Charged pion production in Au on Au collisions at 1 AGeV

Charged pions are measured with the 4 pi detector FOPI at GSI using the Au on Au reaction at 1.06 AGeV bombarding energy. The pion multiplicities n(pi) increase with the number of participants A(part). The average pion multiplicities per participant are <n(pi-)>/<A(part)> = 0.0308 and <n(pi+)>/<A(part)> = 0.0182. These values are only half as large as extrapolated from the low-mass systems studied by Harris et al. The ratio n(pi-)/n(pi+) increases with A(part) and decreases with the pion kinetic energies. The pion kinetic energy spectra have concave shapes, their parametrization in terms of thermal Boltzmann distributions yields a low (T-l,T- pi) and high (T-h,T- pi) temperature which change with the cm emission angle Theta of the pions. In the angular range 45 degrees < Theta < 135 degrees the low temperature <T-l,T- pi- > is larger than <T-l,T- pi+>, the high temperatures <T-h,T- pi->, <T-h,T- pi+> are, within experimental uncertainties, the same. The inclusive polar angular distributions of pions are anisotropic, d sigma/d Omega increases for forward and backward angles. The forward - backward enhancements are independent of the pion kinetic energies or the number of participants. In addition to the preferred forward - backward emission, also the enhanced emission into the transverse direction Theta = 90 degrees is observed for pions with high energies or for pions from near-central collisions. These observations and the shape of the rapidity spectra suggest that pions, emitted from the central rapidity region, are partly rescattered by spectator matter. The strength of the rescattering process depends only weakly on the number of participants. The experimental data are compared to the results of IQMD/GEANT calculations using momentum dependent NN interactions and a hard equation of state. The calculated pion multiplicities are approximately 50% larger than experimentally determined; the existence of secondary pion sources is reproduced by the calculation, but their predicted strengths are larger than experimentally observed.