Speakers 2023

Felix Karbstein

Title of the talk:

Direct Accessibility of the Fundamental Constants Governing Light-by-Light Scattering

Abstract:

Quantum field theory predicts that the vacuum exhibits a nonlinear response to strong electromagnetic fields. This fundamental tenet has remained experimentally challenging and is yet to be tested in the laboratory. We present proof of concept and detailed theoretical analysis of an experimental setup for precision measurements of the quantum vacuum signal generated by the collision of a brilliant x-ray probe with a high-intensity pump laser. The signal features components polarized parallel and perpendicularly to the incident x-ray probe. Our proof-of-concept measurements show that the background can be efficiently suppressed by many orders of magnitude which should not only facilitate a detection of the perpendicularly polarized component of the nonlinear vacuum response, but even make the parallel polarized component experimentally accessible. The angular separation of the signal from the intense x-ray probe should enable precision measurements even in the presence of pump fluctuations and alignment jitter. This provides direct access to the low-energy constants governing light-by-light scattering. In my talk, I will mainly focus on the theoretical considerations underlying this work.

Petr Satunin

Title of the talk:

Neutral particle decays and scatterings with Worldline Instantons

Abstract:

We apply the semiclassical method of Worldline Instanons to the calculation for the width of neutral particle decays to charged ones in an external electromagnetic field: photon decay to an electron-positron pair in a constant external electric or magnetic field, neutrino decay to W-boson and electron in a constant magnetic field. Additionally, we consider the cross-section for the Breight-Wheeler process (two photon scattering to electron-positron pair) in a constant external electric or magnetic field below the perturbative threshold with Worldline instantons.

Presentation:

Satunin.pdf

Stefan Evans

Title of the talk:

Study of QED singular properties for variable gyromagnetic ratio g

Abstract:

We evaluate the QED Euler Heisenberg type effective action for an arbitrarily strong constant and homogeneous field, with arbitrary gyromagnetic ratio g. We explore nonperturbative properties and find a cusp at g=2 in the QED beta-function and in the pseudoscalar (\vec E \cdot \vec B)^{2n}) dependent contributions to the effective action. The cusp produces a nonperturbative in g suppression of particle production, where the vacuum subject to a critical electric field strength is stabilized in magnetic-dominated 'magnetar' environments.

Anton Ilderton

Title of the talk:

Links between strong field QED and gravity

Abstract:

I will discuss some of the work we have done over the past few years at the interface of gauge theory and gravity. This includes exploring the similarities and differences between these theories when strong field effects are present, and using strong field techniques to access results and physical effects which are not easily captured by standard perturbative methods.

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Presentation:

13-02-23 - Anton Ilderton - worldliners-presentation.pdf

Muddu Saketh

Title of the talk:

Compton amplitude to Worldline action

Abstract:

The amplitude for the process of a gravitational wave scattering off a compact body (Compton amplitude) contains crucial information detailing its coupling with the gravitational field. In particular, this can be used to construct/constrain effective worldline actions through a comparison of the Compton amplitude obtained between the real and effective theories. In the real theory, the scattering amplitude can be obtained by solving the metric perturbation equations and imposing the appropriate boundary conditions, for e.g., by solving the Teukolsky equation for a Kerr black hole. In the effective theory, the scattering amplitude may be obtained by writing down a sufficiently general worldline action and subjecting the "particle" to an external gravitational wave, and solving the Einstein equations for the scattered wave. Following this, a comparison between the two may fix the undetermined coefficients in the effective action, yielding a properly constrained worldline action for the desired compact body. The latter can then be used for solving for the dynamics of the compact body in an arbitrary gravitational field, for e.g., in a binary. We will present this idea in two contexts, focusing first on the conservative part of the Compton amplitude and then on the dissipative part respectively. The conservative part of the Compton amplitude helps fix the conservative terms in the effective worldline theory, corresponding to spin-induced multipole moments, love numbers, etc. It also aids in the construction and constraining of higher spin quantum field theories. The dissipative part corresponds to the absorption of a portion of the gravitational wave at the horizon and may be modeled in the effective theory as tidal heating by including additional degrees of freedom in the worldline corresponding to tidally induced multipole moments. Comparison with the actual Compton amplitude can then fix the tidal response function relating the external tidal fields to the induced multipole moments. Subsequently, one can compute the horizon fluxes for a binary black hole system and its contribution to the gravitational waveform. We will then conclude with a discussion of future scope and projects within this approach.

