Positron Pair Creation Research Articles

Time domain optimization of pair production during vacuum breakdown triggered by frequency chirped external fields

The electron–positron pair creation process during the Dirac vacuum breakdown triggered by strong external fields is analyzed. A frequency chirped, subcritical force field can significantly enhance the pair production yield under the identical total energy. This enhancement effect is caused by different chirp frequencies. We pay special attention to optimal field conditions as chirping parameter and phase of the time dependent field are varied. And the total number of created electrons with frequency weighted average under different chirp parameters are different from the theoretically calculated values. And no matter what the main mechanism is, the laser has the highest energy utilization rate when the electric field is symmetrical.

Production of Positrons by Cosmic Rays

It is shown that electromagnetic interactions of cosmic ray protons provide a noticeable contribution to positrons production. This is due to a combination of low energy threshold of electron–positron pair creation compared to the thresholds of pion creation in strong interactions and rapid decrease of the cosmic rays energy spectrum. Moreover, the electromagnetically produced positrons are very soft, therefore their annihilation with background electrons directly produces the observed 511 keV gamma-line.

Real-Time Observation of Instantaneous ac Stark Shift of a Vacuum Using a Zeptosecond Laser Pulse

Based on the numerical solution of the time-dependent Dirac equation, we propose a method to observe in real time the ac Stark shift of a vacuum driven by an ultra-intense laser field. By overlapping the ultra-intense pump pulse with another zeptosecond probe pulse whose photon energy is smaller than 2mc 2, electron–positron pair creation can be controlled by tuning the time delay between the pump and probe pulses. Since the pair creation rate depends sensitively on the instantaneous vacuum potential, one can reconstruct the ac Stark shift of the vacuum potential according to the time-delay-dependent pair creation rate.

Electron–positron pair creation from the vacuum by the Feshbach resonance

Electron–positron pair (EPP) creation under Gaussian and super-Gaussian potential wells is studied using the computational quantum field theory. We find that the EPP creation rate decreases, while the positron spectra have better monochromaticity when the potential well is wider and gentler. The phenomenon is explained by the Feshbach resonance using the complex scaling method. The width of the Feshbach resonance is narrower in the wider and gentler potential well, and the narrower Feshbach resonance can lead to a lower creation rate and better energy monochromaticity. This study indicates that the width of the Feshbach resonance plays an important role in the EPP creation, and the Gaussian-type potential well has an advantage in tuning the Feshbach resonance width.

Quantum vacuum processes in the extremely intense light of relativistic plasma mirror sources

The advent of petawatt-class laser systems allows generating electromagnetic fields of unprecedented strength in a controlled environment, driving increasingly more efforts to probe yet unobserved processes through their interaction with the quantum vacuum. Still, the lowest intensity scale governing these effects lies orders of magnitude beyond foreseen capabilities, so that such endeavor is expected to remain extremely challenging. In recent years, however, plasma mirrors have emerged as a promising bridge across this gap, by enabling the conversion of intense infrared laser pulses into coherently focused Doppler harmonic beams lying in the X-UV range. In this work, we present predictions on the quantum vacuum signatures produced when such beams are focused to intensities between 1024 and 1028 W cm−2, specifically photon–photon scattering and electron–positron pair creation. These signatures are computed via the stimulated vacuum formalism, combined with a model of perfectly focused beam built from PIC-generated harmonics spectra, and implemented on state-of-the-art massively parallel numerical tools. In view of identifying experimentally favorable configurations, we also consider the coupling of the focused harmonic beam with an auxiliary optical beam, and provide comparison with other established schemes. Our results show that a single coherently focused harmonic beam can produce as much scattered photons as two infrared pulses in head-on collision, and confirm that the coupling of the harmonic beam to an auxiliary beam gives rise to significant levels of inelastic scattering, and hence holds the potential to strongly improve the attainable signal to noise ratios in experiments.

