Phase transition for finite-speed detection among moving particles

Consider the model where particles are initially distributed on $\mathbb{Z}^d, \, d\geq 2$, according to a Poisson point process of intensity $λ>0$, and are moving in continuous time as independent simple symmetric random walks. We study the escape versus detection problem, in which the target, initially placed at the origin of $\mathbb{Z}^d, \, d\geq 2$, and changing its location on the lattice in time according to some rule, is said to be detected if at some finite time its position coincides with the position of a particle. We consider the case where the target can move with speed at most 1, according to any continuous function and can adapt its motion based on the location of the particles. We show that there exists sufficiently small $λ_* > 0$, so that if the initial density of particles $λ< λ_*$, then the target can avoid detection forever.

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