Physics in Intense Fields (PIF24)

26 Aug 2024, 08:00 → 30 Aug 2024, 18:00 Europe/London

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Overview

Physics in intense, or strong, background fields has a rich history, captivating researchers since the early days of QFT. Modern advances in high-intensity lasers have prompted a renewed interest in the area of strong-field QED (SFQED), where non-perturbative methods have been necessitated by increased laser field strength. "Physics in Intense Fields" aims to explore SFQED, high-intensity laser physics, and related themes, by bringing together researchers from theory, experiment, and simulation.

PIF24 is hosted online jointly by University of Plymouth and University of Edinburgh, and is preceded by earlier Physics in Intense Fields conferences:

• PIF22 (virtual) (hosted by the University of Plymouth),
• PIF2013 in DESY, Germany in 2013,
• PIF2010 in KEK, Japan in 2010.

We especially encourage participation and contributions from early career researchers. The first session of the conference will be devoted to tutorials on the three pillars of SFQED: Theory, Simulation and Experiment. These tutorials are aimed at Master's / PhD students who are relatively new to this area of research.

Format

Venue: This year, the conference will be held entirely online via Zoom. Zoom links will be sent directly to registered participants shortly before the conference begins.

There is no registration fee for attendance.

Message Board: Zulip will be used for discussions amongst participants and speakers. The link will be provided to registered participants.

Confirmed Speakers

• Tom Blackburn (Tutorial talk), University of Gothenburg, Sweden
• Sergei Bulanov (Overview talk), ELI Beamlines, Czech Republic
• Antonino Di Piazza (Overview talk), University of Rochester, USA
• Gerald Dunne (Overview talk), University of Connecticut, USA
• Ruth Jacobs (Overview talk), DESY, Germany
• Sebastian Meuren (Tutorial talk), SLAC, USA
• Daniel Seipt (Tutorial talk), Helmholtz-Institut Jena, Germany
• Henri Vincenti (Overview talk), Université Paris-Saclay, France

Organizers

Local Organizing Committee

• Patrick Copinger, University of Plymouth, UK
• James P. Edwards, University of Plymouth, UK
• Anton Ilderton, University of Edinburgh, UK
• Ben King, University of Plymouth, UK
• Karthik Rajeev, University of Edinburgh, UK

International Advisory Committee

• Laura Corner, University of Liverpool, UK
• Tom Heinzl, University of Plymouth, UK
• Holger Gies, Friedrich Schiller University Jena, Germany
• Sang Pyo Kim, Kunsan National University, Korea
• Stuart Mangles, Imperial College London, UK
• Mattias Marklund, University of Gothenburg, Sweden
• Caterina Riconda, Sorbonne Université, France
• Christopher Ridgers, University of York, UK

Monday, 26 August

Welcome

Tutorial

Convener: Chair: James P. Edwards

1 Introduction to Strong-Field QED Theory

A tutorial focussing on the theory of strong field QED - more details to follow.

Speaker: Daniel Seipt

10:05 Break

Tutorial

Convener: Chair: Ben King

2 Introduction to simulations of strong-field QED

Numerical simulations are an essential tool in plasma physics: they help us understand the dynamics of complex systems, interpret experimental results, and design novel radiation and particle sources. This is especially true in the high-intensity regime, where strong-field QED interactions affect, and are affected by, classical, collective plasma processes. In this talk I will give a tutorial overview into how we turn theoretical results into high-performance simulation codes, including the approximations we make, the computational challenges we deal with, the uncertainties we face, as well as some examples of how these simulations are being used in practice.

Speaker: Tom Blackburn (University of Gothenburg)

11:20 Break

Overview

Convener: Chair: Ben King

3 Plasma mirror-boosted lasers as a realistic path to the strong-field and fully non-perturbative regimes of QED

Speaker: Henri Vincenti

Tutorial

Convener: Chair: Anton Ilderton

4 Probing Strong-Field QED with Laser-Based Experiments

QED is often considered the most successful theory in physics, as it permits extremely precise predictions in the perturbative regime. In this tutorial talk, we will review experiments that focus coherent laser light into such small space-time volumes that a charged particle passing through the resulting photon density interacts with much more than just one photon, implying that conventional scattering theory breaks down and that the particle is probing the collective electromagnetic field of the laser. If the charge experiences an electric field that is comparable to or larger than the so-called QED critical (“Schwinger”) field in its own rest frame, the domain of Strong-Field QED is entered [1,2].

This talk will focus on electron-laser collisions which are currently being used in various experiments to probe SFQED, such as E-320 at SLAC/FACET-II [3], LUXE [4], and several LWFA-based campaigns (see, e.g., [5]). To this end, the theoretical description of electron and photon beams will be reviewed with a strong focus on the Gaussian approximation. In
particular, we will discuss the challenges of properly synchronizing these beams in both space and time. Furthermore, diagnostics are covered that are capable of delivering shot-to-shot feedback about the actual collision conditions. Finally, we will consider different observables,
with a focus on the scattered electron spectrum and the emitted gamma radiation, and go over detectors that can measure them in practice.

At the end of the talk we will also comment on challenges and opportunities for measuring more extreme parameters, e.g., QED cascades/showers at multi-PW lasers such as Apollon in
France [6,7] or probing the extreme quantum regime using, e.g., beam-beam collisions [8], as well as opportunities enabled by relativistic plasma mirrors [9].