The talk will be based on the following papers a) https://arxiv.org/abs/2208.03170 and b) https://arxiv.org/abs/2212.13095

Click here to access the recording.

Presentation:

06-03-23 - Muddu Saketh - Compton2wl.pptx

Francesco Comberiati

Title of the talk:

From KMOC to WQFT and the classical double copy of spinning matter

Abstract:

In the first part of the talk I am going to discuss the relation between the KMOC formalism and the WQFT, deriving a direct link between them at tree level, based on the construction of a path integral representation for classical off-shell currents with external matter lines in a gauge background. The derivation makes a crucial use of the KMOC coherent wave-functions, which allows us to relate the classical limit of the currents with their WQFT representation. The WQFT path integral so obtained encodes perturbative solutions of the point particle equations in a gauge background and thus can be used in contexts where the classical limit is relevant. In particular we use such result to evaluate the gravitational Compton amplitude up to quadratic in spin, and to compute hard thermal currents in sQED/sQCD and GR.

In the second part of the talk I will review some of the aspects of the BCJ perturbative double copy and provide some new recent results I achieved in the case of the classical double copy of spinning matter. In particular I will show how using supersymmetric worldline models and requiring the double copy prescription to preserve susy and the R-symmetry on the worldline allows the entire reconstruction of the underlying classical and quantum theory on the gravitational side, and efficiently generates classical integrands at loop level and quadratic in spin. Such integrands correspond to the ones needed for evaluating classical observables related to inspiraling binary black holes.

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Eleanor Harris

Title of the talk:

Complex geodesics in de Sitter space

Abstract:

The two-point function of a free massive scalar field on a fixed background can be evaluated in the large mass limit by using a semiclassical geodesic approximation. In de Sitter space, however, this poses a puzzle. Certain spacelike separated points are not connected by real geodesics despite the corresponding two-point function in the Bunch-Davies state being non-vanishing. We resolve this puzzle by considering complex geodesics after analytically continuing to the sphere. We compute one-loop corrections to the correlator and discuss the implications of our results to de Sitter holography.

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Presentation:

27-03-23 - Eleanor Harris - Worldlines_Meeting_2023.pdf

Ian Moss

Title of the talk:

Mining the quantum vacuum: quantum tunnelling and particle creation

Abstract:

Particle production from the vacuum is a remarkable aspect of particle physics. Prime examples are the Schwinger process of particle production in strong electric fields and the Hawking process of particle production from black holes. These processes can be viewed as quantum tunnelling of particles from the vacuum. The tunnelling approach, and the closely related instanton or complex path approaches, are reviewed here with emphasis on paths in the complex coordinate plane. The method is applied to particle production from a black hole in a magnetic field, where ultra-high energy charged particles are produced.

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Andreas Lindner

Title of the talk:

Simulating the Nonlinear QED Vacuum.

Abstract:

The Heisenberg-Euler theory of the quantum vacuum supplements Maxwell's theory of electromagnetism with nonlinear light-light interactions. These originate in vacuum fluctuations, a key prediction of quantum theory, and can be triggered by high-intensity laser pulses, causing a variety of intriguing phenomena. A highly accurate numerical scheme for solving the nonlinear equations due to the leading orders of the Heisenberg-Euler weak-field expansion is presented. The algorithm possesses an almost linear vacuum dispersion relation even for comparably small wavelengths and incorporates a nonphysical modes filter. The implemented solver is tested in one spatial dimension against a set of known analytical results for vacuum birefringence and harmonic generation. More complex scenarios for harmonic generation are demonstrated in two and three spatial dimensions.

Presentation:

worldline_lindner_230522.pdf

Reinhard Alkofer

Title of the talk:

Identifying the Time Scales in Particle Production from Ultra-Strong Fields

Abstract:

In the first part of my talk I will introduce the Dirac-Heisenberg-Wigner formalism and its application to the Sauter-Schwinger effect, i.e., electron-positron pair production from ultra-strong electric fields. In the second part I will discuss why, on the one hand, particle number is only in the asymptotic limit a well-defined concept but, on the other hand, one can nevertheless in a Gedankenexperiment extract information about the particle production at finite and even relatively short time scales. In the third part I will present the results of some recent calculations and argue that, even for simple forms of the electric fields, three different time scales rule the particle pair production in the Sauter-Schwinger regime. See: Matthias Diez, RA and Christian Kohlfürst, arXiv:2211.07510, and references therein.

