# Seminars at the Landau Institute scientific council

Seminars are held on Fridays in the conference hall of Landau Institute for Theoretical Physics in Chernogolovka, beginning at 11:30.

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## Regimes of matter and radiation interaction in strong electromagnetic fields

11 October in 11:30

Arseny Mironov (Sorbonne University, France)

When considering quantum processes rendered by copious absorptions of soft photons like the non-linear Compton scattering, it is common to postulate the strong-field interaction regime. However, upon closer examination this `strong-field regime' falls into several sub-regimes. They are conditional on invariant parameters such as the non-linearity parameter $a_0$ proportional to the field amplitude, and the dynamical quantum parameter $\chi$, which denotes the field amplitude experienced by a particle in its reference frame in the units of the critical (Schwinger) field of QED $E_S=m^2c^3/e\hbar$ (here, $m$ and $-e$ are the electron mass and charge) [1].

An electron injected into a strong field with high $a_0 >> 1$ rapidly become ultra-relativistic and can emit strong radiation, and at $\chi > 1$ the emission process is essentially quantum. In this regime, radiation dominates particle dynamics, and production of new electron-positron pairs by hard photons in a strong field becomes important. This can reveal, for example, in QED cascades [1], which in some cases can be sustained by the field leading to abundant production of pairs [2,3].

The Furry picture, which is at the core of the strong-field QED approach, allows building the perturbative scattering theory in a strong external electromagnetic field that is accounted for exactly. Under the so-called locally constant field approximation (LCFA) for processes involving relativistic particles in transverse fields, the model of a uniform constant crossed field (CCF) appears to be universal, and at the same time allows analytical treatment for many processes.

At very high $\chi >> 1$, the loop contributions to scattering amplitudes (so-called polarisation and mass operators) in a CCF grow surprisingly fast, namely, as a power of the field strength and particle energy entering the parameter $g = \alpha\chi^{2/3}$. As was initially conjectured by Narozhny [4], and later confirmed by our all-order calculation [5], $g^n$ factors the leading-type n-th loop contributions in the perturbation expansion, meaning that at $g>1$ the Furry expansion breaks down and renders the new fully non-perturbative regime of strong-field QED. Strikingly, this regime arises at scales that could be feasible in future experiments.

In this talk, we review the above mentioned regimes of strong-field QED, the related theoretical framework, and the connection to possible future experiments.

[1] A. Fedotov, A. Ilderton, F. Karbstein, B. King, D. Seipt, H. Taya, and G. Torgrimsson, Phys. Rep. 1010, 1 (2023).

[2] A. R. Bell and J. G. Kirk, PRL 101, 200403 (2008).

[3] A. Mercuri-Baron, A.A, Mironov, C. Riconda, A. Grassi, M. Grech, arXiv:2402.04225 (2024).

[4] N. B. Narozhny, Expansion parameter of perturbation theory in intense-field quantum electrodynamics, Physical Review D 21, 1176 (1980).

[5] A. A. Mironov, S. Meuren, and A. M. Fedotov, PRD 102, 053005 (2020); A. A. Mironov and A. M. Fedotov, PRD 105, 033005 (2022).

An electron injected into a strong field with high $a_0 >> 1$ rapidly become ultra-relativistic and can emit strong radiation, and at $\chi > 1$ the emission process is essentially quantum. In this regime, radiation dominates particle dynamics, and production of new electron-positron pairs by hard photons in a strong field becomes important. This can reveal, for example, in QED cascades [1], which in some cases can be sustained by the field leading to abundant production of pairs [2,3].

The Furry picture, which is at the core of the strong-field QED approach, allows building the perturbative scattering theory in a strong external electromagnetic field that is accounted for exactly. Under the so-called locally constant field approximation (LCFA) for processes involving relativistic particles in transverse fields, the model of a uniform constant crossed field (CCF) appears to be universal, and at the same time allows analytical treatment for many processes.

