Create Electron-positron Pairs Research Articles

Nanophotonics for pair production

The transformation of electromagnetic energy into matter represents a fascinating prediction of relativistic quantum electrodynamics that is paradigmatically exemplified by the creation of electron-positron pairs out of light. However, this phenomenon has a very low probability, so positron sources rely instead on beta decay, which demands elaborate monochromatization and trapping schemes to achieve high-quality beams. Here, we propose to use intense, strongly confined optical near fields supported by a nanostructured material in combination with high-energy photons to create electron-positron pairs. Specifically, we show that the interaction between near-threshold γ-rays and polaritons yields higher pair-production cross sections, largely exceeding those associated with free-space photons. Our work opens an unexplored avenue toward generating tunable pulsed positrons from nanoscale regions at the intersection between particle physics and nanophotonics.

Pair production in an electron collision with a radially polarized laser pulse

Using 3D numerical simulations, we show that in the collision of an electron with a counter-propagating laser beam, the radial polarization of the laser pulse leads to more efficient electron–positron pair production via multiphoton Breit–Wheeler process compared to the linearly polarized one having the same energy, duration, and amplitude. The reason is that in the case of the radially (or azimuthally) polarized laser pulse, the area of a strong field region is greater than that in the case of a linear one. As a consequence, more electrons experience a strong field region with sufficient energy, that, in turn, results in a higher number of created electron–positron pairs.

Creating pair plasmas with observable collective effects

Although existing technology cannot yet directly produce fields at the Schwinger level, experimental facilities can already explore strong-field quantum electrodynamics (QED) phenomena by taking advantage of the Lorentz boost of energetic electron beams. Recent studies show that QED cascades can create electron–positron pairs at sufficiently high density to exhibit collective plasma effects. Signatures of collective pair plasma effects can appear in exquisite detail through plasma-induced frequency upshifts and chirps in the laser spectrum. Maximizing the magnitude of the QED plasma signature demands high pair density and low pair energy, which suits the configuration of colliding an over energy-density electron beam with a 1022– intensity laser pulse. The collision creates pairs that have a large plasma frequency, made even larger as they slow down or reverse direction due to both the radiation reaction and laser pressure. This paper explains at a tutorial level the key properties of the QED cascades and laser frequency upshift, and at the same time finds the minimum parameters that can be used to produce observable QED plasma.

Enhancement of electron–positron pairs in combined potential wells with linear chirp frequency

Effect of linear chirp frequency on the process of electron–positron pairs production from vacuum is investigated by the computational quantum field theory. With appropriate chirp parameters, the number of electrons created under combined potential wells can be increased by two or three times. In the low frequency region, frequency modulation excites interference effect and multiphoton processes, which promotes the generation of electron–positron pairs. In the high frequency region, high frequency suppression inhibits the generation of electron–positron pairs. In addition, for a single potential well, the number of created electron–positron pairs can be enhanced by several orders of magnitude in the low frequency region.

Resolving rapidly chirped external fields with Dirac vacuum

We study the dynamical response of the Dirac vacuum state to a very strong time-dependent electric field pulse, whose frequency is chirped in time. The resulting field-induced electron-positron pair-creation process can be used to examine various proposals for time-dependent frequency spectra of the external field. It turns out that the Dirac vacuum can be used as sensitive probe that can respond to the instantaneous values of the frequency at each moment of time by producing electrons with a characteristic energy. This almost instantaneous response feature of the vacuum state permits us to introduce a generalized rate equation. It is based on the concept of a time-dependent decay rate and can provide semianalytical solutions to predict the number of created electron-positron pairs during the interaction with arbitrarily chirped electric field pulses.

Electron-positron pair production in frequency modulated laser fields

The momentum spectrum and the number density of created electron-positron pairs in a frequency modulated laser field are investigated using quantum kinetic equation. It is found that the momentum spectrum presents obvious interference pattern. This is an imprint of the frequency modulated field on the momentum spectrum, because the momentum peaks correspond to the pair production process by absorbing different frequency component photons. Moreover, the interference effect can also be understood qualitatively by analyzing turning point structures. The study of the pair number density shows that the number density is very sensitive to modulation parameters and can be enhanced by over two orders of magnitude for certain modulation parameters, which may provide a new way to increase the number of created electron-positron pairs in future experiments.

Optical analog of particle production in gravitational fields

A strong enough electric field is predicted to spontaneously create electron-positron pairs in vacuum via the Schwinger effect. A somewhat similar effect is predicted to occur in sufficiently strong inhomogeneous gravitational fields. However, due to the necessity of very large field strength, these effects were never observed in the experiment. Here we demonstrate that optical analogs of very strong gravitational fields (up to ) and very strong gravitational field gradients (up to ) may be created in electromagnetic metamaterial waveguides, leading to an optical analog of particle production in gravitational fields. Such waveguide geometries may also potentially be used to search for axion-like particles weakly interacting with an electromagnetic field.