Carsten Müller

Title of the talk:

Nonlinear Breit-Wheeler pair production in high-intensity laser fields

Abstract:

The talk addresses the production of electron-positron pairs from vacuum by high-energy gamma photons colliding with high-intensity laser pulses (nonlinear Breit-Wheeler effect). After a general introduction into the topic, we first discuss the process when the gamma photons are generated through bremsstrahlung, as is envisaged in upcoming strong-field experiments. The influence of the broad energy spectrum of bremsstrahlung is analyzed, along with effects stemming from laser pulse focussing. Afterwards, we present some new results on dynamically assisted Breit-Wheeler pair production in two-color laser fields, where the pair yields are significantly enhanced by the presence of an assisting high-frequency laser mode.

Presentation:

Dresden_2023_CMueller.pdf

Silvia Pla García

Title of the talk:

Partial resummation of QED Effective Lagrangian: Conjecture and Implications

Abstract:

In Quantum Electrodynamics (QED), the one-loop effective Lagrangian remains unknown for arbitrary background configurations. However, we can use its asymptotic (proper-time) expansion to understand some of its characteristics. During my talk, I will present a conjecture that suggests that this expansion can be partially summed in all terms containing the field-strength invariants F_{\mu \nu}F^{\mu \nu} and \tilde{F}_{\mu \nu} F^{\mu \nu}. I will explain the reasoning behind this conjecture and the potential implications it may have.

Scott Robertson

Title of the talk:

DeLLight: Towards an observation of the nonlinear optical index of vacuum

Abstract:

Quantum electrodynamics predicts that the vacuum must behave as a nonlinear optical medium: the speed of light should be modified when the vacuum is stressed by intense electromagnetic fields. The DeLLight (Deflection of Light by Light) experiment at IJCLab, Orsay, aims to observe this nonlinearity via the refraction of a weak probe pulse by an intense pump pulse. In this talk, I will introduce the theory underlying this idea, and report on the current status of the experiment.

Neil Zaim

Title of the talk:

Doppler-boosted lasers as a tool for probing strong-field QED

Abstract:

The new generation of multi-petawatt lasers will produce intensities so high that strong-field quantum electrodynamics (SF-QED) phenomena are predicted to occur in laser-matter interactions. The expected SF-QED processes are the emission of high-energy photons and the generation of electron-positron pairs in the presence of a strong background field. In our group at CEA Saclay, we propose to use Doppler-boosted lasers as a tool for probing SF-QED. Doppler-boosted lasers can be obtained by reflecting a high-intensity laser on a solid-density target and can have a peak intensity that is orders of magnitude higher than that of the initial laser. Our numerical results indicate that this intensification results in a strong enhancement of SF-QED phenomena. At the highest intensities that can be considered, Doppler-boosted lasers even appear as a promising pathway for exploring the fully nonperturbative regime of SF-QED and for reaching scenarios in which SF-QED dominates light-matter interactions. In this presentation, I will present an overview of the recent results that have been obtained with Doppler-boosted lasers.

Click here to access the recording:

Presentation:

25-09-23 - Neil Zaim - SeminarZaim.pdf

Christian Schubert

Title of the talk:

String-based vs. string-inspired in quantum gravity

Abstract:

It has been understood since the eighties that string theory amplitudes are in various aspects better organized than amplitudes in ordinary quantum field theory. One can try to exploit this fact either by representing field theory amplitudes as the infinite string-tension limit of string amplitudes (``string-based formalism’’), or by representing them by worldline path integrals and evaluating those in a way analogous

to string theory (``string-inspired formalism’’). Here I review and compare both approaches with an emphasis on gravity amplitudes and aspects of Yang-Mills theory that bear a relevance for gravity such as color-kinematics duality and multi-particle states. A central role is played by the cycle-and-tail decomposed worldline integrands of the one-loop N-gluon amplitudes that have recently tuned out to

provide a natural route to the construction of Berends-Giele currents in BCJ gauge for Yang-Mills theory as well as gravity.