[1] A. Fedotov et al., Advances in QED with intense background fields, Phys. Rep. 1010, 1 (2023).
[2] A. Gonoskov et al., Charged particle motion and radiation in strong electromagnetic fields, Rev. Mod. Phys. 94, 045001 (2022).
[3] T. Smorodnikova (for the E-320 collaboration), talk at this meeting (2024).
[4] H. Abramowicz et al., Technical Design Report for the LUXE Experiment, arXiv:2308.00515 (2023).
[5] E. E. Los et al., Observation of quantum effects on radiation reaction in strong fields, arXiv:2407.12071 (2023).
[6] D. N. Papadopoulos et al., The Apollon laser facility upgrade to the multi-PW level, High-Brightness Sources and Light-Driven Interactions Congress HTu2B.2 (2024).
[7] M. Pouyez et al., Multiplicity of electron- and photon-seeded electromagnetic showers at multi-petawatt laser facilities, arXiv:2402.04501 (2023).
[8] V. Yakimenko et al., Prospect of Studying Nonperturbative QED with Beam-Beam Collisions, Phys. Rev. Lett. 122, 190404 (2019).
[9] N. Za¨ım et al., Light-Matter Interaction near the Schwinger Limit Using Tightly Focused Doppler-Boosted Lasers, Phys. Rev. Lett. 132, 175002 (2024).

Speaker: Sebastian Meuren

Tuesday, 27 August

Overview

5 Flying focus fields as a tool for strong-field QED and the multipetawatt laser facility NSF OPAL

Speaker: Antonino Di Piazza

General Talk 1

6 S-matrix approach to Sauter-Schwinger pair production

We present a newly developed method for describing dynamical Sauter-Schwinger process using the S-matrix approach and reduction formulas [1]. The method is based on solving the Dirac equation with Feynmann or anti-Feynmann boundary conditions [2]. It leads to spin-resolved (helicity-resolved) probability amplitudes of produced electrons and positrons. With this approach, after summing up over spin (helicity) configurations, we are able to reproduce
the momentum distributions of particles calculated with other methods used in this context, such as the Dirac-Heisenberg-Wigner approach. Our method, however, provides access to
the information about the probability amplitude phase, which allows us to investigate vortex structures in pair creation [3]. In addition, we gain the information about the electron and
positron spins (helicities), which makes it possible to study their correlations.

[1] M. M. Majczak, K. Krajewska, J. Z. Kamiński, and A. Bechler, Scattering matrix approach to dynamical Sauter-Schwinger process: Spin- and helicity-resolved momentum distributions, arXiv:2403.15206 (2024).
[2] I. Białynicki-Birula and Z. Białynicka-Birula, Quantum Electrodynamics (Pergamon, Oxford,1975).
[3] A. Bechler, F. Cajiao Vélez, K. Krajewska, and J. Z. Kamiński, Vortex Structures and Momentum Sharing in Dynamic Sauter–Schwinger Process, Acta Phys. Pol. 143, S18 (2023).

Speaker: Kasia Krajewska

14:55 Break

General Talk 1

Convener: Chair: Kasia Krajewska

7 Time Scales in Particle Production from Quantum Transport Theory

Strong-field quantum electrodynamics is the epitome of what happens when a quantum many-body system is pushed to the extreme. It tests our understanding of non-equilibrium physics, fundamental particle physics and everything we think we know about emergent phenomena and collective dynamics [1]. In particular, identifying the formation time of an electron-positron pair within a background field remains a challenge [2,3].

We propose a new interpretation of the time evolution of a quantum system in which the quantum vacuum is driven out of equilibrium by strong electromagnetic fields, creating massive particles in the process [4]. As a demonstration, we show a particle distribution at finite times where we identify the time scales relevant for particle formation, in particular for the Schwinger effect. In this context, the merits of quantum transport theories, and in particular the Wigner formalism as a quantum kinetic theory, are discussed [5,6].

[1] C. Kohlfürst, N. Ahmadiniaz, J. Oertel, and R. Schützhold, Phys. Rev. Lett. 129, 241801 (2022).
[2] A. Ilderton, Phys. Rev. D 105, 016021 (2022).
[3] C. Gong, Q. Su, and R. Grobe, Phys. Rev. A 109, 013102 (2024).
[4] M. Diez, R. Alkofer, and C. Kohlfürst, Phys. Lett. B 844, 138063 (2023).
[5] D. Vasak, M. Gyulassy and H.T. Elze, Ann. Phys. (N.Y.) 173, 462 (1987).
[6] I. Bialynicki-Birula, P. Górnicki, and J. Rafelski, Phys. Rev. D 44, 1825 (1991).

Speaker: Dr Christian Kohlfürst (Helmholtz-Zentrum Dresden-Rossendorf)

8 Dynamically-assisted Breit-Wheeler pair production in high-intensity laser fields

Production of electron-positron pairs by a high-energy γ photon and a bichromatic laser wave is considered where the latter is composed of a strong low-frequency and a weak high-frequency component, both with circular polarization. An expression for the production rate is derived that accounts for the strong laser mode to all orders and for the weak laser mode to first order. The structure of this formula resembles the well-known expression for the nonlinear Breit-Wheeler process in a strong laser field, but includes the dynamical assistance from the weak laser mode. We analyze the dependence of the dynamical rate enhancement on the applied field parameters and show, in particular, that it is substantially higher when the two laser modes have opposite helicity.

Speaker: Selym Villalba Chavez (Heinrich Heine U., Dusseldorf)

16:05 Break

General Talk 1

Convener: Chair: Arseny Mironov

9 SFQED at future lepton colliders: science case and design contributions

It is widely accepted that the next lepton collider beyond a Higgs factory would require
center-of-mass energy of the order of up to 15 TeV. Since, given reasonable space and cost restrictions,
conventional accelerator technology reaches its limits near this energy, high-gradient advanced
acceleration concepts are attractive. Advanced and novel accelerators (ANAs) are leading candidates
due to their ability to produce acceleration gradients on the order of 1–100GV/m, leading to compact
acceleration facilities. However, intermediate energy facilities (IEF) are required to test the critical technology elements on the way towards multi-TeV-class collliders. Here we discuss a science case for a 20–100 GeV center-of-mass energy ANA-based lepton collider that can be a candidate for an intermediate energy facility is presented as well as the contributions of the SFQED processes studies to this science case and the design of a future lepton collider.

