  In crowded systems, particle currents can be mediated by propagating
collective excitations which are generated as rare events, are localized and
have a finite lifetime. The theoretical description of such excitations is
hampered by the problem of identifying complex many-particle transition states,
calculation of their free energies, and the evaluation of propagation
mechanisms and velocities. Here we show that these problems can be tackled for
a highly jammed system of hard spheres in a periodic potential. We derive
generation rates of collective excitations, their anomalously high velocities,
explain the occurrence of an apparent jamming transition and its strong
dependence on the system size. The particle currents follow a scaling behavior,
where for small systems the current is proportional to the generation rate and
for large systems given by the geometric mean of the generation rate and
velocity. Our theoretical approach is widely applicable to dense nonequilibrium
systems in confined geometries. It provides new perspectives for studying
dynamics of collective excitations in experiments.
