### Attention : désormais les séminaires ont lieu tous les lundis à 10h45 en salle 523 du LPTMC - Tour 12-13

**Freddy Bouchet (LMD, ENS)**

### Statistical physics for climate sciences: application to wave turbulence, extreme heat waves, and extremes of renewable energy production

I will discuss several examples where statistical physics and large deviation theory can be useful to solve fundamental problems for the dynamics of the climate system.

The first example will be a theoretical contribution to the kinetic theory of wave turbulence. Wave turbulence plays an important role for atmosphere/ocean physical exchanges and for mixing of the ocean interior. I will explain how large deviation theory allows to extend this classical theory to compute effects of typical and rare spontaneous fluctuations. I will explain how this can be used for stochastic parameterization for wave energy propagation.

A large part of the talk will be dedicated to extreme heat waves. Extreme events or transitions between climate attractors are of primarily importance for understanding the impact of climate change. Recent extreme heat waves, with huge impact, are striking examples. However, they cannot be studied with conventional approaches, because they are too rare and realistic models are too complex. We will discuss several new algorithms and theoretical approaches, based on large deviation theory, rare event simulations, and machine learning for stochastic processes, which we have specifically designed for the prediction of extreme heat waves. Using the best available climate models, our approach sheds new light on the fluid mechanics processes which lead to these events. We will describe quasi-stationary patterns of turbulent Rossby waves that lead to global teleconnection patterns in connection with heat waves and analyze their dynamics.

At the end of the talk, I will briefly outline current projects where we use the same tools to study extremes of renewable energy production and their connection with climate dynamics. Those rare events are key for the future of the European electricity system.

**Francesco Mori (Oxford University)**

### Optimal control of living systems: from insect navigation to oscillating active fluids

**Ruben Zakine (LadHyX)**

### Socioeconomic agents as active matter and nucleation paths in active field theories

In this seminar, we will tackle two subjects whose common thread is active matter.

In a first part, I will focus on a socio-economic occupation model in the spirit of the Sakoda-Schelling model, historically introduced to shed light on segregation dynamics among human groups. For a large class of utility functions and decision rules that drive the system out of equilibrium, we recover an equilibrium-like phase separation phenomenology. Within the mean-field approximation I will show how the model can be mapped, to some extent, onto an active matter field description, paving the way for a unifying framework which considers population and price dynamics within a field theoretic approach.

In a second part, using the nucleation-induced active phase separation as a starting observation, I will ask a general question: How can one predict the final phase of a nonequilibrium system where several phases compete? Since resorting to the free energy minimization is impossible, the transition depends crucially on the system’s dynamics. By using a minimum action method, I will pinpoint the first-order phase transition in some spatially-extended nonequilibrium systems, including the Active Model B, the natural nonequilibrium extension of the Cahn-Hilliard dynamics. The paths of the transitions and their critical nuclei are notably identified.

References

Socioeconomic agents as active matter in nonequilibrium Sakoda-Schelling models

R Zakine, J Garnier-Brun, AC Becharat, M Benzaquen

arXiv:2307.14270

Minimum Action Method for Nonequilibrium Phase Transitions

R Zakine, E Vanden-Eijnden

arXiv:2202.06936

Unveiling the Phase Diagram and Reaction Paths of the Active Model B with the Deep Minimum Action Method

R Zakine, E Simonnet, E Vanden-Eijnden

arXiv:2309.15033

**Ivan Khaymovich (Nordita, Stockholm University)**

### “Equipartition and Entanglement”. Relation between ergodicity measures

[1] G. De Tomasi, I. M. K., “Multifractality meets entanglement: relation for non-ergodic extended states”, Phys. Rev. Lett. 124, 200602 (2020) [arXiv:2001.03173]

[2] I. M. K., M. Haque, and P. McClarty, “Eigenstate Thermalization, Random Matrix Theory and Behemoths”, Phys. Rev. Lett. 122, 070601 (2019) [arXiv:1806.09631].

[3] M. Haque, P. A. McClarty, I. M. K. , “Entanglement of mid-spectrum eigenstates of chaotic many-body systems—deviation from random ensembles.” [arXiv:2008.12782].

[4] A. Bäcker, I. M. K., M. Haque,, “Multifractal dimensions for chaotic quantum maps and many-body systems”, Phys. Rev. E 100, 032117 (2019) [arxiv:1905.03099].

**Clément Delcamp (IHES)**

### Topological symmetries and dualities in one-dimensional spin models

Focusing on familiar one-dimensional spin models, I will motivate the modern perspective on symmetries in terms of topological operators. Embracing this perspective, I will sketch a systematic approach to dualities in spin models. I will further explain how to deal with the mapping of super-selection sectors of dual models onto one another, which in turn allows to explicitly relate their spectra.