Speaker: Stepan Bulanov

10 Assisted neutrino pair production in combined external fields

Neutrino–antineutrino pair production is a key process contributing to energy loss in stars. While previous studies have examined pair creation via the collision of two (γγ→ννˉ) or three (γγγ→ννˉ) real photons, as well as photon collisions in the presence of nuclear Coulomb fields or external magnetic fields, this talk explores a new scenario.

We investigate the pair production of neutrinos and antineutrinos from a low-energy photon in the presence of a combined homogeneous magnetic field and the Coulomb field of a nucleus with charge Z.

Speaker: Naser Ahmadiniaz

Wednesday, 28 August

Overview

Convener: Chair: Laura Corner

11 Probing non-perturbative QED with LUXE and prospects for future SF-QED experiments at DESY

The proposed LUXE experiment (Laser Und XFEL Experiment) at DESY, Hamburg, using the 16.5 GeV electron beam extracted from the European XFEL, aims to study collisions between high-intensity laser pulses and high-energy electron or secondary photon beams. This will elucidate quantum electrodynamics (QED) at the strong-field frontier, where the electromagnetic field of the laser in the probe particle rest frame is above the Schwinger limit. In this regime, QED is non-perturbative. This manifests itself in the creation of physical electron-positron pairs from the QED vacuum, similar to Hawking radiation from black holes. LUXE intends to measure the positron production rate in an unprecedented laser intensity regime. This talk will discuss the physics goals and experimental challenges of LUXE, and give an overview of ideas for further on-site experiments on strong-field QED at DESY.

Speaker: Ruth Jacobs

General Talk 2

Convener: Chair: Laura Corner

12 Observation of quantum effects on radiation reaction in strong fields

Radiation reaction is the force experienced by an accelerated charge due to radiation emission. Quantum effects dominate charge dynamics, radiation production and radiation reaction for charges accelerated by strong fields. Such fields exist in astrophysical environments such as pulsar magnetospheres, may be accessed by high-power laser systems, and are expected at the interaction point of next generation particle colliders. Classical radiation reaction theories violate energy-momentum conservation and omit stochastic effects inherent in photon emission, thus demanding a quantum treatment. Two quantum radiation reaction models, the quantum-continuous and quantum-stochastic models, correct the former issue, while only the quantum-stochastic model incorporates stochasticity. Such models are of fundamental importance, providing insight into the effect of the electron self-force on its dynamics in electromagnetic fields. The difficulty of accessing conditions where quantum effects dominate inhibited previous efforts to observe such effects on charged particles with high significance.
We report the first highly significant (> 5σ) direct observation of strong-field radiation reaction on charged particles. Furthermore, we obtain strong evidence favouring the quantum radiation reaction models, which perform equivalently, over the classical model. Robust model comparison was facilitated by a novel Bayesian framework which inferred unknown collision parameters. This framework has widespread utility for experiments in which parameters governing lepton-laser collisions cannot be directly measured, including those utilising conventional accelerators.

Speaker: Eva Los

09:25 Break

General Talk 2

Convener: Chair: Matteo Tamburini

13 Growth rate and steady-sate distribution function of a QED cascade

Quantum electrodynamics (QED) cascades describe the exponential growth of an electron-positron-photon plasma in an intense electromagnetic (EM) field. Exponential growth is only possible if both electric and magnetic fields are present. In a nutshell, the electric field allows for every new generation of leptons to be re-accelerated after their creation. In contrast, the magnetic field plays a key role in the emitted radiation as well as in the decay of the photons. In the laboratory, QED cascades can be triggered in a standing EM wave which results from the overlap of two lasers [1]. In Astrophysics, the magnetospheres of rotating astrophysical compact objects such as neutron stars can become charge-starved, giving rise to the formation of vacuum gaps in which pair plasma is produced [2].
There has been a great endeavor in the past decade to study these cascades (i) analytically with semi-phenomenological models or (ii) numerically from first principles with PIC codes. The main conclusions of these numerical works indicate that a sort of universal behavior is reached during the exponential phase (before screening) characterized by a steady-state distribution function.
However, a rigorous kinetic description is still lacking. We will show here that it is possible in some special cases (purely rotating field for the laboratory and static electric field and curved magnetic field for astrophysics) to integrate the Boltzmann equation and extract perturbatively exact solutions for the growth rate and the steady-state distribution function. Beyond the academic technicity of the problem, this result is important to better understand the development of the cascade, to provide analytical results for codes benchmark, and finally allows for finding heuristic rates for pair creation in given EM fields which can be further used in plasma code (thus alleviating the complexity of full QED implementation).
[1] A. R. Bell & J. G. Kirk, PRL 101, 200403 (2008), A. M. Fedotov et al., PRL 105, 080402 (2010), V. F. Bashmakov et al., Phys. Plasmas 21, 013105 (2014), T. Grismayer et al., Phys. Rev. E 95, 023210 (2017).
[2] A.N. Timokhin, MNRAS. 408, 2092 (2010), F. Cruz et al., ApJ, 908, 149 (2021), A. Philippov et al., PRL 124, 245101 (2020), F. Cruz et al., ApJ Lett. 124, 245101 (2021)

Speaker: Thomas Grismayer (Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico-Universidade de Lisboa, Lisbon, Portugal)

14 Dynamical Screening of ultra-intense Fields in QED plasmas

Plasmas immersed in ultra-intense electromagnetic fields radiate through QED channels high-energy photons which can decay into pairs. These pairs, if created in sufficient amounts, can significantly impact the background field. This is especially true in the context of QED avalanches created by intense lasers, where the exponentially growing density will eventually reach a critical value whereupon the laser field starts to be depleted [1-2].

