  In the past two decades, pions created in the high density regions of heavy
ion collisions have been predicted to be sensitive at high densities to the
symmetry energy term in the nuclear equation of state, a property that is key
to our understanding of neutron stars. In a new experiment designed to study
the symmetry energy, the multiplicities of negatively and positively charged
pions have been measured with high accuracy for central $^{132}$Sn+$^{124}$Sn,
$^{112}$Sn+$^{124}$Sn, and $^{108}$Sn+$^{112}$Sn collisions at
$E/A=270~\mathrm{MeV}$ with the S$\pi$RIT Time Projection Chamber. While the
uncertainties of individual pion multiplicities are measured to 4\%, those of
the charged pion multiplicity ratios are measured to 2\%. We compare these data
to predictions from seven major transport models. The calculations reproduce
qualitatively the dependence of the multiplicities and their ratios on the
total neutron to proton number in the colliding systems. However, the
predictions of the transport models from different codes differ too much to
allow extraction of reliable constraints on the symmetry energy from the data.
This finding may explain previous contradictory conclusions on symmetry energy
constraints obtained from pion data in Au+Au system. These new results call for
better understanding of the differences among transport codes, and new
observables that are more sensitive to the density dependence of the symmetry
energy.