I conclude with a sketch of the recent adaption of the worldline formalism to classical black hole scattering due to Mogull, Plefka, and Steinhoff.

Click here to access the recording.

Presentation:

talk_wlseminar_ChristianSchubert.pdf

Mattia Damia Paciarini

Title of the talk:

Worldline path integrals for the graviton

Abstract:

In perturbative quantum gravity, the first three Seeley-DeWitt coefficients represent the counterterms needed to renormalize the graviton one-loop effective action in $D=4$ dimensions. A standard procedure to compute them is by means of the traditional heat kernel method. However, these coefficients can be studied also through first quantization using the so-called $\mathcal{N} = 4$ spinning particle model.

In the present work, a different worldline model, able to correctly reproduce the Seeley-DeWitt coefficients in arbitrary dimensions, is developed. After a covariant gauge-fixing procedure of the Einstein-Hilbert action with cosmological constant, a worldline representation of the field kinetic operators identified by its quadratic approximation is found. This quantum mechanical representation can be presented in different but equivalent forms. Some of these different forms are discussed, and their equivalence is verified by deriving the gauge invariant counterterms needed to renormalize a quantum gravity theory with cosmological constant at one-loop order.

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Presentation:

Worldline Presentation_Mattia.pdf

Uwe Hernandez Acosta

Title of the talk:

Concepts of strong-field QED in momentum space: gauge invariance and soft-photon theorems

Abstract:

Usually, quantum electrodynamics acts as the prime example, when it comes to a well-understood and outstandingly precise description of elementary particle processes. However, modern laser facilities provide highly intense light with a non-trivial temporal structure, where an arbitrary number of ‘photons’ from the light source may interact with the colliding particles. In this case, the standard perturbative treatment known from quantum electrodynamics becomes cumbersome and impractical. Accordingly, there are, among others, wide theoretical investigations w.r.t. scattering processes of particles impinging these extreme light sources. This has been done by applying the strong-field quantum electrodynamics, a theory of electromagnetic interactions within coherent highly intense light treated as a classical background field. Here, the distinction between a classical background field and a quantized photon field revealed a vast amount of novel non-linear structures and non-perturbative phenomena. In this seminar, we introduce the basic concepts of strong-field QED and derive the Feynman rules for the theory in momentum space. Then, we explore their general structure and implications from fundamental principles, namely gauge invariance.

Click here to access the recording.

Presentation:

worldline_theory_seminar2023_Uwe.pdf

Sascha Lang

Title of the talk:

Quantum vacuum phenomena in an alternative model for dissipative dielectrics

Abstract:

Quantum vacuum fluctuations have a number of well-observable consequences including the Lamb shift of atomic energy levels or Casimir forces. Apart from this indirect evidence for the non-trivial nature of the electromagnetic ground state, quantum vacuum fluctuations can also be converted into real photons under suitable external conditions - for instance in set-ups with rapidly time-varying system parameters. Tuneable dielectric media constitute a promising platform for systematically investigating such conversion processes. Unlike in empty space, quantum vacuum fluctuations in dielectric bodies are affected by material properties such as dispersion and dissipation but the established models for dispersive and dissipative media do not easily extend to time-dependent systems.

This talk presents an alternative approach which models a dielectric medium with a continuous set of harmonic oscillators and provides a pathway for dissipation by coupling each of these oscillators to a scalar environment field. Besides introducing this model and its quantisation, we will also consider photon production in a medium with a time-dependent dissipation strength.

As another application, we will consider an inertial photon detector moving through a time-independent dispersive and dissipative medium in its quantum vacuum state. Close to material resonances, phase velocities in dielectrics grow very small and may allow inertial photon detectors to move `superluminally' with respect to parts of the surrounding quantum fluctuations, which can have interesting implications.

Click here to access the recording.

