  We perform a theoretical analysis of the $a_1$ resonance mass spectrum in
ultra-relativistic heavy ion collisions within a hadron/string transport
approach. Predictions for the $a_1$ yield and its mass distribution are given
for the GSI-FAIR and the critRHIC energy regime. The potential of the $a_1$
meson as a signal for chiral symmetry restoration is explored. In view of the
latest discussion, we investigate the decay channel $a_1 \to \gamma\pi$ in
detail and find a strong bias towards low $a_1$ masses. This apparent mass
shift of the $a_1$ if observed in the $\gamma\pi$ channel might render a
possible mass shift due to chiral symmetry restoration very difficult to
disentangle from the decay kinematics.
