  Driving a many-body system out of equilibrium induces phenomena such as the
emergence and decay of transient states, which can manifest itself as pattern
and domain formation. The understanding of these phenomena expands the scope of
established thermodynamics into the out-of-equilibrium domain. Here, we
experimentally and theoretically study the out-of-equilibrium dynamics of a
bosonic lattice model subjected to a strong DC field, realized as ultracold
atoms in a strongly tilted triangular optical lattice. We observe the emergence
of pronounced density wave patterns -- which spontaneously break the underlying
lattice symmetry -- using a novel single-shot imaging technique with
two-dimensional single-site resolution in three-dimensional systems, which also
resolves the domain structure. Our study suggests that the short-time dynamics
arises from resonant pair tunneling processes within an effective description
of the tilted Hubbard model. More broadly, we establish the far
out-of-equilibrium regime of lattice models subjected to a strong DC field, as
an exemplary and paradigmatic scenario for transient pattern formation.
