Progress in laser wakefield acceleration (LWFA) has led to the production of a multi-GeV electron beam from a cm-length plasma. Such beams are collocated together with high intensity laser pulses at petawatt (PW) laser facilities, allowing the study of laser-electron collisions in all-optical setups. This configuration opens up the possibility to test strong-field quantum electrodynamics...
Utilizing the relativistic invariance of the parameter $\chi=\gamma E/E_{0}$, ultrarelativistic particles in strong crystalline fields of the order $10^{11}$ V/cm enable investigations of processes in fields of the order the QED critical field $E_{0}=m^{2}c^{3}$/eħ=1.32·$10^{16}$ V/cm (with a corresponding magnetic field of $B_{0}$=4.41·$10^{9}$ T) in the particle rest frame. In the framework...
The quest of the perturbative post-Minkowskian study of the gravitational two body problem has recently seen advances upon employing quantum field theory techniques. I report on a novel approach based on a worldline quantum field theory that provides an efficient way to study the classical scattering of two massive objects (black holes, neutron stars or stars) in GR.
We are able to directly...
The techniques of scattering amplitudes in quantum field theory have been applied successfully to the description of black hole scattering in recent years. In this talk, I will analyze three instances where QED can be used to great advantage as a toy model (or building block) of the gravity case.
We argue that measurements of X-ray polarization using the recently launched Imaging X-ray Polarimetry Explorer will answer many open questions about magnetars in particular the physical state of their surfaces, whether vacuum birefringence exists, and the nature of the hard X-ray emission from these objects. We outline the capabilities of the instrument, specific models and the results of...
In this talk I will review the role of QED effects in magnetospheres of astrophysical compact objects, black holes and neutron stars. I will focus on the dynamics of electron-positron pair discharges in large unscreened electric fields, as well as on magnetic reconnection with strong radiative cooling and pair production. Finally, I will highlight observable emission signatures of these...
A major computational challenge in the physics of intense fields is the ability to do reliable non-perturbative computations for fields that are both strong and have large gradients. In this talk I discuss the use of ideas from resurgence to make precise resummations and analytic continuations into the intense field regime beginning with modest amounts of perturbative weak-field information....
We study electron-positron pair creation induced by the field of two colliding (transversal and linearly polarized) laser pulses ranging from the Sauter-Schwinger regime at small laser frequencies to the Breit-Wheeler regime at large photon energies.
On the basis of a generalized WKB approach, we find that the pair creation rate along the symmetry axis (where one would expect the maximum...
We discuss spatially- and temporally-resolved, non-perturbative pair production in colliding, linearly polarized laser pulses on the basis of two numerical approaches; a generalized WKB approach culminating in solving a modified Riccati equation and large-scale simulations based on the Dirac-Heisenberg-Wigner formalism.
We discuss how Dirac-Heisenberg-Wigner (quantum kinetic theory) as well...
There is a renewed interest in the physics of radiation reaction (RR), largely driven by high-power laser systems where particles are subject to RR forces at least as strong as the Lorentz force. The Lorentz-Abraham Dirac (LAD) equation of motion with RR has, however, unphysical runaway solutions. The Landau-Lifshitz (LL) equation obtained from the Lorentz-Abraham-Dirac equation through...
The quantum vacuum nonlinearity allows for the effect of laser photon merging in the collision of two (or more) laser beams. As the merged photons origin from a manifestly inelastic process, their energy differs significantly from the background photons of the driving lasers, making them accessible for experiments. However, the number of merged photons is typically considered to be very small....
The most general QED four-photon amplitude that has been computed, so far, is the one with two photons on-shell and two off-shell. The generalization of this work to a fully off-shell calculation is presently still lacking. Here we present the result of such a calculation, although still with at least one of the legs taken in the low-energy limit, unifying the scalar and spinor QED cases....
The standard formalism for calculating the S-matrix in quantum field theory is based on path integrals over field configurations, and can be formulated as a diagrammatic perturbative method. Alternative to this approach is the Worldline formalism, based on first-quantized relativistic point particle path integrals. Recently, a novel representation of the fermion propagator dressed with an...
In this talk, we consider the scattering of an x-ray free-electron laser (XFEL) beam on the superposition of a strong magnetic field $B_{\textrm{ext}}$ with the Coulomb field $E_{\textrm{ext}}$ of a nucleus with charge number $Z$. In contrast to Delbrück scattering (Coulomb field only), the magnetic field $B_{\textrm{ext}}$ introduces an asymmetry (i.e., polarization dependence) and renders...
