FEDERICA FERRETTI

Dottoressa di ricerca

ciclo: XXXIV



Titolo della tesi: Microscopic Dynamics of Polar Active Systems: Inference Methods and Signatures of Irreversibility for Stochastic Models

Flocking denotes the spontaneous onset of collective motion in systems of self- propelled agents, of which groups of birds are a prototypical example. A pretty coherent corpus of theoretical models has been introduced over the past three decades to explain how this collective behavior arises from microscopic interactions, revealing that the emergence of polar order is a manifestation of the non-equilibrium character of the dynamics. The confront with experimental data allowed for the validation and refinement of those models, and, in some cases, even for the application of quantitative inference approaches. In this thesis we employ standard methods from stochastic calculus to study problems related to the microscopic dynamics of such systems. Motivated by the availability of data, we firstly derive a novel Bayesian inference method for the inertial dynamics of flocks. Our inference scheme is based on a previously introduced model (the Inertial Spin Model), which is non-Markovian in the observed variables’ space. This feature raises technical problems, when combined with discrete-time recordings, and is common to many stochastic dynamic systems. The method we propose for the Inertial Spin Model applies in fact to a larger class of processes; examples are illustrated. We also exploit an analogy between the Renormalization Group and augmentation techniques used to infer partially observed SDEs to provide an alternative proof of the lack of finite-dimensional delay vector embeddings for stochastic dynamic systems. The second focus of this thesis concerns the study of non-equilibrium effects in simple models for polar active matter. It is known that the emergence of polar order in systems of aligning self-propelled particles is due to the non-equilibrium character of the dynamics. We quantify the distance from equilibrium through the entropy production rate, which we measure from numerical simulations of interacting active Brownian particles. We investigate two kinds of short-ranged interaction rules, based on different notions of metrics. We find that the entropy production rate is maximal close to the transition, while two equilibrium limits are reached in the deeply ordered phase (perfect flock) or completely disordered phase (ideal active gas). We pivot on the entropy production rate to study how irre- versibility constrains asymmetries in the steady state distribution of microstates. In the presence of pairwise forces, robust signatures of irreversibility are visible in the two-particle density, as confirmed by numerical simulations. On the contrary, in the presence of multi-particle interactions, irreversibility directly constrains only correlations among a higher number of particles. All these correlations are typically neglected in the derivation of hydrodynamic equations for polar active matter through kinetic approaches.

Produzione scientifica

11573/1668529 - 2022 - Renormalization group approach to connect discrete- and continuous-time descriptions of Gaussian processes
Ferretti, Federica; Chardès, Victor; Mora, Thierry; Walczak, Aleksandra M.; Giardina, Irene Rosana - 01a Articolo in rivista
rivista: PHYSICAL REVIEW. E (Ridge, NY: American Physical Society, [2016]-) pp. 044133- - issn: 2470-0045 - wos: WOS:000798274600005 (1) - scopus: 2-s2.0-85129776571 (1)

11573/1438870 - 2020 - Building general Langevin models from discrete datasets
Ferretti, Federica; Chard(`(E))S, Victor; Mora, Thierry; Walczak, Aleksandra M.; Giardina, Irene Rosana - 01a Articolo in rivista
rivista: PHYSICAL REVIEW. X (College Park, Md. : American Physical Society) pp. - - issn: 2160-3308 - wos: WOS:000551343100001 (23) - scopus: 2-s2.0-85092715710 (24)

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