  Mapping out the equation of state (EOS) of nuclear matter is a long standing
problem in nuclear physics. Recent emphasis is onto the density dependence of
the symmetry energy, with experiments needing dedicated symmetry-energy
observables. Towards the latter goal, we employ pBUU transport model to
simulate pion production in heavy ion collision (HIC). We find that the net
pion yield tests the momentum dependence of nuclear mean field (MF). In
exploring the sensitivity of pion observables to the symmetry energy at higher
than normal densities, we find that our calculations of pion ratios contradict,
at some level, predictions from both IBUU and ImIQMD models. We propose to
employ the pion ratio in the high-energy tail of spectra in future experiments,
to distinguish between different variants of high-density symmetry energy.