Strong fields are ubiquitous in physics, occurring on scales from quarks to the entire universe, and encompassing all fundamental interactions. Insights gained from one area of strong field physics can be of great value in other areas. This is (relatively) straightforward with vector and scalar forces, but the very different nature of gravity can make it hard to translate results from one...
Over the last 30 years, there has been intensive work and incredible progress in our understanding of perturbative scattering amplitudes in gauge theory and gravity, but so far these advances have not addressed strong background fields. In this talk, I will try to convince you that there is actually an un-tapped wealth of connections between the amplitudes and strong field communities, which...
Quantum Electrodynamics (QED) serves as a useful toy model for classical observables in gravitational two-body systems with reduced complexity due to the linearity of QED. We investigate scattering observables in scalar QED at the sixth order in the charges (two-loop order) in a classical regime analogous to the post-Minkowskian expansion in General Relativity. We use modern methods to compute...
We derive a quantum kinetic theory for QED based on Kadanoff-Baym equation [1]. It generalizes the well-known classical kinetic theory to the polarized case, with spin polarization entering in the next order of gradient expansion. We also discuss generalization to QCD. We use this framework to study polarization of probe massive fermion in QED plasma with shear [2]. We find new collisional...
The Euler-Heisenberg Lagrangian is discussed in homogeneous electromagnetic fields with a constant axial gauge coupling, to one-loop order. Two special configurations, namely a magnetic field with chiral chemical potential as well as an electric field with spatial axial gauge, are argued to possess an exact eigenspectrum, whose sum leads to a defining Lagrangian.
In the case of an electric...
In recent years, the sole light-based interaction with the spin of a free electron (polarization and spin detection) has been discussed for the case of electron diffraction in counterpropagating laser beams [1,2]. The quantum dynamics of the electron in such a standing light wave of the so-called Kapitza-Dirac effect [3,4] is commonly solved for the approximation of two counterpropagating...
The KMOC approach is a formalism that expresses classical observables on flat backgrounds in terms of quantum scattering amplitudes. After a first review, I will show two generalizations of the original framework by extending its range of application to classical wave physics and observables on a curved background. Using these, I will prove how to compute the bending of light and waveforms...
We study the space of quantum and classical observables for the radiation emitted by a scalar moving in gauge and gravitational plane-wave backgrounds. We explore the structure of new localised observables such as the momentum and angular momentum flow, as well as their global analogues. We observe that classical observables exhibit a power-law divergence in QED and a logarithmic divergence...
There is a famous $\alpha \chi^{2/3} \sim 1$ problem of a perturbation theory for QED in a strong electromagnetic field. It leads to the situation when radiation losses of highly energetic electrons in a sufficiently high intensity electromagnetic wave cannot be calculated reliably in the frame of the perturbation theory because of divergence of its series at $\alpha \chi^{2/3} \sim 1$. We...
The acceleration of spin-polarized particle beams from laser-plasma interaction has gained a lot of interest in recent years due to the availability of high-intensity lasers and their applications for investigating strong-field phenomena. In particular, probing the nuclear structure of protons and neutrons requires polarized particle beams [1]. In this talk, we will present a setup consisting...
New laser facilities will reach intensities of $10^{23} \textrm{W cm}^{-2}$. This advance enables novel experimental setups in the study of laser-plasma interaction. In these setups with extreme fields quantum electrodynamic (QED) effects like photon emission via non-linear Compton scattering and Breit-Wheeler pair production become important.
We study high-intensity lasers grazing the...
Creation of electrons and positrons via binary photon collisions, i.e. the linear Breit-Wheeler (BW) process, is a basic prediction of quantum electrodynamics, but it is yet to be observed in the laboratory. Motivated by experimental capabilities of newly constructed laser facilities and by recent developments in target fabrication, we have performed PIC simulations supplemented by a...
The LUXE experiment (Laser Und XFEL Experiment) is a new experiment in planning at DESY Hamburg using the electron beam of the European XFEL. At LUXE, the aim is to study collisions between a high-intensity optical laser and up to $16.5\,$GeV electrons from the Eu.XFEL electron beam, or, alternatively, high-energy secondary photons. The physics objectives of LUXE are to measure processes of...
The fast-paced advance in the high-power laser technology has recently allowed reaching focussed intensities exceeding $10^{21}$ Wcm$^{−2}$, with realistic plans to reach $> 10^{23}$ Wcm$^{−2}$ in near-term largescale laser facilities worldwide. While these intensities are still orders of magnitude lower than those needed to produce an electron-positron pair from the vacuum, this limitation...
