Energy dependence for pair production with electron capture in relativistic heavy-ion collisions

In 'Charge transfer from the negative-energy continuum: alternative mechanism for pair production in relativistic atomic collisions', Eichler (1995 Phys. Rev. Lett. 75 3653) proposes an alternative mechanism for capture by pair production, and from it derives an analytic expression for the total cross section with a surprisingly strong energy dependence. We show that, in fact, there is no alternative mechanism; rather the above mechanism may be more transparently viewed as an ionization-like transition in one centre with inclusion of continuum distortion by the second centre. We further show that to Centre the initial and final states on the target and projectile leads to confusion in the momentum transfer vectors, and hence, respectively that the alleged high-energy behaviour is erroneous.

Pair production and electron capture in relativistic heavy-ion collisions

Results are presented for simulations of electron-positron pair production in relativistic heavy-ion collisions leading to electron capture and positron ejection. We apply a two-center relativistic continuum distorted-wave model to represent the electron or positron dynamics during the collision process. The results are compared with experimental cross-section data for La57+ and Au79+ impact on gold, silver, and copper targets. The theory is in good agreement with experiment for La57+ impact, verifying the result that the process increases in importance with both collision energy and target atomic number, and improves upon previous simulations of this process.

Generation of overdense and high-energy electron-positron-pair plasmas by irradiation of a thin foil with two ultraintense lasers

A scheme for enhanced quantum electrodynamics (QED) production of electron-positron-pair plasmas is proposed that uses two ultraintense lasers irradiating a thin solid foil from opposite sides. In the scheme, under a proper matching condition, in addition to the skin-depth emission of gamma-ray photons and Breit-Wheeler creation of pairs on each side of the foil, a large number of high-energy electrons and photons from one side can propagate through it and interact with the laser on the other side, leading to much enhanced gamma-ray emission and pair production. More importantly, the created pairs can be collected later and confined to the center by opposite laser radiation pressures when the foil becomes transparent, resulting in the formation of unprecedentedly overdense and high-energy pair plasmas. Two-dimensional QED particle-in-cell simulations show that electron-positron-pair plasmas with overcritical density 10(22) cm(-3) and a high energy of 100s of MeV are obtained with 10 PW lasers at intensities 10(23) W/cm(2), which are of key significance for laboratory astrophysics studies.

COMPUTATION OF SYNTHETIC SPECTRA FROM SIMULATIONS OF RELATIVISTIC SHOCKS

Particle-in-cell (PIC) simulations of relativistic shocks are in principle capable of predicting the spectra of photons that are radiated incoherently by the accelerated particles. The most direct method evaluates the spectrum using the fields given by the Lienard-Wiechart potentials. However, for relativistic particles this procedure is computationally expensive. Here we present an alternative method that uses the concept of the photon formation length. The algorithm is suitable for evaluating spectra both from particles moving in a specific realization of a turbulent electromagnetic field or from trajectories given as a finite, discrete time series by a PIC simulation. The main advantage of the method is that it identifies the intrinsic spectral features and filters out those that are artifacts of the limited time resolution and finite duration of input trajectories.

Testing numerical implementations of strong-field electrodynamics

We test current numerical implementations of laser-matter interactions by comparison with exact analytical results. Focusing on photon emission processes, it is found that the numerics accurately reproduce analytical emission spectra in all considered regimes, except for the harmonic structures often singled out as the most significant high-intensity (multiphoton) effects. We find that this discrepancy originates in the use of the locally constant field approximation.

A table-top laser-based source of short, collimated, ultra-relativistic positron beams

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-wake field 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.