A single-laser scheme for observation of linear Breit–Wheeler electron–positron pair creation

We show that a single laser pulse, traveling through a dense plasma, produces a population of MeV photons of sufficient density to generate a large number of electron–positron pairs via the linear Breit–Wheeler process. While it may be expected that the photons are emitted predominantly in the forward direction, parallel to the laser propagation, we find that a longitudinal plasma electric field drives the emission of photons in the backwards direction. This enables the collision of oppositely directed, MeV-level photons necessary to overcome the mass threshold for the linear Breit–Wheeler process. Our calculations predict the production of 107 electron–positron pairs, per shot, by a laser with peak intensity of just 3 × 1022 W cm−2. By using only a single laser pulse, the scheme sidesteps the practical difficulties associated with the multiple-laser schemes previously investigated.

Nonlinear Axion Electrodynamics: Axionically Induced Electric Flares in the Early Magnetized Universe

We consider the nonlinearly extended Einstein–Maxwell-axion theory, which is based on the account for two symmetries: first, the discrete symmetry associated with the properties of the axion field, and second, the Jackson’s symmetry, prescribing to the electrodynamics to be invariant with respect to the rotation in the plane coordinated by the electric and magnetic fields. We derive the master equations of the nonlinearly extended theory and apply them to the Bianchi-I model with magnetic field. The main result, describing the behavior of the nonlinearly coupled axion, electromagnetic, and gravitational fields is the anomalous growth of the axionically induced electric field in the early magnetized Universe. The character of behavior of this anomalous electric field can be indicated by the term flare. We expect, that these electric flares can produce the electron–positron pair creation, significant acceleration of the born charged particles, and the emission of the electromagnetic waves by these accelerated particles.

Pair Drizzle around Sub-Eddington Supermassive Black Holes

Abstract Electron–positron pair creation near sub-Eddington accretion rate black holes is believed to be dominated by the Breit–Wheeler process (photon–photon collisions). The interacting high-energy photons are produced when unscreened electric fields accelerate leptons either in coherent, macroscopic gaps, or in incoherent structures embedded in the turbulent plasma flow. The latter type of acceleration results in a drizzle of pair production sourced by photons from the background radiation field whose energies are near the pair-production threshold. In this work, we use radiation general relativistic magnetohydrodynamic simulations to extend an earlier study of pair drizzle by Mościbrodzka et al. We focus on low-magnetization (standard and normal evolution) accretion onto supermassive Kerr black holes and consider radiation due to synchrotron, bremsstrahlung, and Compton upscattering processes. We confirm that pair drizzle in M87 is sufficient to keep the magnetospheric charge density orders of magnitude above the Goldreich–Julian density. We also find that pair production peaks along the jet–disk boundary.

Gamma photons and electron-positron pairs from ultra-intense laser-matter interaction: A comparative study of proposed configurations

High-energy γ-photon generation via nonlinear Compton scattering and electron–positron pair creation via the Breit–Wheeler process driven by laser–plasma interaction are modeled, and a number of mechanisms are proposed. Owing to the small cross section, these processes require both an ultra-intense laser field and a relativistic electron bunch. The extreme conditions for such scenarios can be achieved through recent developments in laser technology. Photon emission via nonlinear Thomson and Compton scattering has been observed experimentally. High-energy positron beams generated via a multiphoton process have recently been observed too. This paper reviews the principles of γ-ray emission and e+e− pair creation in the context of laser–plasma interaction. Several proposed experimental setups for γ-ray emission and e+e− pair creation by ultra-intense laser pulses are compared in terms of their efficiency and the quality of the γ-photon and positron beams produced for ultrashort (15 fs) and longer (150 fs) multi-petawatt laser beams.

