In the presence of an electromagnetic background plane-wave field, electron, positron, and photon
states are not stable, because electrons and positrons emit photons and photons decay into electronpositron pairs. This decay of the particle states leads to an exponential damping term in the
probabilities of single nonlinear Compton scattering and nonlinear Breit-Wheeler pair production.
We present analytical and numerical investigations for the probabilities of nonlinear Compton
scattering and nonlinear Breit-Wheeler pair production including the particle states decay.
The local-constant-field approximation (LCFA) is an essential theoretical
tool for investigating strong-field QED phenomena in background electromagnetic
fields with complex spacetime structure. In [1] we have analyzed the shortcomings
of the LCFA in nonlinear Compton scattering for the case of a background plane-wave
field. Here, we generalize that analysis to background fields, which can feature a
virtually arbitrary spacetime structure [2]. In addition, we provide an explicit and simple
implementation of an improved expression of the nonlinear Compton scattering
differential probability that solves the main shortcomings of the standard LCFA, and
is suitable for background electromagnetic fields with arbitrary spacetime structure
such as those of particle-in-cell (PIC) codes. Finally, we carry out a systematic procedure
to calculate the probability of nonlinear Compton scattering per unit of emitted photon
light-cone energy and of nonlinear Breit-Wheeler pair production per unit of produced
positron light-cone energy beyond the LCFA in a plane-wave background field, which
allows us to identify the limits of validity of this approximation quantitatively.
[1] A. Di Piazza, M. Tamburini, S. Meuren, and C. H. Keitel, Phys. Rev. A vol. 98, 012134 (2018)
[2] A. Di Piazza, M. Tamburini, S. Meuren, and C. H. Keitel, arXiv:1811.05834
An in-depth analysis of two recent studies on nonlinear QED effects relevant to large-scale numerical simulations of laser-plasma interactions is presented. We demonstrate how accounting for a plasma as a background different from vacuum affects fundamental effects of nonlinear QED, quantify hitherto neglected coherences and propose an improved approach to better account for such effects in simulations. In particular, we show how the background plasma can be included in the calculation of nonlinear QED amplitudes on the example of the emission of a single high-energy photon by a laser-driven electron with the laser experiencing a non-trivial dispersion relation due to its propagation through a background plasma. Second, we discuss the failure of the so-called local-constant- field approximation, which is employed in all state-of-the-art numerical codes implementing QED effects both in nonlinear Compton scattering and in laser-assisted pair production. Finally, we show how in laser-assisted pair production by a high-energy photon emitted from a laser-driven electron the usually employed incoherent assumption, i.e., that the two QED processes of photon emission and pair production occur at separate points in space-time, can become invalid.
Compact γ-ray sources are of key importance not only for fundamental research but also for paramount practical applications such as cancer radiotherapy, active interrogation of materials, and high-energy radiography. Particular characteristics are required for meaningful implementation: multi-MeV energies per photon, a high degree of collimation, and a high peak brilliance. Laser-driven sources are theoretically expected to deliver such capabilities but experiments to date have reported either sub-MeV photon energies, or relatively low brilliance. By entering the non-linear regime of Thomson scattering, we report here on the first experimental realisation of a compact laser-driven γ-ray source that simultaneously ensures ultra-high brilliance (≈1019 photons s-1 mm-2 mrad-2 0.1% BW), low divergence (≈ mrad), and high photon energy (up to 18 MeV). The reported brilliance exceeds by two orders of magnitudes those of alternative mechanisms and it is the highest ever achieved in the multi-MeV regime in a laboratory experiment.
An ultra-relativistic electron beam passing through a thick, high-Z solid target triggers an electromagnetic cascade, whereby a large number of high energy photons and electron-positron pairs are produced. By exploiting this physical process, we present here the first experimental evidence of the generation of ultra-short, highly collimated and ultra-relativistic positron beams following the interaction of a laser-wakefield accelerated electron beam with high-Z solid targets. Clear evidence has also been obtained of the generation of GeV electron-positron jets with variable composition depending on the solid target material and thickness. The percentage of positrons in the overall leptonic beam has been observed to vary from a few per cent up to almost fifty per cent, implying a quasi-neutral electron-positron beam. We anticipate that these beams will be of direct relevance to the laboratory study of astrophysical leptonic jets and their interaction with the interstellar medium.
We show that a strong laser pulse combined with a strong x-ray pulse can be employed in a detection scheme
for characterizing high-energy γ-ray pulses down to the zeptosecond timescale. The scheme employs streak
imaging technique built upon the high-energy process of electron-positron pair production in vacuum through
the collision of a test pulse with intense laser pulses. The role of quantum radiation reaction in multiphoton
Compton scattering process and limitations imposed by it on the detection scheme are examined.
The possibility of employing strong optical and x-ray laser fields to investigate processes in the realm of classical
and quantum electrodynamics as well as nuclear quantum optics is considered. In the first part we show on
the theoretical side how modern strong optical laser fields can be employed to test the fundamental classical
equations of motion of the electron which include radiation reaction, i.e., the effect of the radiation emitted
by the electron on its own motion. Then, we clarify the quantum origin of radiation reaction and discuss a
new radiation regime where both quantum and radiation effects dominate the electron dynamics. The second
part is dedicated to the possibility of controlling nuclear transitions with coherent x-ray light. In particular, we
investigate the resonant driving of nuclear transitions by super-intense x-ray laser fields considering parameters
of upcoming high-frequency coherent light sources. As relevant application, the controlled pumping or release of
energy stored in long-lived nuclear states is discussed.
Quantum electrodynamics (QED) in strong laser fields is a vivid area of research, which will soon become systematically
testable experimentally. We present analyses of three aspects of strong-field QED: First we show that the precise shape of
ultra-intense few-cycle laser pulses is determinable via nonlinear Compton scattering and second that due to photon-photon
interactions mediated by the quantum vacuum it is possible to set up a double slit experiment comprising only light. Finally
we consider nonperturbative electron-positron pair creation.
The interaction of ultra-high intensity laser pulses with solid targets is studied theoretically and with Particle-
In-Cell (PIC) simulations. The regime of Radiation Pressure Acceleration of ultrathin foil targets is investigated
within an improved "Light Sail" or "accelerating mirror" model. The latter provides simple and useful scalings
for the characteristics of accelerated ions. The underlying dynamics, unfolded by PIC simulations, is however
more complex than the simple model may suggest. An important issue is the heating of electrons that, even if
strongly reduced by the use of circulary polarized (CP) pulses, may lead to a significant broadening of the ion
spectrum. Radiation Reaction (RR) effects in the ultra-relativistic regime of extreme intensities are included
in the PIC simulations via the Landau-Lifshitz formula. Apparently, for linearly polarized pulses RR slightly
reduces the ion energy but also contributes to cooling the electrons, while RR effects are rather weak for CP
pulses.
Different physical processes using strong laser fields with intensities likely available in the near future are studied.
We focus on the possibility of probing experimentally the nonlinear properties of the quantum vacuum that arise
due to the existence of the so-called "quantum vacuum fluctuations", as predicted by quantum electrodynamics
(QED). In particular, we consider the laser-assisted photon-photon scattering process and the diffractive effects
arising during the interaction between an x-ray probe and a strong, focused optical standing wave. Also, the
enhancement of vacuum polarization effects due to the presence of a cold relativistic plasma is pointed out.
Finally, direct nuclear excitation by an intense, high-frequency laser field is studied.
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