  The goal of this paper is to better understand the structure of fission
fragment distributions by investigating the competition between the static
structure of the collective manifold and stochastic dynamics. In particular, we
study the characteristics of the tails of yield distributions, which correspond
to very asymmetric fission. We use the stochastic Langevin framework to
simulate the nuclear evolution after the system tunnels through the
multi-dimensional potential barrier. For a representative sample of different
initial configurations along the outer turning-point line, we define effective
fission paths by computing a large number of Langevin trajectories. We extract
the relative contribution of each such path to the fragment distribution. We
then use nucleon localization functions along effective fission pathways to
analyze the characteristics of prefragments at pre-scission configurations. We
find that non-Newtonian Langevin trajectories, strongly impacted by the random
force, produce the tails of the fission fragment distribution. The prefragments
deduced from nucleon localizations are formed early and change little as the
nucleus evolves towards scission. On the other hand, the system contains many
nucleons that are not localized in the prefragments, even near the scission
point. Such nucleons are rapidly distributed at scission to form the final
fragments. Our study shows that only theoretical models of fission that account
for some form of dissipative/stochastic dynamics can give an accurate
description of the structure of fragment distributions. In particular, it
should be nearly impossible to predict the tails of these distributions within
the standard formulation of time-dependent density functional theory. At the
same time, the large number of non-localized nucleons during fission suggests
that adiabatic approaches are ill-suited to describe fission fragment
properties.