Reaching supercritical field strengths with intense lasers

It is conjectured that all perturbative approaches to quantum electrodynamics (QED) break down in the collision of a high-energy electron beam with an intense laser, when the laser fields are boosted to ‘supercritical’ strengths far greater than the critical field of QED. As field strengths increase toward this regime, cascades of photon emission and electron–positron pair creation are expected, as well as the onset of substantial radiative corrections. Here we identify the important role played by the collision angle in mitigating energy losses to photon emission that would otherwise prevent the electrons reaching the supercritical regime. We show that a collision between an electron beam with energy in the tens of GeV and a laser pulse of intensity at oblique, or even normal, incidence is a viable platform for studying the breakdown of perturbative strong-field QED. Our results have implications for the design of near-term experiments as they predict that certain quantum effects are enhanced at oblique incidence.

Relationship of the pair creation yield during and after the interaction

We compare the predictions of two different approaches of studying the electron–positron pair creation process triggered by a supercritical field inside the interaction region before the field is turned off. These two approaches are based on the projection of the electron–positron field operator on the sub-space spanned by either field-free or instantaneous energy eigenstates. For the case of an infinitely extended static electric field, we suggest that the type of the chosen turn-off of the external field determines which of the two approaches is physically more meaningful. We also derive an alternative quantum kinetic Vlasov equation, which is more meaningful than the traditional one to predict the interaction if the field is turned off sufficiently rapidly.

Minimum requirements for electron–positron pair creation in the interaction of ultra-short laser pulses with thin foils

Current laser technology allows the production of extremely high laser intensities and brings ever closer experimental probing of quantum electro-dynamic effects e.g. radiation reaction and electron–positron pair creation via the multiphoton Breit–Wheeler process. The exponential dependence of the latter process on laser intensity means that the process appears suddenly above some threshold, which is still not well defined in the case of laser-plasma interactions. The threshold intensity for the generation of a significant number of positrons is shown to be in the order 1022 W cm−2, when optimal target properties, as presented in this paper, are considered. With the help of a modified particle-in-cell code, the detailed angular-energy distribution of positrons is presented, which is in good agreement with our simple analytical model.

Electron–Positron Pair Creation Close to a Black Hole Horizon: Redshifted Annihilation Line in the Emergent X-Ray Spectra of a Black Hole. I.

Abstract We consider a Compton cloud (CC) surrounding a black hole (BH) in an accreting BH system, where electrons propagate with thermal and bulk velocities. In that cloud, soft (disk) photons may be upscattered off these energetic electrons and attain energies of several MeV. They could then create pairs due to photon–photon interactions. In this paper, we study the formation of the 511 keV annihilation line due to this photon–photon interaction, which results in the creation of electron–positron pairs, followed by the annihilation of the created positrons with the CC electrons. The appropriate conditions for annihilation-line generation take place very close to a BH horizon within (103–104)m cm from it, where m is the BH hole mass in solar units. As a result, the created annihilation line should be seen by the Earth observer as a blackbody bump, or the so-called reflection bump at energies around (511/20) (20/z) keV, where z ∼ 20 is a typical gravitational redshift experienced by the created annihilation-line photons when they emerge. This transient feature should occur in any accreting BH system, either galactic or extragalactic. Observational evidences for this feature in several galactic BH systems is detailed in an accompanying paper. An extended hard tail of the spectrum up to 1 MeV may also be formed due to X-ray photons upscattering off created pairs.

On Electron–Positron Pair Production by a Spatially Inhomogeneous Electric Field

A detailed analysis of electron–positron pair creation induced by a spatially non-uniform and static electric field from vacuum is presented. A typical example is provided by the Sauter potential. For this potential, we derive the analytic expressions for vacuum decay and pair production rate accounted for the entire range of spatial variations. In the limit of a sharp step, we recover the divergent result due to the singular electric field at the origin. The limit of a constant field reproduces the classical result of Euler, Heisenberg and Schwinger, if the latter is properly averaged over the width of a spatial variation. The pair production by the Sauter potential is described for different regimes from weak to strong fields. For all these regimes, the locally constant-field rate is shown to be the upper limit.

