  We study, both experimentally and theoretically, short-time modifications of
the decay of excitations in a Bose-Einstein Condensate (BEC) embedded in an
optical lattice. Strong enhancement of the decay is observed compared to the
Golden Rule results. This enhancement of decay increases with the lattice
depth. It indicates that the description of decay modifications of few-body
quantum systems also holds for decay of many-body excitations of a BEC.
