
chair: Naser Ahmadiniaz

09:00  09:45

Stepan Bulanov
(Lawrence Berkeley National Laboratory)
SF QED effects and plasma based collider for high energy physics studies
It is widely accepted that the next lepton collider beyond a Higgs factory would require centerofmass 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, highgradient 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 1–100 GV/m, leading to compact acceleration structures. However, intermediate facilities are required to test the technology and demonstrate key subsystems. A 20100 GeV centerofmass energy ANAbased lepton collider can be a possible candidate for an intermediate facility. Apart from being a test beam facility for accelerator and detector studies, this collider will provide opportunities to study muon and proton beam acceleration, make precision Quantum Chromodynamics and Beyond the Standard Model physics measurements, and investigate charged particle interactions with extreme electromagnetic fields in the quantum regime. There is a number of questions that need to be addressed in the course of designing such a facility. One of the most important is the positron beam generation, capture, and acceleration.

09:45  10:30

Marija Vranic (virtual)
(University of Lisbon)
Positron creation and acceleration with intense lasers
The next generation of pulsed lasers will have intensities in excess of $10^{23}~\mathrm{W/cm^2}$.
While propagating through a preformed plasma channel, a laser of such intensity allows for
direct laser acceleration (DLA) of leptons in the radiation reactiondominated regime.
The DLA scheme has already been demonstrated to provide highcharge electron beams (at a $\sim \mathrm{nC}$ level) with moderate
laser intensities ($\sim 10^{20}~\mathrm{W/cm^2}$).
In this work, we show what can be accomplished with nearfuture laser facilities.
We have found that increasing the laser power is bound to augment the charge content even further.
The field structure formed due to electron beam loading allows for accelerating positrons.
What is more, the interaction in the radiationdominated regime will provide a high flux of emitted photons, in the hard xray and gammaray range.
These photons can then be used as a seed for electronpositron pair creation, as well as a radiation source for applications. \\
This work was supported by FCT grants CEECIND/01906/2018, PTDC/FISPLA/3800/2021, FCT UI/BD/151560/2021 and ERC2015AdG Grant 695088. We acknowledge PRACE for granting access to MareNostrum in BSC, Spain.

10:30  11:00

Coffee break


chair: J. Tito Mendonca

11:00  11:45

Uwe Hernandez Acosta
(HelmholtzZentrum DresdenRossendorf)
First steps in QED cascades  the onset stage
While the high frequency of modern xray freeelectron lasers has the benefit of requiring less energy of a seed electron for triggering the development of a QED cascade, the nonlinearity parameter obeys a_0 < 1, in contrast to highintensity optical lasers.
Accordingly, we analyse the phenomenon of multiphoton effects in trident pairproduction in pulsed xray laser fields at such values of a_0.
The impact of the energy spectrum and its temporal structure and the coherence of the laser field on the emergent particle distribution at the onset of further cascading is discussed. Besides the evolution of mean multiplicities in the course of an energypowered cascade, we seek for characteristic fluctuation patterns.

11:45  12:30

Sergei Bulanov
(ELI Beamlines Dolni Brezany)
Nonlinear Electromagnetic Waves in Quantum Vacuum
Extremely intense electromagnetic fields modify the Maxwell equations due to the photonphoton scattering that makes the vacuum refraction index to depend on the field amplitude. The refraction index modification makes the ultrarelativistic electron to emit high energy photons via the Synergic CherenkovCompton radiation mechanism. With high power lasers the Synergic CherenkovCompton process can be observed by colliding the laser accelerated electrons with high intensity electromagnetic pulse. In the presence of electromagnetic waves with small but finite wavenumbers the vacuum behaves as a dispersive medium. The finite amplitude electromagnetic wave evolution in QED vacuum results in wave steepening, subsequent generation of high order harmonics and shock wave formation with electronpositron pair generation at the shock front. At extremely high photon energy end the vacuum refraction index tends to unity quenching the Cherenkov radiation. The interplay between the vacuum dispersion and the nonlinear effects in the interaction of electromagnetic waves results in the formation of solitons that can propagate without changing their shape.