Presentation:

talk_Sascha.pdf

Yuchen Du

Title of the talk:

Tree-level Graviton Scattering in the Worldline Formalism

Abstract:

We use the worldline formalism to study tree-level scattering processes involving gravitons. A massless spin 2 particle is described by an N=4 supersymmetric worldline action which is also O(4) symmetric. More generally, N=2S supersymmetric worldline actions exhibiting O(N) symmetry describe free spin S particles. Recently a BRST approach was used to construct the on-shell background graviton emission vertex from a graviton worldline. Nonetheless, an action describing the coupling of higher spin (S≥2) particles with generic background gravity is unknown.

In our work, we found that in order to reproduce Einstein's general relativity 3-point graviton vertex, interpreted as the emission of an off-shell graviton from the worldline, the coupling to background gravity must break the O(4) symmetry to O(2)×O(2). In addition to this symmetry-breaking feature, we also found that the coefficient β of the worldline action counterterm βR differs from previous results in the literature. By comparing the linearized graviton and photon emission vertex operators from different worldlines, we noticed that they obey a squaring relation. For MHV (Maximal Helicity Violating) amplitudes, these squaring relations among the linearized vertex operators directly result in double-copy-like relations between the scattering amplitudes.

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Presentation:

Wolfgang Bietenholz

Title of the talk:

Hadron Physics from Lattice QCD

Abstract:

We first sketch the general concepts of the lattice regularisation in quantum field theory. This formulation does not require gauge fixing, and it provides a link to statistical mechanics, which enables Monte Carlo simulations. They lead to non-perturbative numerical measurements of observables, such as the hadron spectrum, from first principle of Quantum Chromodynamics (QCD). Next we summarise the status of lattice QCD with dynamical quarks,where hadronic observables can now be evaluated to percent level. We also comment on open problems, in particular topological freezing and the sign problem at high baryon density. Finally we show recent lattice results for models out of thermal equilibrium.

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Davide Vadacchino

Title of the talk:

On the structural similarity of N-meson scattering amplitudes in strongly coupled lattice gauge theory in the worldline approach and flat space string theory.

Abstract:

The lattice N-points meson scattering amplitude in the lattice strong coupling regime is computed via a hopping expansion and in the worldline formalism. These results are found to coincide in the 't Hooft limit. Under the assumption of confinement, they are shown to lead to the Veneziano amplitude of flat space string theory. Arguments from gauge/gravity correspondence are used to hint that this structural similarity is not entirely accidental.

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Mohammed Alkhateeb

Title of the talk:

Pair creation and Klein tunneling of fermions and bosons in a space-time resolved quantum field approach

Abstract:

Klein tunneling is the undamped propagation of wave packets whose dynamics is governed by relativistic wave equations as they cross regions with sufficiently strong fields, where the potential height exceeds the energy of the wave packets by more than the rest energy of the particle under study i.e. supercrtical barriers. The first-quantized description of this phenomenon gives rise to paradoxes that necessitate a second quantized approach accounting for pair creation.

In this presentation, I will introduce a non-perturbative space-time-resolved quantum field approach to pair creation induced by supercritical electric potential barriers and Klein tunneling through such barriers. The advantage of this approach, compared to others relying on Bogoliubov transformations between asymptotic states, lies in its ability to account for dynamics at intermediate times. With this space-time resolution, this approach has successfully unveiled the dynamics of Klein tunneling for both fermions and bosons. Notably, we have discovered that in both cases, particles do not actually "tunnel" through the potential barrier. Instead, it is the modulation of the number densities of created anti-particles (increase for bosons and decrease for fermions) that is responsible for the generation of the "tunneled" particle outside the barrier. A comparison between the results of this formalism for Klein tunneling and the first quantized approach reveals that the density calculated inside the barrier in the first quantized approach precisely matches the modulation of anti-particle numbers obtained in the space-time QFT approach.

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Friedemann Queisser

Title of the talk:

Dynamically assisted tunneling in the impulse regime

Abstract:

We investigate how tunneling through a potential barrier, V(x), can be intensified by time-varying electrical fields, whether they take a pulse-shaped form or adhere to harmonic oscillations. To facilitate numerical computations significantly, we employ the Kramers-Henneberger frame. In the context of a periodically driven system, we aim to identify clear resonance signatures when the incident energy E matches the driving frequency, ω=E, revealing the breakdown of the time-averaged potential approximation. Regarding the dependence on a pulse-shaped electrical field, we discover that, in addition to the known effects of pre-acceleration and potential deformation present in the adiabatic regime, there is also energy mixing, reminiscent of the Franz-Keldysh effect in the nonadiabatic (impulse) regime. Specifically, the pulse Ax(t) can enhance tunneling by effectively propelling a portion of the wave function beyond the rear end of the barrier.

