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 $n_{\rm \pi}$ increase with the number of participants $A_{\rm part}$. The average pion multiplicities per participant are $\frac{}{} = 0.0308$ and $\frac{}{} =0.0182$. These values are only half as large as extrapolated from the low-mass systems studied by Harris et al. The ratio $\frac{n_{\rm \pi^-}} {n_{\rm \pi^+}}$ increases with $A_{\rm part}$ 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 $(T_{\rm l,\pi})$ and high $(T_{\rm h,\pi})$ temperature which change with the cm emission angle $\Theta$ of the pions. In the angular range $45^\circ < \Theta