In this work, we propose a model to address the response of the pair plasma on the background field in the weak field limit ($\chi \ll 1$), where the effect of radiation can be treated with classical friction. Starting from Ampère’s equation and the equations of motions of charges, we obtain a system of differential equations on particle momenta and the total electric field. The analytical model is compared with Particle-In-Cell simulations and is found to be in good agreement.

Furthermore, using dynamical systems analysis tools, we can provide analytical predictions about the asymptotic response of the plasma. For instance, we can give the necessary conditions for significant screening of in constant and oscillating external field, which is consistent with previous numerical works [1-2].

[1] Grismayer, T., et al. (2016). Physics of Plasmas, 23(5), 056706
[2] Nerush, E. N., et al. (2011) Phys. Rev. Lett. 106, 035001

Speaker: Anthony Mercuri-Baron (GoLP / IPFN, Instituto Superior Técnico)

15 Finite beaming effect on QED cascades

The quantum electrodynamic (QED) theory predicts the photon emission and pair creation in QED cascades mainly occur in a forward cone with finite angular spread $\Delta\theta \sim 1/\gamma_{i}$ along the momenta of incoming particles. This finite beaming effect has been assumed to be negligible because of the particles' ultra-relativistic Lorentz factor $\gamma_{i}\gg1$ in laser-driven QED cascades. We develop an energy- and angularly resolved particle-tracking code, and prove that accounting the finite angular spread in the outgoing particle momentum can improve substantially the agreement between the simulation and exact QED results. The narrow beaming in each QED event could be accumulated to significantly reduce the growth of particle number in the long-term development of QED cascades, but can hardly affect the early formation of the cascades. For QED cascades driven by two counter-propagating circularly polarized laser pulses, the finite beaming effect could decrease the final yield of the particles, especially at ultrahigh intensities by more than one order of magnitude.

Speaker: Dr Suo Tang (Ocean University of China)

11:00 Break

General Talk 2

Convener: Chair: Patrick Copinger

16 On the theory of avalanche-type QED cascades in strong electromagnetic fields

Since the initial prediction of the onset of selfsustained QED cascades upon injection of particles in certain electromagnetic field configurations [1], the pivotal theory questions remain the same: (i) What are the general conditions required to trigger such cascades? (ii) How many electron-positron pairs can be produced? (iii) What is the threshold of the phenomenon, particularly in the context of future experiments with ultra-high-intensity lasers? These questions appear to be surprisingly hard to tackle analytically beyond estimates proposed for simple field configurations (like a rotating electric field) [2, 3].

We will talk about our recent advancement in the avalanche-type cascade theory [4]. We consider the problem in a general field. While we base on familiar grounds - the kinetic approach [2] and semiclassic analysis of particle trajectories [5] within LCFA - careful treatment of these ingredients allowed us to build a new predictive model for the particle growth rates. We provide a simple formula applicable to a broad class of field configurations and robust in the full range of field strengths (e.g. $a_0$ ranging from $500$ to $10^5$). It shows excellent agreement with simulations for realistic field models that combine one or more strongly focused laser beams and can be used to identify the conditions required to generate dense electron-positron plasma.

In a general field configuration accounting for the electron and photon migration from a finite-sized strong field region appears to be crucial, in particular in the (relatively) low-field interaction regime, e.g. $a_0 < 1000$. As a consequence, a hard threshold can be derived for the cascade onset in focused laser fields. This effect was seen in simulations and not predicted by preexisting cascade models and is particularly important for planning upcoming experiments at multi-petawatt laser facilities, which will have intensity near the threshold.

[1] A. R. Bell and J. G. Kirk, PRL 101, 200403 (2008).
[2] N. V. Elkina, A. M. Fedotov, I. Y. Kostyukov, M. V. Legkov, N. B. Narozhny, PR STAB 14, 054401 (2011).
[3] T. Grismayer, M. Vranic, J. L. Martins, R. A. Fonseca, and L. O. Silva, PRE 95, 023210 (2017).
[4] A. Mercuri-Baron, A.A, Mironov, C. Riconda, A. Grassi, M. Grech, arXiv:2402.04225 (2024).
[5] A. A. Mironov, E. G. Gelfer, and A. M. Fedotov, PRA 104, 012221 (2021).

Speaker: Dr Arseny Mironov (LULI, Sorbonne University, CNRS, CEA, Ecole Polytechnique, Paris, France)

17 Vacuum HHG in strong-field QED

I discuss high-harmonic generation from the vacuum in strong-field QED. Using the semi-classical approximation, I explicitly calculate the number of photons and the resulting harmonic spectrum in the presence of a spatially uniform linearly-polarized AC electric field, at the leading order in the fine-structure constant $\alpha$. I briefly compare the obtained results with the previous ones, e.g., a tadpole contribution in two-dimensional Dirac material [1] and the ${\mathcal O}(\alpha^2)$ contribution within the Euler-Heisenberg approach [2,3].

[1] Taya, Hongo, Ikeda, Phys. Rev. B 104, L140305 (2021).
[2] Piazza, Hatsagortsyan, Keitel, Phys. Rev. D 72, 085005 (2005).
[3] Fedotov, Narozhny, Phys. Lett. A 362, 1 (2007).