12:30  13:30

Lunch break

13:30  14:30

Informal discussions


chair: Zheng Gong

14:30  15:00

Elias Gerstmayr
(Queen's University Belfast and SLAC National Laboratory)
Strongfield QED experiments at FACETII (E320)
The E320 collaboration at SLAC plans to collide the FACETII highenergy electron beam ($1013\,\mathrm{GeV}$) with intense laser pulses ($a_0 \sim 110) to access the nonperturbative regime of strongfield QED. In this regime the vacuum becomes unstable to pair production and novel phenomena are expected to occur. E320 aims to perform precision measurements of the fundamental strongfield processes pair production and photon emission from the perturbative to the nonperturbative regime. This talk will summarise the goals of the experimental program, describe the experimental setup, and provide an update on its ongoing commissioning and first beam times.

15:00  15:30

Christoffer Olofsson
(University of Gothenburg)
Concepts for studying strongfield QED using laserelectron colliders
Laser radiation of extreme intensities in conjunction with highenergy electron beams permit the study of radiation reaction (RR) in the quantum regime as well as effects of strongfield quantum electrodynamics (SFQED). These are quantified by the electron acceleration within its rest frame normalized to the Schwinger critical field, denoted $\chi$, where $1 \lesssim \chi \lesssim 1600$ delimit the quantum domain of RR and SFQED phenomena where the upper limit is the conjectured breakdown of perturbative nonlinear QED [1]. Probing theories from measurements in the high$\chi$ limit can be intractable since electrons can emit multiple times during interaction while high$\chi$ emissions from electrons may be indistinguishable to those at low$\chi$. Moreover, rapid cascade of electronpositron pairs inhibit the peak field being reached [2]. We present simulations of an experimental scheme using an optimal laser geometry that attain high values of $\chi$ and provide strategies to isolate the signal of electron emission at large $\chi$ [3].
[1] Fedotov, “Conjecture of perturbative QED breakdown at $\alpha \chi^{2/3} \gtrsim 1$,” Journal of Physics: Conference
Series, vol. 826, p. 012027, Apr. 2017. [Online]. Available: https://doi.org/10.1088/17426596/826/1/012027
[2] S. Bulanov, V. D. Mur, N. B. Narozhny, J. Nees, and V. S. Popov, “Multiple colliding electromagnetic
pulses: A way to lower the threshold of $e^+ e^$ pair production from vacuum,” Physical Review Letters, vol. 104,
no. 22, Jun. 2010. [Online]. Available: https://doi.org/10.1103/physrevlett.104.220404
[3] C. Olofsson and A. Gonoskov, “Bidipole wave: optimum for attaining extreme regimes at matterlight collid
ers,” 2 2022.

15:30  16:00

Anthony MercuriBaron
(Sorbonne Université)
Impact of field configurations for QED cascade development in counterpropagating LaguerreGauss beams
When charges interact with a laser pulse in the regime of strong fields, two main phenomena are important, hard photon emission by nonlinear Compton scattering, and electronpositron pair creation by nonlinear BreitWheeler. In favorable conditions of intensity and field configurations, those two processes can in principle couple to each other to reach a regime called a cascade or an avalanche, an exponential increase in the number of electronpositron pairs. Although a recent work suggests that a cascade could be initiated with one laser pulse [1], the required intensity would be far beyond what is expected in the near future experimental facilities.
More promising configurations involve two counterpropagating laser pulses, as discussed in numerical and theoretical studies [2, 3, 4, 5]. However these works only consider Gaussian laser pulses. Here we present the result of 3D Particle In Cell simulations using the code SMILEI [6] in which we explore the optimal configurations to produce a cascade using LaguerreGauss (LG) pulses. We discuss the effect of polarisation and LG beam order in the field configuration
and then on particles dynamics. Finally we identify the physical quantities which are relevant to trigger the cascade development, so as to be able to predict if a given field configuration is favorable or not, and in case improve its efficiency to maximize the number of produced pairs.
References :
[1] A. A. Mironov, E. G. Gelfer, and A. M. Fedotov, Onset of electronseeded cascades in generic electromagnetic
fields, Phys. Rev. A 104, 012221
[2] Grismayer T, Vranic M, Martins J L, Fonseca R A and Silva L O 2017, Seeded QED cascades in counterpropagating
laser pulses, Phys. Rev. E 95 023210
[3] Elkina N V, Fedotov A M, Kostyukov I Y, Legkov MV, Narozhny N B, Nerush E N and Ruhl H 2011, QED
cascades induced by circularly polarized laser fields, Phys. Rev. Spec. Top. Accel. Beams 14 054401
[4] TamburiniM, Di Piazza A and Keitel C H 2017, Laserpulseshape control of seeded QED cascades, Sci.
Rep. 7 5694
[5] Jirka M, Klimo O, VranicM, Weber S and Korn G 2017, QED cascade with 10 PWclass lasers, Sci. Rep. 7
15302
[6] Derouillat J et al 2018, Smilei: a collaborative, opensource, multipurpose particleincell code for plasma simulation, Comput. Phys. Commun. 222 351–73, https://smileipic.github.io/Smilei