Electron-positron pair production in an oscillating Sauter potential

Electron-positron pair production in an oscillating Sauter potential is investigated in the framework of the computational quantum field theory. It is found that for a Sauter potential well with oscillating width and depth simultaneously, the phase difference between them has a great impact on the number of created electron-positron pairs. Optimal values of the phase difference corresponding to different oscillation frequencies are obtained. The optimal phase difference has a strong nonlinear dependence on oscillating frequency. When the potential well changes slowly, our results can be explained by the instantaneous bound states. For the higher frequency case, however, multiphoton effect is enhanced and the Pauli blocking effect has a strong inhibitory effect on pair creations. These results can provide a theoretical reference for the experimental creation of the electron-positron pairs.

Dirac Vacuum as a Transport Medium for Information.

Usually, the transport of information requires either an electromagnetic field or matter as a carrier. It turns out that the Dirac vacuum modes could be exploited as a potentially loss-free carrier of information between two distant locations in space. At the first location, a spatially localized electric field is placed, whose temporal shape is modulated, for example, as a binary sequence of distinguishable high and low values of the amplitude. The resulting distortion of the vacuum state reflecting this information propagates then to a second location, where this digital signal can be read off sequentially by a static electric field pulse. If this second field is supercritical, it can create electron-positron pairs from the manipulated vacuum state. The original information transported by the vacuum mode is then imprinted on the temporal behavior of the created particle yield for a selected energy.

Pair creation in heavy ion channeling

Heavy ions channeling through crystals with multi-GeV kinetic energies can create electron-positron pairs. In the framework of the ion, the energy of virtual photons arising from the periodic crystal potential may exceed the threshold $2mc^2$. The repeated periodic collisions with the crystal ions yield high pair production rates. When the virtual photon frequency matches a nuclear transition in the ion, the production rate can be resonantly increased. In this two-step excitation-pair conversion scheme, the excitation rates are coherently enhanced, and they scale approximately quadratically with the number of crystal sites along the channel.

Enhanced pair production in strong fields by multiple-slit interference effect with dynamically assisted Schwinger mechanism

In the quantum kinetic framework, we investigate the momentum spectrum and the number density of created electron-positron pairs from vacuum in the combined electric fields composed of two sets of alternating-sign electric field pulse trains, a strong but slowly varying one and a weak but rapidly changing one. It is found that the pair production can be strongly enhanced by combining the multiple-slit interference effect with the dynamically assisted Schwinger mechanism. By considering the contribution of the transverse momentum, we find that the number density of created particles depends linearly on the electric field pulse number, i.e., a power law with index 1. Moreover, we study the effect of interpulse time delay on pair production and find that the number density is very sensitive to the interpulse time delay. The symmetry of the momentum spectrum for an $N$-pulse electric field and the difference of the multiple-slit interference effect between the nonperturbative Schwinger mechanism and the perturbative multiphoton electron-positron pair creation are also studied. Some possible explanations for observed results of momentum spectrum and number density are also given and discussed.

Effects of laser pulse shape and carrier envelope phase on pair production

For different fields of supercycle and subcycle laser pulses, the effects of laser pulse shape and carrier envelope phase on pair production are investigated by solving quantum Vlasov equation. By changing the pulse width and shape, the nonlinear behaviors of momentum distribution function and number density of created electron–positron pairs are obtained. It is found that there exist the multiphoton processes and stabilization phenomenon in pair production for supercycle situation. Our study may shed a light on optimizing the form of applied laser field to enhance the created pairs number, for example, when other conditions are fixed, the flat-top super-Gaussian laser pulse has advantage on pairs production.

Momentum Signatures for Schwinger Pair Production in Short Laser Pulses with a Subcycle Structure

We investigate electron-positron pair production from vacuum for short laser pulses with a subcycle structure, in the nonperturbative regime (Schwinger pair production). We use the nonequilibrium quantum kinetic approach and show that the momentum spectrum of the created electron-positron pairs is extremely sensitive to the subcycle dynamics-depending on the laser frequency omega, the pulse length tau, and the carrier phase varphi-and shows several distinctive new signatures. This observation could not only help in the design of laser pulses to optimize the experimental signature of Schwinger pair production but also ultimately lead to new probes of light pulses at extremely short time scales.

Electrodynamics of Magnetars. III. Pair Creation Processes in an Ultrastrong Magnetic Field and Particle Heating in a Dynamic Magnetosphere

We first examine the QED processes that create electron-positron pairs in magnetic fields approaching and exceeding 1014 G. The formation of free and bound pairs is addressed, and the importance of positronium dissociation by thermal X-rays is noted. We calculate the collision cross section between an X-ray and a gamma ray, and point out a resonance in the cross section when the gamma ray is close to the threshold for pair conversion. We then show how the pair creation rate in the open-field circuit and the outer magnetosphere can be strongly enhanced by current-driven instabilities near the light cylinder. A cascade develops when the current has a strong fluctuating component. We examine the mechanism of particle heating and show that a high rate of pair creation can be sustained close to the star, but only if the spin period is shorter than several seconds. The dissipation rate in this turbulent state can easily accommodate the observed radio output of the transient radio-emitting magnetars and even their infrared emission. Finally, we outline how a very high rate of pair creation on the open magnetic field lines can help to stabilize a static twist in the closed magnetosphere and to regulate the loss of magnetic helicity by reconnection at the light cylinder.