Speaker: Hidetoshi Taya (Keio University)

Conference Photo

Overview

Convener: Chair: Holger Gies

18 tba

Speaker: Gerald Dunne

General Talk 3

Convener: Chair: Holger Gies

19 A glimpse on higher-loop contributions to the Heisenberg-Euler effective action

In an attempt to obtain any controlled insights into the low-energy effective theory governing the physics of classical electromagnetic fields in the quantum vacuum beyond low loop orders, we study the Heisenberg-Euler effective action in constant electromagnetic fields $\bar{F}$ for quantum electrodynamics (QED) with $N$ charged particle flavors of the same mass and charge $e$ in the large $N$ limit characterized by sending $N\to\infty$ while keeping $Ne^2\sim e\bar{F}\sim N^0$ fixed. This immediately implies that contributions that scale with inverse powers of $N$ can be neglected and the resulting effective action scales linearly with $N$. Interestingly, due to the presence of one-particle reducible diagrams, even in this specific limit the Heisenberg-Euler effective action receives contributions of arbitrary loop order. In particular for the special cases of electric- and magnetic-like field configurations we construct an explicit expression for the associated effective Lagrangian that, upon extremization for two constant scalar coefficients, allows to evaluate its full, all-order result at arbitrarily large field strengths. We also comment on relations to physically realized $N=1$ QED.

Speaker: Felix Karbstein

14:55 Break

General Talk 3

Convener: Chair: Karthik Rajeev

20 Coherent radiation of electron bunch traversing laser pulse

Coherency can crucially modify the properties of radiation emitted by many particles compared to a single particle. We study the conditions for coherent radiation of an electron bunch driven by a counterpropagating strong pulsed electromagnetic plane wave [1,2]. We derive the spectrum of the coherent radiation and show that it is emitted backwards with respect to the laser propagation direction and has a very narrow angular spread. We demonstrate that for a solid density plasma coherent radiation extends to frequencies up to hundreds of keV thereby enhancing the low-frequency part of the spectrum by many orders of magnitude. Our analytical findings are tested with 3D particle-in-cell simulations of an electron bunch passing through a laser pulse, clearly demonstrating how the coherence can essentially modify the observed radiation spectrum.

References
[1] E.G. Gelfer, A.M. Fedotov, O. Klimo, S. Weber, arXiv:2306.16945 (2023).
[2] E.G. Gelfer, A.M. Fedotov, O. Klimo, S. Weber, MRE 9, 024201 (2024).
[3] J. Derouillat,et. al., Comput. Phys. Commun. 222, 351 (2018).

Speaker: Evgeny Gelfer (ELI Beamlines facility, The Extreme Light Infrastructure ERIC)

21 Quantum simulations for strong field QED

Quantum field theory in the presence of strong background fields contains interesting problems where quantum computers may someday provide a valuable computational resource. In the noisy intermediate-scale quantum (NISQ) era it is useful to consider simpler benchmark problems in order to develop feasible approaches, identify critical limitations of current hardware, and build new simulation tools. In this talk, I will demonstrate quantum simulations of strong-field QED (SFQED) in 3+1 dimensions, using real-time nonlinear Breit-Wheeler pair-production as a prototypical process. The SFQED Hamiltonian is derived and truncated in the Furry-Volkov mode expansion, and the interactions relevant for Breit-Wheeler are transformed into a quantum circuit. Quantum simulations of a “null double slit” experiment are found to agree well with classical simulations following the application of various error mitigation strategies, including an asymmetric depolarization algorithm which we develop and adapt to the case of Trotterization with a time-dependent Hamiltonian. We also discuss longer-term goals for the quantum simulation of SFQED.

Speaker: Luis Hidalgo (The University of Illinois)

16:05 Break

Short Talk 1

Convener: Chair: Alexander Macleod

22 Experiment 320 at SLAC: Probing Strong-Field QED in Laser-Ultrarelativistic Electron Beam Collisions

The SLAC experiment 320 (E-320) collaboration is probing Strong-Field QED (SFQED), by colliding a $\sim 10\, \textrm{GeV}$ electron beam, generated by the FACET-II linear accelerator, with $\sim 10\, \textrm{TW}$ NIR laser pulses [1,2]. The main objectives of E-320 are: a) the investigation of the transition from the multiphoton to the strong background-field regime, where the laser can no longer be treated perturbatively, and b) the generation of electron-positron pairs through the nonlinear Breit-Wheeler mechanism in the deep tunneling regime [3,4].

In this talk, we will report our preliminary analysis of the experimental data obtained in the spring of 2024. We have clearly observed a strongly nonlinear Compton signal in the spectra of scattered electrons and an intensity-dependent redshift of the Compton edges, leading to a quasi-continuous, quantum-corrected synchrotron spectrum. Based on our preliminary estimates, we observed collisions with an averaged quantum nonlinearity parameter $\chi$ of at least 0.12, which implies that we have likely observed quantum radiation reaction [4].

[1] V. Yakimenko et al., Phys. Rev. Accel. Beams, 101301 (2019)
[2] S. Meuren (for the E-320 collaboration), talk at FACET-II PAC Meeting(2022)
[3] A. Fedotov et al., Phys. Rep. 1010, 1 (2023)
[4] A. Gonoskov et al., Rev. Mod. Phys. 94, 045001 (2022)

This work was supported by the U.S. Department of Energy under award DE-SC0020076. FACET-II is supported by the U.S. Department of Energy under Contract No. DE-AC02-76SF00515.