16:00  16:30

Coffee break


chair: Nina Elkina

16:30  17:00

Felix Karbstein
(Helmholtz Institute Jena)
Alloptical signatures of quantum vacuum nonlinearities in laser fields
Alloptical experiments offer a promising route to unprecedented precision tests of quantum electrodynamics in strong macroscopic electromagnetic fields. So far, most theoretical studies of alloptical signatures of quantum vacuum nonlinearity are based on simplifying approximations of the beam profiles and pulse shapes of the driving laser fields. However, precise experimental tests require quantitatively accurate theoretical predictions. Our approach is based on the vacuum emission picture, and makes use of the fact that the dynamics of the driving laser fields are to an excellent approximation governed by classical Maxwell theory in vacuum. In combination with a Maxwell solver, which selfconsistently propagates any given laser field configuration, this allows for accurate theoretical predictions of photonic signatures of vacuum nonlinearity in highintensity laser experiments from first principles.

17:00  17:30

Arseny Mironov
(Sorbonne Université)
Photon emission in a fully nonperturbative regime of strongfield QED
At extremely high strength of the external field, which exceeds ~1600 SauterSchwinger fields in the reference frame of a particle, the standard strongfield QED perturbative approach based on the Furry picture is known to be problematic. As a result, any scattering problem requires a summation of loop radiative corrections to all orders. According to the RitusNarozhny conjecture, the leading order contribution (for a given process) can be obtained by the summation over all the Feynman diagrams with photon lines replaced by the bubblechains. They are given by combining the 1loop photon polarization operators with the bare photon lines. However, there is a caveat: in effect, such dressed `photons' obtain a dynamical mass depending on the quantum parameter $\chi$. Unless $\chi$ is small, they are violently unstable. This makes formulation of the photon emission probability quite intricate. In this talk, we review this issue in detail and the possible ways to resolve it. Specifically, we discuss application of the optical theorem to SFQED with unstable states.

17:30  18:15

Alexey Arefiev (virtual)
(University of California, San Diego)
Achieving pair creation via photonphoton collisions in dense laserirradiated plasmas
Highpower highintensity laser facilities have been designed to push the frontier of high field science, so finding regimes involving these lasers that are relevant to high field science and quantum electrodynamics research is paramount. The processes of matter/antimatter production from light alone stand out because they have yet to be observed in the laboratory. While most attention has been focused on the multiphoton process, the process that involves two gammarays, the linear BreitWheeler process, has been overlooked due to a misconception that it is more difficult to observe. To objectively assess the linear BW process, we have developed a firstever fully kinetic code for predictive simulations of the electronpositron pair production via this process in highintensity lasermatter interactions and the subsequent positron acceleration. Using this new tool, we have discovered several regimes where one or two laser pulses propagating through a dense plasma form an effective selforganized collider of gammarays and an adjoining accelerator for the generated positrons. In contrast to the regimes proposed for the multiphoton process, our regime only requires a peak intensity that is already accessible at most flagship laser facilities to produce more than $10^7$ electronpositron pairs. Positrons are emitted as energetic beams with a narrow divergence angle, which is likely to facilitate their detection.

18:15  19:15

Dinner

19:15  20:00

Informal discussions