Ground State and Charge Renormalization in a Nonlinear Model of Relativistic Atoms

We study the reduced Bogoliubov-Dirac-Fock (BDF) energy which allows to describe relativistic electrons interacting with the Dirac sea, in an external electrostatic potential. The model can be seen as a mean-field approximation of Quantum Electrodynamics (QED) where photons and the so-called exchange term are neglected. A state of the system is described by its one-body density matrix, an infinite rank self-adjoint operator which is a compact perturbation of the negative spectral projector of the free Dirac operator (the Dirac sea). We study the minimization of the reduced BDF energy under a charge constraint. We prove the existence of minimizers for a large range of values of the charge, and any positive value of the coupling constant α. Our result covers neutral and positively charged molecules, provided that the positive charge is not large enough to create electron-positron pairs. We also prove that the density of any minimizer is an L 1 function and compute the effective charge of the system, recovering the usual renormalization of charge: the physical coupling constant is related to α by the formula αphys ≃ α(1 + 2α/(3π) log Λ)−1, where Λ is the ultraviolet cut-off. We eventually prove an estimate on the highest number of electrons which can be bound by a nucleus of charge Z. In the nonrelativistic limit, we obtain that this number is ≤ 2Z, recovering a result of Lieb. This work is based on a series of papers by Hainzl, Lewin, Séré and Solovej on the mean-field approximation of no-photon QED.

Oscillatory pair-creation probabilities in time-dependent subcritical forces

We examine the pair-production process from vacuum in a subcritical force field. If this force field is turned on linearly in time, the number of created electron-positron pairs does not increase monotonically but in a rather oscillatory way with a universal frequency related to twice the electron’s rest mass energy. As a consequence of this oscillatory growth pattern, the final number of created pairs could be controlled by an optimized temporal dependence of the pulse shape of the force.

Monte Carlo analysis of electron-positron pair creation by powerful laser-ion impact

We consider electron-positron pair creation by the impact of very powerful laser pulses with highly charged ions. In contrast to our foregoing work with rather limited angular configurations of pair creation, we extend these calculations to even higher laser intensities, and we use the Monte Carlo method to numerically analyze the rates of pair creation for arbitrary angular distributions. We also evaluate the intensity dependence of the total rates of pair creation. Thus we demonstrate that our laser-induced process shows stabilization, because beyond a specific laser power the total rates of pair creation decreases. Our analysis of the angular distributions of the created electron-positron pairs leads to the conclusion that pairs are predominantly emitted in the direction of laser pulse propagation.

Monte-Carlo-based studies of a polarized positron source for International Linear Collider (ILC)

The full exploitation of the physics potential of an International Linear Collider (ILC) requires the development of a polarized positron beam. New concepts of polarized positron sources are based on the development of circularly polarized photon sources. The polarized photons create electron–positron pairs in a thin target and transfer their polarization state to the outgoing leptons. To achieve a high level of positron polarization the understanding of the production mechanisms in the target is crucial. Therefore, a general framework for the simulation of polarized processes with Geant4 is under development. In this contribution the current status of the project and its application to a study of the positron production process for the ILC is presented.

Electric field screening by a proton counterflow in the pulsar polar cap

We propose a new mechanism to screen the electric field in the pulsar polar cap. Previous studies have shown that if an electron beam from the stellar surface is accelerated to energies high enough to create electron-positron pairs, the required electric field parallel to the magnetic field lines is too strong to be screened out by the produced pairs. We argue here that if non-relativistic protons are supplied from the magnetosphere to flow towards the stellar surface, they can provide an anode to screen out such a strong electric field. Injected electron-positron pairs yield an asymmetry of the electrostatic potential around the screening point. The required pair creation rate in this model is consistent with the conventional models.

Numerical study of pair creation by ultraintense lasers

Now that intensity of lasers has reached 1020 W/cm2, electron–positron pairs can be created by the irradiation of such ultraintense lasers on a thin gold foil. The energy of electrons produced by ultraintense lasers reaches more than several tens of MeV. Such high energy electrons become a source for creating electron–positron pairs via interaction with nuclei. There are a few processes that create electron–positron pairs in this situation. Two processes, call the trident process and the Bethe–Heitler process, are considered in this study. A numerical simulation code based on a relativistic Fokker–Planck equation is developed for studying the hot electron transport. The equation is solved by assuming one-dimensional real space and two-dimensional momentum space with axial symmetry. It is found that the total positron yield increases logarithmically with the increase of the laser intensity, and the resultant energy distribution of the created positron is found to have a peak near the energy of 1–2 MeV.