Speaker: Tatiana Smorodnikova (Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, CA, USA)

23 tba

Speaker: Miles Radford

Thursday, 29 August

Overview

Convener: Chair: Antonino DiPiazza

24 tba

Speaker: Sergei Bulanov

General Talk 4

Convener: Chair: Antonino DiPiazza

25 Spectral and spatial high-resolution characterisation of high-flux and high-energy gamma-ray beams from intense laser-electron beam interactions

At the frontier of ultra-high electromagnetic intensities, it is now possible to access peak laser intensities of up to ∼ $10^{23}$ W/cm$^2$ [1], with even higher intensities envisaged at upcoming multi-petawatt class facilities [2]. The interaction of an ultra-relativistic electron beam with electromagnetic fields of this magnitude represents an ideal experimental configuration to access unexplored regimes of strong-field quantum electrodynamics (SFQED), where the transition from perturbative to non-perturbative processes, as well as the transition from classical to quantum dynamics, occurs [3,4]. Remarkably, formulation and calculation of a first-principle and accurate theory for the dynamics of an electron in an external electromagnetic field of arbitrary intensity is still one of the most fundamental outstanding problems in electrodynamics. The spectral and spatial properties of the Compton photon beams [5] during this type of interactions are known to carry precious information on the interaction; however, a high-resolution detection of the spatial and spectral distribution of such photon beams is experimentally challenging due to their high energy per photon and their high flux.
Here we present the recent development of a high-energy gamma-ray spectrometer [6,7] and a gamma-ray profiler [8,9] suited for this challenging task. Numerical modelling and proof-of-concept experimental tests indicate the possibility of measuring the spectrum and spatial profile of GeV-scale gamma-ray beams with %-level and micron-scale precision, respectively. These diagnostics will be included in several SFQED experimental platforms [10,11] and are expected to represent key diagnostics for this class of experiments.

[1] J. W. Yoon et al., Optica 8,630 (2021)
[2] C. N. Danson et al., High Power Laser Sci. Eng. 7,e54 (2019)
[3] K. Poder et al., Phys. Rev. X 8, 031004 (2018)
[4] J. Cole et al., Phys. Rev. X 8, 011020 (2018)
[5] G. Sarri et al., Phys. Rev. Lett. 113, 224801 (2014)
[6] K. Fleck et al., Scientific Reports 10, 9894 (2020)
[7] N. Cavanagh et al., Phys. Rev. Res. 5, 043046 (2023)
[8] K. Fleck et al., Phys. Rev. A accepted (2024)
[9] P. Grutta et al., NIM-A submitted (2024)
[10] H. Abramowicz et al., Eur. Phys. J. Special Topics 230, 2445 (2021)
[11] S. Meuren et al., Probing Strong-field QED at FACET-II (SLAC E320)

Speaker: Prof. Gianlucca Sarri (Queen's College Belfast)

14:55 Break

General Talk 4

Convener: Chair: Tom Heinzl

26 Searching for axion resonances in vacuum birefringence with three-beam collisions

We consider birefringent (i.e., polarization changing) scattering of x-ray photons at the superposition of two optical laser beams of ultrahigh intensity and study the resonant contributions of axions or axionlike particles, which could also be short-lived. Applying the specifications of the Helmholtz International Beamline for Extreme Fields (HIBEF), we find that this setup can be more sensitive than previous light-by-light scattering (birefringence) or light-shining-through-wall experiments in a certain domain of parameter space. By changing the pump and probe laser orientations and frequencies, one can even scan different axion masses, i.e., chart the axion propagator.

Speaker: Ralf Schützhold (Institut für Theoretische Physik, Technische Universität Dresden, Dresden, Germany)

27 Fundamental constants from photon-photon scattering in three-beam collisions

Direct measurement of the elastic scattering of real photons on an electromagnetic field would allow the fundamental low-energy constants of quantum electrodynamics (QED) to be experimentally determined. We show that scenarios involving the collision of three laser beams have several advantages over conventional two-beam scenarios. The kinematics of a three-beam collision allows for a higher signal-to-noise ratio in the detection region, without the need for polarimetry and separates out contributions from different orders of photon scattering. A planar configuration of colliding a photon beam from an x-ray free electron laser with two optical beams is studied in detail. We show a measurement of elastic photon scattering and vacuum birefringence is possible with current technology.

Speaker: Alexander Macleod (ELI Beamlines Facility, ELI ERIC)

16:05 Break

General Talk 4

Convener: Chair: Felix Karbstein

28 Photon amplitudes in strong background fields

I summarize recent progress in the application of the worldline formalism to the calculation of one-loop photon amplitudes in strong-field QED with a non-perturbative treatment of the background field. The examples include constant, plane-wave, Redmond, Coulomb and Sauter fields.

Speaker: Dr Christian Schubert (Facultad de Ciencias Físico-Matemáticas, Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Michoacán, Mexico)

29 Analytical formula for signal optimization in stimulated photon-photon scattering setup with three laser pulses

Strong electromagnetic fields perturb the quantum fluctuations, thereby modifying the properties of the vacuum. This effect is called vacuum polarization. In QED, the thus emergent interaction is described by the second and the omitted higher-order terms in the Heisenberg-Euler action
$\qquad \qquad \qquad S = \! \int d^4x \left( \frac{\mathfrak{F}}{4\pi} + \frac{\alpha}{360 \pi^2 E_c^2} \times (4 \mathfrak{F}^2 + 7 \mathfrak{G}^2) + \dots \right),$
where $\mathfrak{F} = (E^2 - H^2) / 2$ and $\mathfrak{G} = \mathbf{E} \cdot \mathbf{H}$ are electromagnetic field invariants, ${E_c = 1.3 \cdot 10^{16} \, \text{V/cm}}$ is the critical field, and $\alpha$ is the fine structure constant. Radiative corrections $\mathcal{O}(\alpha)$ in $S$ introduce nonlinearity into the Maxwell's equations which results in a wide range of nonlinear optical phenomena, in particular photon-photon scattering. Their direct observation and measurement remain a long-standing challenge.

