  Several models have been proposed to simulate heavy ion reactions beyond the
mean field level. The lack of data in phase space regions which may be
sensitive to different treatments of fluctuations made it difficult to judge
these approaches. The recently published high energy proton spectra, measured
in the reaction 94 AMeV Ar + Ta, allow for the first time for a comparison of
the models with data. We find that these spectra are reproduced by Quantum
Molecular Dynamics (QMD) and Boltzmann Uehling Uhlenbeck (BUU) calculations.
Models like Boltzmann Langevin (BL) in which additional fluctuations in
momentum space are introduced overpredict the proton yield at very high
energies. The BL approach has been successfully used to describe the recently
measured very subthreshold kaon production assuming that the fluctuations
provide the necessary energy to overcome the threshold in two body collisions.
Our new findings suggest that the very subthreshold kaon production cannot be
due to two body scattering and thus remains a open problem.
