  We study the dynamics of micron-sized particles on a layer of motile cells.
This cell carpet acts as an active bath that propels passive tracer particles
via direct mechanical contact. The resulting nonequilibrium transport shows a
crossover from superdiffusive to normal-diffusive dynamics. The particle
displacement distribution is distinctly non-Gaussian even in the limit of long
measurement times -- different from typically reported Fickian yet non-Gaussian
transport, for which Gaussianity is restored beyond some system-specific
correlation time. We obtain the distribution of diffusion coefficients from the
experimental data and introduce a model for the displacement distribution that
matches the experimentally observed non-Gaussian statistics and argue why
similar transport properties are expected for many composite active matter
systems.
