Total progeny in killed branching random walk

International audienceWe consider a branching random walk for which the maximum position of a particle in the $n$'th generation, $M_n$, has zero speed on the linear scale: $M_n/n \to 0$ as $n\to\infty$. We further remove (``kill'') any particle whose displacement is negative, together with its entire descendence. The size $Z$ of the set of un-killed particles is almost surely finite. In this paper, we confirm a conjecture of Aldous that $\mathbf E[Z]<\infty$ while $\mathbf E [Z\log Z]=\infty$. The proofs rely on precise large deviations estimates and ballot theorem-style results for the sample paths of random walks