  Branching is an essential element of streamer discharge dynamics but today it
is understood only qualitatively. The variability and irregularity observed in
branched streamer trees suggest that stochastic terms are relevant for the
description of streamer branching. We here consider electron density
fluctuations due to the discrete particle number as a source of stochasticity
in positive streamers in air at standard temperature and pressure. We derive a
quantitative estimate for the branching distance that agrees within a factor of
2 with experimental values. As branching without noise would occur later, if at
all, we conclude that stochastic particle noise is relevant for streamer
branching in air at atmospheric pressure.

Plasma Physics