We show that an all-optical configuration of the laser-electron collision in the $\lambda^3$ configuration based on 10 PW-class lasers presents a viable platform for reaching the range of parameters where a perturbative QED in strong external electromagnetic field breaks. This case is contingently referred to as a case of the non-perturbative QED, and this range of parameters is the intriguing...
We investigate the optimization of photon polarization to increase the yield of Breit-Wheeler pair production in arbitrarily polarized plane wave backgrounds. We show that the optimized photon polarization can improve the positron yield by more than $20\%$ compared to the unpolarized case, in the intensity regime of current laser-particle experiments.
The seed photon optimal polarization...
We consider the scattering of probe particles on an ultraboosted beam of charge, in the case that the fields of the beam are strong and must be treated nonperturbatively.
We show that the fields of the ultraboosted beam act as stochastic plane waves-scattering amplitudes (of elastic scattering, nonlinear Compton and nonlinear Breit-Wheeler) are obtained without approximation by averaging...
The locally constant crossed field approximation (LCFA) is a powerful tool for studies of various strong field QED phenomena. It is common that numerical codes for simulating strong laser-matter interaction rely on LCFA for taking into account QED effects, and therefore it is crucial to establish the limits of applicability of the approximation and develop possible extensions.
We explore...
Time-dependent discharges of electron-positron pairs have recently been proposed as a primary ingredient to explain the nature of pulsar radio emission, a long-standing open problem in high-energy astrophysics. During these discharges - positive feedback loops of gamma-ray photon emission via curvature radiation by TeV electrons and positrons and pair production - the plasma self-consistently...
We discuss the fermion-antifermion polarization phenomena in strong magnetic fields that can be realized in heavy-ion collisions, laser fields, and neutron stars. We elaborate on its effects on birefringence when a photon is traversing the magnetic-field region [1].
The medium effects can drastically change the vacuum birefringence due to the Pauli blocking effect and the medium-specific...
In inflation models driven by an axion-like particle the inflaton may have a Chern-Simons coupling to the Standard Model (SM) U$(1)_Y$. In this talk we show that this setup is a highly predictive baryogenesis model without further ingredients other than the SM and the inflaton (and the origin of neutrino mass). During inflation this Chern-Simons coupling sources a dual production of the SM...
Strong magnetic fields show up in various context in high-energy physics such as heavy-ion collisions, compact stars, and the early universe. Thus it is of interest to understand the behavior of strongly interacting matter in such extreme conditions. In this talk, I will give an overview of some of the progress that has been made over the past decade. In particular, I will discuss the...
Strong electromagnetic fields, as they arise in high-energy heavy-ion collisions, in the interior of magnetars and potentially during the evolution of the early universe, have a significant impact on the physics of quarks and gluons. First-principles lattice simulations of this non-perturbative system have revealed a highly nontrivial response to the background fields and a corresponding phase...
The large-scale structure of our Universe is seen as a result of quantum field fluctuations amplified by the evolution of space-time itself. Quantum fields in curved spacetimes have many tantalizing theoretical properties, for example particles being produced by the time-dependence of the geometry. I will describe how quantum fields in geometries with spacetime curvature and different...
I’ll discuss the role of the triangle anomaly in polarized deep inelastic scattering (DIS) employing a worldline formalism. I’ll demonstrate that the structure function $g_1(x_B,Q^2)$ measured in polarized DIS, as well as its first moment which defines the proton's helicity $\Sigma(Q^2)$, is dominated by the chiral anomaly in both Bjorken ($Q^2\rightarrow \infty)$ and Regge ($x_B\rightarrow...
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 electron-positron 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 [1]. We...
Accounting for loop radiative corrections, such as photon polarization and electron mass operators, become essential as the strength of the external field reaches the limit of ~1600 Sauter-Schwinger fields in the reference frame of a particle, namely, $\alpha\chi^{2/3}\sim 1$ where $\chi$ denotes the (dimensionless) field. It is sufficient to consider just 1-loop corrections below the limit...
Quantum electrodynamics (QED) is a theory of the interacting fermionic electron-positron and electromagnetic fields. However, it may be expedient to isolate or neglect contributions of spin effects. Also, there do exist charged scalar particles. Therefore, it is worth studying a scalar version of QED as well. At first glance, such a theory should be simpler due to the absence of spin...
Experimental efforts toward the detection of the nonperturbative strong-field regime of the Breit-Wheeler pair creation process plan to combine incoherent sources of GeV $\gamma$ quanta and the coherent fields of tightly focussed optical laser pulses. This endeavour calls for a theoretical understanding of how the pair yields depend on the applied laser field profile. We provide estimates...
