  Streamer discharges are the primary mode of electric breakdown of air in
lightning and high voltage technology. Streamer channels branch many times,
which determines the developing tree-like discharge structure. Understanding
these branched structures is for example important to describe streamer coronas
in lightning research. We simulate branching of positive streamers in air using
a 3D fluid model where photoionization is included as a discrete and stochastic
process. The probability and morphology of branching are in good agreement with
dedicated experiments. This demonstrates that photoionization indeed provides
the noise that triggers branching, and we show that branching is remarkably
sensitive to the amount of photoionization. Our comparison is therefore one of
the first sensitive tests for Zheleznyak's photoionization model, confirming
its validity.
