We present transverse momentum and rapidity spectra of charged pions in central $\mathrm{Ru}+\mathrm{Ru}$ collisions at $400A$ and $1528A\mathrm{MeV}$. The data exhibit enhanced production at low transverse momenta compared to expectations from a thermal model that includes the decay of $\Delta (1232)$ resonances and thermal pions. Modifications of the $\Delta$ spectral function and the Coulomb interaction are necessary to describe the detailed shape of the transverse momentum spectra. Within the framework of the thermal model, the freeze-out radii of pions are similar at both beam energies. The isospin quantum molecular dynamics model reproduces the shapes of the transverse momentum and rapidity spectra of pions, but the predicted absolute yields are larger than in the measurements, especially at lower beam energy.

• Received 12 October 2004

DOI:https://doi.org/10.1103/PhysRevC.71.034902