Total number of births on the negative half-line of the binary branching Brownian motion in the boundary case

The binary branching Brownian motion in the boundary case is a particle system on the real line behaving as follows. It starts with a unique particle positioned at the origin at time 0 . The particle moves according to a Brownian motion with drift µ = 2 and diffusion coefficient σ 2 = 2 , until an independent exponential time of parameter 1 . At that time, the particle dies giving birth to two children who then start independent copies of the same process from their birth place. It is well-known that in this system, the cloud of particles eventually drifts to ∞ . The aim of this note is to provide a precise estimate for the total number of particles that were born on the negative half-line, investigating in particular the tail decay of this random variable.