Critical branching random walks on $\mathbb{Z}^2$: local survival probabilities and Yaglom limit theorems

Abstract

We consider a branching random walk on $\mathbb{Z}^2$ with critical offspring of mean $1$ and spatial motion governed by the jumps of a lazy simple random walk. For every site $x\in\mathbb{Z}^2$, we obtain a uniform asymptotical estimate for the local survival probability, i.e., the probability that there are particles at $x$ at large time $n$. Using Stein's method, we establish a Yaglom-type theorem for the number of particles at $x$ at time $n$ when $x$ is at distance of order $\sqrt{n}$ from the origin. Moreover, at the position occupied by a typical particle at time $n$, the number of particles at that site, normalized by $\log n$, converges in law to a Gamma distribution, thereby confirming a conjecture of Lalley and Zheng [Ann. Probab. 39 (2010), 327-368]. Finally, we prove that, conditional on local survival, the total number of particles at time $n$, divided by $n$, also converges weakly to a Gamma distribution.