October 1, 2009: Jason Schweinsberg (University of California, San Diego)
Title:  The genealogy of branching Brownian motion with absorption.

Abstract:  We consider a system of particles which perform branching Brownian motion with negative drift and are killed upon reaching zero, in the near-critical regime where the total population stays roughly constant with approximately N particles. We show that the characteristic time scale for the evolution of this population is of order (log N)3, in the sense that when time is measured in these units, the scaled number of particles converges to a version of Neveu's continuous-state branching process. Furthermore, the genealogy of the particles is then governed by a coalescent process known as the Bolthausen-Sznitman coalescent. This validates the non-rigorous predictions by Brunet, Derrida, Muller, and Munier for a closely related model.  This is joint work with Julien Berestycki and Nathanael Berestycki.