming negligible occupations. Known on-shell descriptions are recovered in the case of simply peaked ultrarelativistic fermion occupations. Collisional strong-field equations are necessary to describe the dynamics to thermal equilibrium starting from strong-field initial conditions.

Recoil momentum effects in quantum processes induced by twisted photons

We consider physical processes caused by the twisted photons for a wide range of energy scales, from optical (eV) to nuclear (MeV) to high-energy gamma-rays (TeV). We demonstrate that in order to satisfy angular momentum conservation, absorption of a twisted photon leads to a transverse recoil of the final particle or a system of particles leading to increased threshold energy required for the reaction to proceed. Modification of the threshold energy is predicted for (a) Photo-absorption on colds trapped ions of $^{40}$Ca, along with emerging new transverse-motion sidebands, (b) photo-disintegration of deuterium and (c) photo-production of electron-positron pairs in astrophysics environment.

Resonant Effects in a Photoproduction of Ultrarelativistic Electron-Positron Pairs on a Nucleus in the Field of the X-ray Pulsar

The resonant photoproduction of the electron-positron pairs on a nucleus near a surface of the X-ray pulsar was studied theoretically. The main feature of the processes, which are responsible for the formation of the electron-positron fluxes, is a capability to occur in a resonant way in the electromagnetic field of the X-ray pulsar. One of the properties of the resonant case is that the initial process of second order in the fine structure constant in an external field effectively reduces into two successive processes of the first order due to the fact that in the resonant conditions intermediate virtual electron (positron) becomes a real particle. It is shown that the resonances are possible only when the initial gamma quantum energy is more than the threshold energy, which significantly depends on the number of absorbed photons of an external electromagnetic field. Additionally, in the resonant conditions, the energies of the particles depend on the outgoing angle of a positron (channel A) or an electron (channel B). It is shown that the resonant differential cross section has an extremely large magnitude in units αZ2re2. A mechanism to explain the presence of anomalous fluxes of ultrarelativistic positrons near the surface of an X-ray pulsar was proposed.

Parametric interference effect in nonresonant pair photoproduction on a nucleus in the field of two pulsed light waves

Nonresonant electron-positron pair photoproduction on a nucleus in the field of two pulsed light waves is studied theoretically. The process is considered in detail within the interference kinematic region, when stimulated absorption and emission of photons of external pulsed waves by an electron and a positron occurs in a correlated manner. Within this region, a correspondence between the emission angle and energy of the produced particles appears. The distribution of the obtained differential cross section over the pair kinetic energy is characterized by presence of oscillations, within the interference region. Each of the maxima corresponds to the definite partial process with emission and absorption of an equal number of photons of both waves. It was shown that the differential cross section within the interference region for certain values of the pair energy may exceed the cross section in other scattering kinematics in two orders of the magnitude. Obtained results may be experimentally verified, for example, by scientific facilities at sources of pulsed laser radiation (SLAC, FAIR, ELI, XCELS).

Strong-field Breit–Wheeler pair production in two consecutive laser pulses with variable time delay

Photoproduction of electron-positron pairs by the strong-field Breit-Wheeler process in an intense laser field is studied. The laser field is assumed to consist of two consecutive short pulses, with a variable time delay in between. By numerical calculations within the framework of scalar quantum electrodynamics, we demonstrate that the time delay exerts a strong impact on the pair-creation probability. For the case when both pulses are identical, the effect is traced back to the relative quantum phase of the interfering S-matrix amplitudes and explained within a simplified analytical model. Conversely, when the two laser pulses differ from each other, the pair-creation probability depends not only on the time delay but, in general, also on the temporal order of the pulses.

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.

Resonant electron-positron pair photoproduction on a nucleus in a pulsed light field

The resonant photoproduction of an electron-positron pair on a nucleus in the field of a pulsed light wave is studied theoretically. The approximation where the electromagnetic pulse duration is much longer than the characteristic time of wave oscillations is considered. The interaction of the electron and positron with the Coulomb potential of the nucleus is considered in the Born approximation. An analytical expression for the resonant differential cross section is derived for the range of moderately strong external fields. This cross section contains a resonant peak whose height and width are determined by the external pulsed wave characteristics. The resonant cross section for pair photoproduction on a nucleus in a pulsed laser field can exceed the corresponding cross section for pair photoproduction on a nucleus in the absence of an external field by an order of magnitude.

Polarized inelastic electromagnetic processes in crystals with allowance for a new real distribution of nucleus charges and atomic electrons

The process of polarized electron-positron pair photoproduction via a polarized γ quantum in a crystalline medium is investigated using a new method that considers the distribution of nucleus charges and atomic electrons. Real changes in the cross sections of both polarized and nonpolarized electron pair production, and in the degrees of particle polarization in silicon crystal, are obtained.

