# Phase transitions and macroscopic limits in a BGK model of body-attitude coordination

## TL;DR

This paper studies phase transitions in a kinetic model of body-attitude coordination, revealing how self-organization depends on local density and deriving macroscopic limits through stability and equilibrium analysis.

## Contribution

It introduces a novel analysis of phase transitions in a BGK model of body-attitude coordination, linking it to nematic polymer models and deriving associated macroscopic equations.

## Key findings

- Self-organization depends on local density.
- Complete equilibrium description via nematic alignment models.
- Convergence and stability proven for different regimes.

## Abstract

In this article we investigate the phase transition phenomena that occur in a model of self-organisation through body-attitude coordination. Here, the body-attitude of an agent is modelled by a rotation matrix in $\mathbb{R}^3$ as in [Degond, Frouvelle, Merino-Aceituno, 2017]. The starting point of this study is a BGK equation modelling the evolution of the distribution function of the system at a kinetic level. The main novelty of this work is to show that in the spatially homogeneous case, self-organisation may appear or not depending on the local density of agents involved. We first exhibit a connection between body-orientation models and models of nematic alignment of polymers in higher dimensional space from which we deduce the complete description of the possible equilibria Then, thanks to a gradient-flow structure specific to this BGK model, we are able to prove the stability and the convergence towards the equilibria in the different regimes. We then derive the macroscopic models associated to the stable equilibria in the spirit of [Degond, Frouvelle, Merino-Aceituno, 2017] and [Degond, Frouvelle, Liu, 2015].