For practical experimental applications, especially in solid-state physics, we examine a simplified model utilizing a rectangular potential. This model can also be validated by comparing it with analytical results. Additionally, we explore the truncated Coulomb potential, which holds relevance in the context of nuclear fusion.

Click here to access the recording.

Worldline_FriedemannQueisser.pdf

Francois Fillion-Gourdeau

Title of the talk:

Graphene, a quantum simulator for some high-energy physics processes

Abstract:

The Schwinger effect and pair production from supercritical fields (vacuum collapse) are long-lasting predictions of quantum electrodynamics (QED) which involve the spontaneous creation of electron-positron pairs in the presence of strong electric fields. Numerous theoretical and experimental proposals, employing various field configurations, have been advanced to detect these mechanisms. However, thus far, they have remained elusive in experimental investigations primarily due to the
insignificant particle production rate achievable with current laboratory-generated field configurations.

In this presentation, I will present how graphene, the quintessential 2D Dirac material, can serve as a
platform for studying these processes, exemplifying a quantum simulation 'à la Feynman.' I will elaborate the formal analogy that exists between this condensed matter system and QED, while also
highlighting some of its limitations and potential challenges. Additionally, I will describe the correspondence between certain quantum gravity models with minimal length and the tight-binding model of graphene, when including contributions from next-nearest neighbors. Leveraging this analogy, I will explore 'quantum-gravity-like' corrections to Klein tunneling.

Click here to access the recording.

Talk_Worldlineseminar_Francois.pdf

Matteo Tamburini

Title of the talk:

Theory and Numerical Methods of Strong-Field QED

Abstract:

Strong-field QED processes are central in determining the dynamics of particles and plasmas in extreme electromagnetic fields such as those generated with ultraintense lasers, dense charged particle beams, or present in the vicinity of compact astrophysical objects such as pulsars and magnetars. The advent of a new generation of multi-petawatt lasers as well as accelerator facilities capable of delivering high-energy and high-density beams enables for the first time the investigation of SFQED processes in the laboratory. In this talk I will present recent

simulation results in a so far unexplored regime, where a mildly relativistic electron-positron bunch collides with an intense laser pulse, showing that the inclusion of interparticle fields can coherently amplify emission in a broad range of radiated frequencies. In addition, I will discuss a new route for probing strong-field QED, where a single dense electron beam collides with a solid-density plasma thereby creating strong fields capable of triggering a QED cascade with efficient conversion of the beam kinetic energy into energetic photons and electron-positron pairs.

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Daniel Seipt

Title of the talk:

Nonlinear Compton Scattering: From fundamental strong-field QED to radiation sources

Abstract:

In Nonlinear Compton (NLC) scattering an electron emits a high-energy photon while interacting with an ultra-strong electromagnetic field. This is one of the fundamental processes of strong-field QED. Experimentally this process has been investigated in the famous SLAC E-144 experiment and more recently with LWFA electrons at various laser facilities, with several new experiments being in the planning stage. In this talk I will discuss the phenomena occurring due to the finite duration and how they affect the spectra of the emitted photons. These effects have profound consequences for the application of NLC as a narrowband radiation source since they typically lead to massive spectral broadening. We have developed a novel scheme in which optimally chirped laser pulses can be used to reduce the broadening effects. I will discuss several options to implement this concept, some of which make use of catastrophe theory to find the optimal parameters.

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Gianluca Degli Esposti

Title of the talk:

On the pair production spectrum from open worldline instantons

Abstract:

We show how to use the worldline-instanton formalism to calculate the momentum spectrum of the electron-positron pairs produced by an electric field that depends on both space and time. Using the LSZ reduction formula with a worldline representation for the propagator in a spacetime field, we make use of the saddle-point method to obtain a semiclassical approximation of the pair-production spectrum. In order to check the final result, we integrate the spectrum and compare with the results obtained using a previous instanton method for the imaginary part of the effective action.

Click here to access the recording.

Worldline_presentation_Gianluca.pdf

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