  We present the first successful simulation of a neutrino-driven supernova
explosion in three dimensions (3D), using the Prometheus-Vertex code with an
axis-free Yin-Yang grid and a sophisticated treatment of three-flavor,
energy-dependent neutrino transport. The progenitor is a nonrotating,
zero-metallicity 9.6 Msun star with an iron core. While in spherical symmetry
outward shock acceleration sets in later than 300 ms after bounce, a successful
explosion starts at ~130 ms postbounce in two dimensions (2D). The 3D model
explodes at about the same time but with faster shock expansion than in 2D and
a more quickly increasing and roughly 10 percent higher explosion energy of
>10^50 erg. The more favorable explosion conditions in 3D are explained by
lower temperatures and thus reduced neutrino emission in the cooling layer
below the gain radius. This moves the gain radius inward and leads to a bigger
mass in the gain layer, whose larger recombination energy boosts the explosion
energy in 3D. These differences are caused by less coherent, less massive, and
less rapid convective downdrafts associated with postshock convection in 3D.
The less violent impact of these accretion downflows in the cooling layer
produces less shock heating and therefore diminishes energy losses by neutrino
emission. We thus have, for the first time, identified a reduced mass accretion
rate, lower infall velocities, and a smaller surface filling factor of
convective downdrafts as consequences of 3D postshock turbulence that
facilitate neutrino-driven explosions and strengthen them compared to the 2D
case.
