  Interactions with neutrons and protons play a crucial role for the neutrino
opacity of matter in the supernova core. Their current implementation in many
simulation codes, however, is rather schematic and ignores not only
modifications for the correlated nuclear medium of the nascent neutron star,
but also free-space corrections from nucleon recoil, weak magnetism or strange
quarks, which can easily add up to changes of several 10% for neutrino energies
in the spectral peak. In the Garching supernova simulations with the
Prometheus-Vertex code, such sophistications have been included for a long time
except for the strange-quark contributions to the nucleon spin, which affect
neutral-current neutrino scattering. We demonstrate on the basis of a 20 M_sun
progenitor star that a moderate strangeness-dependent contribution of g_a^s =
-0.2 to the axial-vector coupling constant g_a = 1.26 can turn an unsuccessful
three-dimensional (3D) model into a successful explosion. Such a modification
is in the direction of current experimental results and reduces the
neutral-current scattering opacity of neutrons, which dominate in the medium
around and above the neutrinosphere. This leads to increased luminosities and
mean energies of all neutrino species and strengthens the neutrino-energy
deposition in the heating layer. Higher nonradial kinetic energy in the gain
layer signals enhanced buoyancy activity that enables the onset of the
explosion at ~300 ms after bounce, in contrast to the model with vanishing
strangeness contributions to neutrino-nucleon scattering. Our results
demonstrate the close proximity to explosion of the previously published,
unsuccessful 3D models of the Garching group.
