  A granular system confined in a quasi two-dimensional box that is vertically
vibrated can transit to an absorbing state in which all particles bounce
vertically in phase with the box, with no horizontal motion. In principle, this
state can be reached for any density lower than the one corresponding to one
complete monolayer, which is then the critical density. Below this critical
value, the transition to the absorbing state is of first order, with long
metastable periods, followed by rapid transitions driven by homogeneous
nucleation. Molecular dynamics simulations and experiments show that there is a
dramatic increase on the metastable times far below the critical density; in
practice, it is impossible to observe spontaneous transitions close to the
critical density. This peculiar feature is a consequence of the non-equilibrium
nature of this first order transition to the absorbing state. A Ginzburg-Landau
model, with multiplicative noise, describes qualitatively the observed
phenomena and explains the macroscopic size of the critical nuclei. The nuclei
become of small size only close to a second critical point where the active
phase becomes unstable via a saddle node bifurcation. It is only close to this
second critical point that experiments and simulations can evidence spontaneous
transitions to the absorbing state while the metastable times grow dramatically
moving away from it.
