  Recent experiments on macroscopic quantum tunneling reveal a non-exponential
decay of the number of atoms trapped in a quasibound state behind a potential
barrier. Through both experiment and theory, we demonstrate this
non-exponential decay results from interactions between atoms. Quantum
tunneling of tens of thousands of 87 Rb atoms in a Bose-Einstein condensate is
modeled by a modified Jeffreys-Wentzel-Kramers-Brillouin model, taking into
account the effective time-dependent barrier induced by the mean-field.
Three-dimensional Gross-Pitaevskii simulations corroborate a mean-field result
when compared with experiments. However, with one-dimensional modeling using
time-evolving block decimation, we present an effective renormalized mean-field
theory that suggests many-body dynamics for which a bare mean-field theory may
not apply.
