  Quantum many-body systems may defy thermalization even without disorder.
Intriguingly, non-ergodicity may be caused by a fragmentation of the many-body
Hilbert-space into dynamically disconnected subspaces. The tilted
one-dimensional Fermi-Hubbard model was proposed as a platform to realize
fragmented models perturbatively in the limit of large tilt. Here, we
demonstrate the validity of this effective description for the transient
dynamics using ultracold fermions. The effective analytic model allows for a
detailed understanding of the emergent microscopic processes, which in our case
exhibit a pronounced doublon-number dependence. We study this experimentally by
tuning the doublon fraction in the initial state.
