Continuous and discontinuous shape transitions in the spatial distribution of self-propelling particles in power-law potential wells

Abstract: We study the stationary states of an active Brownian particle and run-and-tumble particle in a two dimensional power-law potential well, in the limit where translational diffusion is negligible. The potential energy of the particle is taken to have the form $U(r)\propto r^{n}$, where $n\geq 2$ and even. We derive an exact equation for the positional probability distribution $\phi({\bf r})$ in two dimensions, and solve for the same, under the assumption that the particle's orientation angle is a Gaussian variable. We show that $\phi({\bf r})$ has compact support and undergoes a phase transition-like change in shape as the active velocity increases. For active Brownian particle, our theory predicts a continuous transition in shape for $n=2$ and a discontinuous transition for $n>2$, both of which agree with simulation results. In the strongly active regime, the orientational probability distribution is unimodal near the outer boundary but becomes bimodal towards the interior, signifying orbiting motion. The unimodal-bimodal transition in the angular distribution is nearly absent for run-and-tumble particle.