The most prospective all-optical setup involving three overlapping focused laser pulses has been studied since quite long ago [1]. Here a detectable signature reveals as an emission of the polarized vacuum which can be also viewed as a stimulated elastic photon-photon scattering. Combining three incoming pulses drastically stimulates the effect as compared to the earlier proposed two-pulse setups making it potentially observable at the currently or near-future available laser facilities. However, the collision geometries studied earlier were not optimized with respect to the direction of the emitted signal photons. Furthermore, the distributions of signal photons and their total number have been calculated only numerically for a partially or completely specified configuration of the collision scheme [2,3,4].

Our work generalizes previous considerations of three-pulse setups for detecting photon-photon scattering by deriving a fully analytical and general formula [5,6] for the yield of signal photons.
$~$
[1] R. L. Dewar, Physical Review A 10, 2107 (1974).
[2] E. Lundström et al., Physical review letters 96, 083602 (2006).
[3] H. Gies et al., Physical Review D 97, 076002 (2018).
[4] B. King et al., Physical Review A 98, 023817 (2018).
[5] A. V. Berezin and A. M. Fedotov, Bulletin of the Lebedev Physics Institute, 50(6), 641-651 (2023).
[6] A. V. Berezin and A. M. Fedotov, Physical Review D 110, 016009 (2024).

Speaker: Arseniy Berezin (National Research Nuclear University MEPhI)

Friday, 30 August

General Talk 5

30 From one to many particles: Coherent emission and radiation reaction

The trajectories of relativistic particles in an intense electromagnetic field can be described by the Landau-Lifshitz equation, where the effect of radiation emission is accounted for via a self-force, and interparticle fields are often neglected as an approximation. Yet, the inclusion of interparticle fields is necessary to ensure energy-momentum conservation, particularly during coherent emission. It has been suggested that a bunch of particles radiating coherently would experience a coherently enhanced self-force. By simulating a neutral, relativistic bunch of electrons and positrons colliding with a laser pulse, we show that the inclusion of interparticle fields can significantly affect the particle dynamics primarily through the Lorentz force for the range of parameters studied. In these simulations, no coherently enhanced self-force was observed or needed to satisfy energy-momentum conservation.

Speaker: Matteo Tamburini (Max Planck Institute for Nuclear Physics)

31 Strong-field QED effects in pulsar emission

Highly magnetized neutron stars have quantum refraction effects on pulsar emission due to the non-
linearity of the quantum electrodynamics (QED) action. In this context, we investigate the strong-field
QED effects on the following properties in pulsar emission: (i) the propagation and polarization
vectors, (ii) the polarization states. With regard to (i), we determine the leading-order corrections to
the vectors due to quantum refraction via perturbation analysis. In addition, the QED effects on the
orthogonality between the vectors and the Faraday rotation angle are determined. With regard to (ii),
we solve a system of evolution equations of the polarization states, where the birefringent vector, in
which the QED effects are encoded, combined with the frequency of emission, acts on the Stokes
vector. At a fixed frequency of emission, depending on the magnitude of the birefringent vector, the
evolution of the polarization states largely exhibits three different patterns: monotonic, or half-
oscillatory, or highly oscillatory behaviours. For some practical examples of rotation-powered pulsars,
these features are shown by fully numerical solutions of the evolution equations, and also understood
and confirmed by means of approximate analytical solutions.

[1] Kim D.-H., Kim C. M., Kim S. P. (2024), Monthly Notices of the Royal Astronomical Society, 531, 2148
[2] Kim D.-H., Kim C. M., Kim S. P. (2024), arXiv:2406.05752

Speaker: Dong-Hoon Kim (Seoul National University)

09:50 Break

General Talk 5

Convener: Chair: Hidetoshi Taya

32 Extending limits of QED-PIC simulations

Despite some methodological limitations, particle-in-cell simulations can be used to analyze laser-driven QED cascades and dynamics of the emergent QED plasmas. These processes, however, can quickly run into extreme regimes in terms of number of particles and their density causing extraordinary computational demands. We discuss possibilities to overcome some key limitations of the PIC method by two improvements. The first improvement is a possibility to enforce exact energy conservation to suppress or even eliminate numerical heating and instabilities without field smoothing, high-order weighting as well as excessive computational costs related to large number of particles per cell and small time and space steps [1]. The second improvement concerns conservative and thereby non destructive down-sampling of particle ensemble during the development of QED cascades [2]. Both improvements are made available in an open-source Python package 𝜋-PIC [3] that is designed to support studies of QED cascades in extreme regimes, as well as data analysis of upcoming laser-based experiments on strong-field QED.

[1] A. Gonoskov, Explicit energy-conserving modification of relativistic PIC method, J. Comput. Phys., 502, 112820 (2024); arXiv:2302.01893
[2] A. Gonoskov, Agnostic conservative down-sampling for optimizing statistical representations and PIC simulations, Comput. Phys. Commun. 271,108200 (2022); arXiv:1607.03755
[3] 𝜋-PIC, https://github.com/hi-chi/pipic

Speaker: Arkady Gonoskov (University of Gothenburg)

33 A path towards seeding QED cascades using direct laser acceleration

Lasers at moderate intensities propagating through a plasma waveguide have demonstrated the potential for generating high-frequency radiation [1] as well as high-charge (>100 nC) electron beams [2] via direct laser acceleration (DLA). Such electron beams in a strong field background can emit multi-MeV photons, which have many potential applications in laboratory astrophysics, photonuclear spectroscopy, radiation therapy and radiosurgery, among others [3]. We are particularly interested in generating high-charge electron beams, and high-number photon beams with multi-GeV energy cutoff to seed electron-positron QED cascades and generate positron beams at high-power laser facilities. The path for optimizing the electron energy cutoff at any given laser facility is presented in our recent work, where we have computed the expected DLA electron energies for laser powers of 1 PW and higher [4]. We have already demonstrated with full-scale, quasi-3D particle-in-cell simulations that, using intense lasers, positrons can be created, injected and accelerated by DLA over hundreds of microns in a plasma channel [5]. We prove that positrons are guided along the channel central axis thanks to a high-charge self-loaded electron beam driven by the high laser intensity. This opens a path toward a new kind of compact and relativistic positron source, based on future-generation laser systems. DLA is therefore expected to play a central role for future QED plasma experiments, as it can couple with QED processes in a way that facilitates the energy transfer from the laser to the newly created particles, which can then be used for controlled seeding of further QED events, towards the ultimate goal of this quest: creating enough pairs to form an electron-positron plasma in the lab.

