Occupation Statistics of Critical Branching Random Walks in Two or Higher Dimensions

Consider a critical nearest neighbor branching random walk on the $d$-dimensional integer lattice initiated by a single particle at the origin. Let $G_n$ be the event that the branching random walk survives to generation $n$. We obtain limit theorems conditional on the event $G_n$ for a variety of occupation statistics: (1) Let $V_n$ be the maximal number of particles at a single site at time $n$. If the offspring distribution has finite $\alpha$th moment for some integer $\alpha \ge 2$, then in dimensions 3 and higher, $V_n=O_p(n^{1\mathrm{/}\alpha })$; and if the offspring distribution has an exponentially decaying tail, then $V_n=O_p(\log n)$ in dimensions 3 and higher, and $V_n=O_p((\log n{)}^2)$ in dimension 2. Furthermore, if the offspring distribution is non-degenerate then $P(V_n\ge \delta \log n\mathrm{\mid }{G}_n)\to 1$ for some $\delta >0$. (2) Let $M_n(j)$ be the number of multiplicity-$j$ sites in the $n$th generation, that is, sites occupied by exactly $j$ particles. In dimensions 3 and higher, the random variables $M_n(j)\mathrm{/}n$ converge jointly to multiples of an exponential random variable. (3) In dimension 2, the number of particles at a “typical” site (that is, at the location of a randomly chosen particle of the $n$th generation) is of order $O_p(\log n)$, and the number of occupied sites is $O_p(n\mathrm{/}\log n)$.