  Granular hydrodynamics predicts symmetry-breaking instability in a
two-dimensional (2D) ensemble of nearly elastically colliding smooth hard
spheres driven, at zero gravity, by a rapidly vibrating sidewall. Super- and
subcritical symmetry-breaking bifurcations of the simple clustered state are
identified, and the supercritical bifurcation curve is computed. The cluster
dynamics proceed as a coarsening process mediated by the gas phase. Far above
the bifurcation point the final steady state, selected by coarsening,
represents a single strongly localized densely packed 2D cluster.