[1] S. Kneip et al, Phys. Rev. Lett. 100, 105006 (2008)
[2] A. E. Hussein et al., New J. Phys. 23, 023031 (2021)
[3] D. J. Stark et al.., Phys. Rev. Lett. 116, 185003 (2016)
[4] R. Babjak et al.Phys. Rev. Lett., 132, 125001 (2024)
[5] B. Martinez et al, Phys. Rev. Accel. Beams 26, 011301 (2023)

Speaker: Prof. Marija Vranic (Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico-Universidade de Lisboa, Lisbon, Portugal)

11:00 Break

Short Talk 2

Convener: Chair: Suo Tang

34 SF-QED showers in strongly magnetized environments

In extreme astrophysical environments such as neutron stars, pulsars and magnetars, magnetic fields can reach strengths as high as 1015 Gauss. Due to the fast rotation of the star, a very large electric field is associated with these strong magnetic fields which accelerates charged particles to energies from GeV to TeV and provides an excellent environment for so-called QED shower [1-2]. Subject to an intense electromagnetic field, an electron can emit high-energy photons (non-linear Compton scattering) that can decay into an electron-positron pair (non-linear Breit-Wheeler process), further contributing to the shower. It will develop until the emitted photon does have not enough energy to decay and the remaining photons will escape thus providing the main source of radiation from the magnetized environments.

An analytical model of the shower characteristics has been proposed in [3] but shown to be inadequate for quantitative predictions [4]. 30 years later, the number of produced pairs as a function of the interaction time, the initial particle energy and the magnetic field intensity is identified using a different approach based on our previous work [5]. Two scaling laws respectively valid at short times (before the electron distribution has significantly cooled down) and at long times (when the majority of the incident particle energy is exhausted) are derived.

A systematic study using a Monte Carlo code shows excellent agreement with our model predictions for the photon energy spectrum and evolution of the number of pairs. The proposed scalings laws are also applied to a time-dependent field and show excellent agreement with the particle-in-cell code SMILEI [5] for laser-particle scattering. The model has practical applications for beam-beam interaction, laser-driven showers in the laboratory, astrophysical observations of pulsar radiation or astrophysics simulations.

[1] Goldreich & Julian (1969). ApJ 157 , 869
[2] Daugherty & Harding (1982). ApJ 252 , 337
[3] Akhiezer et al. (1994). Phys. G Nucl. Part. Phys. 20 ,1499
[4] Anguelov & Vankov (1999) J. Phys. G: Nucl. Part. Phys. 25 , 1755
[5] Pouyez et al. (2024), arXiv preprint arXiv:2402.04501
[6] Derouillat et al. (2018), Comput. Phys. Commun. 222 , 351

Speaker: Mattys Pouyez (Sorbonne Université)

35 The effects of the spin magnetic moment on quantum radiation and radiation reaction in the high-$\chi_e$ regime

Objects such as pulsars, magnetars and black holes all have sufficiently strong magnetic fields to reach the Schwinger limit in which strong field quantum electrodynamics (SF-QED) effects become important. Understanding the effects
of SF-QED on radiation and radiation reaction, the recoil force experienced by a particle as it radiates remains an important investigation in the field of high intensity laser physics. With the advent of lasers capable of intensities of the
order $10^{23} Wcm^{−2}$, and using a relativistic electron beam, we are able to reach the Schwinger limit and begin to explore such effects. When we approach the quantum regime of emission, where the quantum parameter, $\chi_e$≥ 1, the classical model for radiation reaction begins to break down, and thus a quantum treatment is required. One aspect of this is emission due to the intrinsic magnetic moment of the electron. This spin radiation manifests itself through spin-light and spin-flips however previous study has shown the spin radiation contribution to be minimal, or in the case where the strong fields were provided by a crystal lattice rather than a laser, require a large electron beam energy to reach similar $\chi_e$. Here, we use electron-laser pulse collisions to demonstrate the importance of spin-effects by directly comparing identical simulations that differ only by the inclusion of the spin contribution. Furthermore, we construct a functional fit for the gaunt factor excluding the effects of spin, and compare it to the fit with spin to demonstrate the importance of spin effects as a function of $\chi_e$. Using this fit, we then demonstrate that we are able to accurately predict the evolution of the lorentz factor of the electrons to hence predict the difference in the radiation reaction. Using a collision between a 60 GeV electron beam with a laser with intensity $10^{23} Wcm^{−2}$, we demonstrate a qualitative and quantitative difference in the radiated energy spectrum, resulting in a 12% difference in the total number of photons produced when spin is included, with the difference in the number of photons with 20 GeV or higher being 23%. When examining the positrons we observe 14% more positrons when including spin radiation effects. When observing the average Lorentz factor of the electrons after radiating, we demonstrate a 45% reduction when spin emission is included.

Speaker: Louis Ingle (University Of York)

Closing Remarks