Minimal Positions in a Branching Random Walk

We consider a branching random walk on the real line, with mean family size greater than 1. Let $B_n$ denote the minimal position of a member of the $n$th generation. It is known that (under a weak condition) there is a finite constant $\gamma$, defined in terms of the distributions specifying the process, such that as $n \rightarrow \infty$, we have $B_n = \gamma n + o(n)$ a.s. on the event $S$ of ultimate survival. Our results here show that (under appropriate conditions), on $S$ the random variable $B_n$ is strongly concentrated and the $o(n)$ error term may be replaced by $O(\log n)$.