Exact position distribution of a harmonically-confined run-and-tumble particle in two dimensions

We consider an overdamped run-and-tumble particle in two dimensions, with self propulsion in an orientation that stochastically rotates by 90 degrees at a constant rate, clockwise or counter-clockwise with equal probabilities. In addition, the particle is confined by an external harmonic potential of stiffness $\mu$, and possibly diffuses. We find the exact time-dependent distribution $P(x,y,t)$ of the particle’s position, and in particular, the steady-state distribution $P_{\text{st}}(x,y)$ that is reached in the long-time limit. We also find $P(x,y,t)$ for a “free” particle, $\mu =0$. We achieve this by showing that, under a proper change of coordinates, the problem decomposes into two statistically-independent one-dimensional problems, whose exact solution has recently been obtained. We then extend these results in several directions, to two such run-and-tumble particles with a harmonic interaction, to analogous systems of dimension three or higher, and by allowing stochastic resetting.