Random search with stochastic resetting: when finding the target is not enough

Abstract: In this paper we consider a random search process with stochastic resetting and a partially accessible target $\calU$. That is, when the searcher finds the target by attaching to its surface $\partial \calU$ it does not have immediate access to the resources within the target interior. After a random waiting time, the searcher either gains access to the resources within or detaches and continues its search process. We also assume that the searcher requires an alternating sequence of periods of bulk diffusion interspersed with local surface interactions before being able to attach to the surface. The attachment, detachment and target entry events are the analogs of adsorption, desorption and absorption of a particle by a partially reactive surface in physical chemistry. In applications to animal foraging, the resources could represent food or shelter while resetting corresponds to an animal returning to its home base. We begin by considering a Brownian particle on the half-line with a partially accessible target at the origin $x=0$. We calculate the non-equilibrium stationary state (NESS) in the case of reversible adsorption and obtain the corresponding first passage time (FPT) density for absorption when adsorption is only partially reversible. We then reformulate the stochastic process in terms of a pair of renewal equations that relate the probability density and FPT density for absorption in terms of the corresponding quantities for irreversible adsorption. The renewal equations allow us to incorporate non-Markovian models of absorption and desorption. They also provide a useful decomposition of quantities such as the mean FPT (MFPT) in terms of the number of desorption events and the statistics of the waiting time density. Finally, we consider various extensions of the theory, including higher-dimensional search processes and an encounter-based model of absorption.