A note on stable point processes occurring in branching Brownian motion

Abstract

We call a point process $Z$ on $\mathbb R$ \emph{exp-1-stable} if for every $α,β\in\mathbb R$ with $e^α+e^β=1$, $Z$ is equal in law to $T_αZ+T_βZ'$, where $Z'$ is an independent copy of $Z$ and $T_x$ is the translation by $x$. Such processes appear in the study of the extremal particles of branching Brownian motion and branching random walk and several authors have proven in that setting the existence of a point process $D$ on $\mathbb R$ such that $Z$ is equal in law to $\sum_{i=1}^\infty T_{ξ_i} D_i$, where $(ξ_i)_{i\ge1}$ are the atoms of a Poisson process of intensity $e^{-x}\,\mathrm d x$ on $\mathbb R$ and $(D_i)_{i\ge 1}$ are independent copies of $D$ and independent of $(ξ_i)_{i\ge1}$. In this note, we show how this decomposition follows from the classic \emph{LePage decomposition} of a (union)-stable point process. Moreover, we give a short proof of it in the general case of random measures on $\mathbb R$.