  The phenomenon of metastability can shape dynamical processes on all temporal
and spatial scales. Here, we induce metastable dynamics by pumping ultracold
bosonic atoms from the lowest band of an optical lattice to an excitation band,
via a sudden quench of the unit cell. The subsequent relaxation process to the
lowest band displays a sequence of stages, which include a metastable stage,
during which the atom loss from the excitation band is strongly suppressed.
Using classical-field simulations and analytical arguments, we provide an
explanation for this experimental observation, in which we show that the
transient condensed state of the atoms in the excitation band is a dark state
with regard to collisional decay and tunneling to a low-energy orbital.
Therefore the metastable state is stabilized by destructive interference due to
the chiral phase pattern of the condensed state. Our experimental and
theoretical study provides a detailed understanding of the different stages of
a paradigmatic example of many-body relaxation dynamics.