Electron–positron pair creation by counterpropagating laser pulses: role of carrier envelope phase

The effect of carrier envelope phase (CEP) on the spatio-temporal distribution of the electron-positron pairs created by untraintense counterpropagating femtosecond laser pulses is studied. When the laser pulses are linearly polarized, the temporal distribution of the pairs is found to be sensitive to CEP. On the hand, it is found to be largely insensitive to CEP for circularly polarized pulses.

Radiative Magnetic Reconnection Near Accreting Black Holes

Abstract A radiative mechanism is proposed for magnetic flares near luminous accreting black holes. It is based on recent first-principle simulations of magnetic reconnection, which show a hierarchical chain of fast-moving plasmoids. The reconnection occurs in a compact region (comparable to the black hole radius), and the chain experiences fast Compton cooling accompanied by electron–positron pair creation. The distribution of plasmoid speeds is shaped by radiative losses, and the self-regulated chain radiates its energy in hard X-rays. The mechanism is illustrated by Monte-Carlo simulations of the transfer of seed soft photons through the reconnection layer. The emerging radiation spectrum has a cutoff near 100 keV similar to the hard-state spectra of X-ray binaries and AGN. We discuss how the chain cooling differs from previous phenomenological emission models, and suggest that it can explain the hard X-ray activity of accreting black holes from first principles. Particles accelerated at the X-points of the chain produce an additional high-energy component, explaining the “hybrid Comptonization” observed in Cyg X-1.

Multi-pair states in electron–positron pair creation

Ultra strong electromagnetic fields can lead to spontaneous creation of single or multiple electron-positron pairs. A quantum field theoretical treatment of the pair creation process combined with numerical methods provides a description of the fermionic quantum field state, from which all observables of the multiple electron-positron pairs can be inferred. This allows to study the complex multi-particle dynamics of electron-positron pair creation in-depth, including multi-pair statistics as well as momentum distributions and spin. To illustrate the potential benefit of this approach, it is applied to the intermediate regime of pair creation between nonperturbative Schwinger pair creation and perturbative multiphoton pair creation where the creation of multi-pair states becomes nonnegligible but cascades do not yet set in. Furthermore, it is demonstrated how spin and helicity of the created electrons and positrons are affected by the polarization of the counterpropagating laser fields, which induce the creation of electron-positron pairs.

Electron-positron pair production in the low-density approximation

Electron–positron pair creation is studied in the low-density approximation by solving the quantum Vlasov equation exactly and the mapping equation approximately. The simpler mapping equation is an approximate treatment of the quantum Vlasov equation in which the continuous external field is regarded as a series of delta kicks. Our study indicates that this new treatment is appropriate because the results of the two methods are in good agreement with each other. However, as the period number increases, interference and a complicated structure in the momentum distribution are observed. Furthermore, we also obtain the square power law relation of the number density to the applied electric field strength.

Layout of new experiments on the observation of spontaneous electron–positron pair creation in supercritical Coulomb fields

Collisions of bare heavy nuclei implemented by means of merging-beam technique are suggested for the study of remarkable phenomena appearing in very strong fields created when distance between the nuclear surfaces approaches zero. Suggested here is to build a collider with two beams of actinide nuclei merging within a common straight section. Principal parameters of this collider are outlined.

Nonperturbative Bethe–Heitler pair creation in combined high- and low-frequency laser fields

The nonperturbative regime of electron–positron pair creation by a relativistic proton beam colliding with a highly intense bichromatic laser field is studied. The laser wave is composed of a strong low-frequency and a weak high-frequency mode, with mutually orthogonal polarization vectors. We show that the presence of the high-frequency field component can strongly enhance the pair-creation rate. Besides, a characteristic influence of the high-frequency mode on the angular and energy distributions of the created particles is demonstrated, both in the nuclear rest frame and the laboratory frame.