  We investigate the decay of highly excited states of ultracold fermions in a
three-dimensional optical lattice. Starting from a repulsive Fermi-Hubbard
system near half filling, we generate additional doubly occupied sites
(doublons) by lattice modulation. The subsequent relaxation back to thermal
equilibrium is monitored over time. The measured doublon lifetime covers two
orders of magnitude. In units of the tunneling time h/J it is found to depend
exponentially on the ratio of on-site interaction energy U to kinetic energy J.
We argue that the dominant mechanism for the relaxation is a high order
scattering process involving several single fermions as scattering partners. A
many-body calculation is carried out using diagrammatic methods, yielding good
agreement with the data.

Quantum Gases;Other Condensed Matter;Strongly Correlated Electrons