Photoproduction of electron-positron pairs in bent single crystals

The process of photoproduction of electron-positron pairs in bent single crystals is considered in this paper. In particular, it is shown that the probability of the process for gamma-quanta with energies from 100 GeV on is significantly higher than the one in an amorphous medium. A possible scenario for the experimental validation of the process is discussed and the positive features of the photoproduction in bent crystals compared to straight ones are underlined from the point of view of possible applications.

Effect of electric field on photoproduction of electron-positron pairs

The cross section of electron-positron pair production by two circularly polarized photons in a static uniform electric field is calculated. The dependence of the cross section on the photon energy is determined. It is shown that the corrections to the cross section of the process involving photons of various polarizations are suppressed as compared to the situation with crossed fields.

TeV astrophysics constraints on Planck scale Lorentz violation

We analyze observational constraints from TeV astrophysics on Lorentz violating nonlinear dispersion for photons and electrons without assuming any a priori equality between the photon and electron parameters. The constraints arise from thresholds for vacuum Cerenkov radiation, photon decay and photo-production of electron-positron pairs. We show that the parameter plane for cubic momentum terms in the dispersion relations is constrained to an order unity region in Planck units. We find that the threshold configuration can occur with an asymmetric distribution of momentum for pair creation, and with a hard photon for vacuum Cerenkov radiation.

Experimental and theoretical features of hard-collimated type-B coherent high energy pair production

We have reexamined the conditions for the enhancement of electron–positron pair photoproduction cross-section to occur due to the type-B coherent effect. Simple formulae to characterize the position and the bandwidth of the enhancement for the hard collimated pairs are derived. We also examined the effect of interference among the atomic strings and found that, in some cases it is almost identical to a random phase approximation while in other cases, phenomena analogous to the extinction law in X-ray diffraction may occur.

Interference effect in the photoproduction of electron-positron pairs on a nucleus in the field of two light waves

In the general relativistic case, the nonresonance photoproduction of electron-positron pairs on a nucleus in the field of two circularly polarized light waves propagating in one direction is theoretically investigated. It is shown that there are two essentially different kinematical regions: the noninterference region, where the Bunkin-Fedorov quantum parameters play the role of multiphoton parameters, and the interference region, where interference effects are important and where quantum interference parameters are treated as multiphoton parameters. The interference effect is found in the photoproduction of an electron-positron pair on a nucleus in the field of two light waves. This effect occurs in the interference region and is due to the production of an electron-positron pair in the plane spanned by the light wave vector and the incident-photon momentum and to stimulated correlated emission and absorption of photons of the two waves. The cross sections for pair photoproduction on a nucleus in the above kinematical regions are determined for moderately strong fields. It is shown that, in the interference region, the partial cross sections for the case where there is a correlated emission (absorption) of the same number of photons of the two waves can significantly exceed the corresponding cross sections in any other geometry.

The diffraction mechanism of electron-positron pair photoproduction

We examine a mechanism of electron-positron pair photoproduction near a giant resonance. The mechanism is based on the possibility of photons being absorbed by nuclei. Our calculations set the cross section of this process at roughly 10−30 cm2. We also discuss the feasibility of observing the effect.

Brilliance of coherent peaks for type-B photoproduction of electron-positron pairs in a crystal

The shape and magnitude of coherent peaks are studied for the type-B coherent photoproduction of electron-positron pairs in a crystal, in the energy range of photons ω = 10 2–10 3 MeV. It is shown that under detection of pairs with restricted emission angles the coherent peaks (as a function of photon energy ω) become more brilliant, with larger value of relation of peak height to background, and lower width. The influence of channeling effect on the position of coherent maxima is estimated.

Modification of gamma-ray spectrum in a photon field

The changes in the high-energy gamma-ray spectra due to electron-photon cascading in the photon field (e.g., microwave background radiation) are investigated. The propagation of gamma-rays with an initial power-law spectrum through an isotropic monoenergetic photon field is considered. Two processes of the cascade particle-photon field interaction are taken into account, namely, the photoproduction of electron-positron pairs and the inverse Compton effect. The effect of the synchrotron energy loss by cascade electrons in a magnetic field on the cascade gamma-quantum spectrum is studied. The results may be used to estimate the absorption and the variations in the shape of the spectrum of radiation from compact gamma-ray sources in the metagalaxy. The cascade characteristics have been obtained by numerically solving the cascade equations by the moment method.

Resonance effects with the photoproduction of electron-positron pairs in the field of a plane electromagnetic wave

The differential cross section is obtained for the formation of electron-positron pairs by a photon at a Coulomb center in the field of a plane electromagnetic wave whose intensity η = eA′/m≪ 1 and frequency Ω′/m≪ 1. Expressions are given for the resonance frequencies of the photon for which the emission of a virtual electron (positron) at a mass shell occurs. It is shown that resonance occurs only for electron or positron energies E > m2/Ω′. The resonance differential cross section under interference conditions is obtained. It is shown that the resonance cross section can exceed by several orders of magnitude the ordinary cross section for the photoproduction of pairs without a field.

Photoproduction of electron-positron pairs in compact x-ray sources

Photoproduction of electron-positron pairs in compact x-ray sources