Branching Brownian motion deactivated at the boundary of an expanding ball

Abstract: We study a $d$-dimensional branching Brownian motion inside subdiffusively expanding balls, where the boundary of the ball is deactivating in the sense that once a particle hits the moving boundary, it is instantly deactivated but is reactivated later if and when its ancestral line becomes fully inside the expanding ball at that later time. That is, at each time, the process consists of the particles of a normal branching Brownian motion whose ancestral lines are fully inside the expanding ball at that time. We obtain a full limit large-deviation result as time tends to infinity on the probability that the mass of the process inside the expanding ball is aytpically small. A phase transition at a critical rate of expansion for the ball is identified, at which the nature of the optimal strategy to realize the large-deviation event changes, and the Lyapunov exponent giving the decay rate of the associated large-deviation probability is continuous. As a corollary, we also obtain a kind of law of large numbers for the mass of the process inside the expanding ball.