Collective motion in large deviations of active particles*

We analyse collective motion that occurs during rare (large deviation) events in systems of active particles. We discuss the associated dynamical phase transition to collective motion, which occurs when the work of non-conservative active forces is biased towards larger values. We show numerically for 2D active Brownian particles that a finite biasing field is needed to induce spontaneous symmetry breaking, even in large systems, and analyse this symmetry breaking by an optimal-control representation of the biased dynamics.