Yariv Kafri: Flocking: Why Continuous Symmetries Break More Readily Than Discrete Ones

The talk will discuss the stability of flocking phases which exhibit symmetry breaking. For models which break a discrete symmetry, both an active Ising model and a hydrodynamic description will be used to show that droplets of particles moving in a direction opposite to that of the ordered phase nucleate and grow ballistically in all directions. The results imply that, in the thermodynamic limit, discrete-symmetry flocks are metastable in all dimensions. Following this the ordered phase of a flocking model which breaks a continuous symmetry will be considered and argued to be stable in parts of the phase diagram. This implies that in flocking models, in contrast to equilibrium systems, breaking a continuous symmetry is easier than a discrete one.