At very high $\chi >> 1$, the loop contributions to scattering amplitudes (so-called polarisation and mass operators) in a CCF grow surprisingly fast, namely, as a power of the field strength and particle energy entering the parameter $g = \alpha\chi^{2/3}$. As was initially conjectured by Narozhny [4], and later confirmed by our all-order calculation [5], $g^n$ factors the leading-type n-th loop contributions in the perturbation expansion, meaning that at $g>1$ the Furry expansion breaks down and renders the new fully non-perturbative regime of strong-field QED. Strikingly, this regime arises at scales that could be feasible in future experiments.

In this talk, we review the above mentioned regimes of strong-field QED, the related theoretical framework, and the connection to possible future experiments.

[1] A. Fedotov, A. Ilderton, F. Karbstein, B. King, D. Seipt, H. Taya, and G. Torgrimsson, Phys. Rep. 1010, 1 (2023).

[2] A. R. Bell and J. G. Kirk, PRL 101, 200403 (2008).

[3] A. Mercuri-Baron, A.A, Mironov, C. Riconda, A. Grassi, M. Grech, arXiv:2402.04225 (2024).

[4] N. B. Narozhny, Expansion parameter of perturbation theory in intense-field quantum electrodynamics, Physical Review D 21, 1176 (1980).

[5] A. A. Mironov, S. Meuren, and A. M. Fedotov, PRD 102, 053005 (2020); A. A. Mironov and A. M. Fedotov, PRD 105, 033005 (2022).

## Equivalence of Deformations of Berglund Hübsch Mirror Pairs

11 October in 11:30 (short)

__Alexander A. Belavin__, Doron R. Gepner

We investigate here the deformations of Berglund Hübsch loop and chain
mirrors where the original manifolds are defined in the same weighted projective space. We show that the deformations are equivalent by two methods. First, we map directly the two models to each other and show that the deformations bare the same for 79 ”Good” models, but not for the 77 ”Bad” ones. We then investigate the orbifold of the mirror pair by the maximal symmetry group and show that the number of deformations is the same and that they are almost thesame, i.e., the first four exponents of the deformations are identical.

https://doi.org/10.1016/j.Nucl-PhysB.2024.116695

http://arxiv.org/abs/2408.15182

https://doi.org/10.1016/j.Nucl-PhysB.2024.116695

http://arxiv.org/abs/2408.15182

## Instability of the engineered dark state in two-band fermions under number-conserving dissipative dynamics

1 November in 11:30

__A. A. Lyublinskaya__, P. A. Nosov, I. S. Burmistrov

We examine a general class of models involving a two-band fermion system subjected to dissipation, conserving the total number of particles. By construction, these models have a guaranteed steady state – a dark state – with a completely filled lower band and an empty upper band. In the limit of weak dissipation, we derive equations governing the dynamics of the fermion densities over long length and time scales. These equations belong to the Fisher-Kolmogorov-Petrovsky-Piskunov reaction-diffusion universality class. Our analysis reveals that the engineered dark state is generically unstable, giving way to a new steady state with a finite density of particles in the upper band. Our results suggest that number-conserving dissipative protocols may not be a reliable universal tool for stabilizing dark states.

## Oceanic Eddy-Induced Transport: Full-Tensor Approach

15 November in 11:30

Pavel S. Berloff (Imperial College London)

In geophysical turbulence effects of small-scale motions on the large-scale ones are often formulated in terms of turbulent (eddy) diffusion characterized by the eddy diffusivity coefficient. Within this context we focused on passive-tracer transport induced by mesoscale oceanic eddies and applied brute-force approach, without any simplifications and extra assumptions. The uncovered new levels of complexity of the corresponding full-tensor eddy diffusivity coefficient, referred to as the "transport tensor", call for reconsideration of how the eddy diffusion and corresponding scale interactions are treated in observations, general circulation models and theory.