Abstract
The doubly differential cross sections in mass-asymmetric $^{nat}$In($^{28}$Si, $\pi ^{\pm}$) reactions were measured at 400, 600, and 800 MeV/nucleon. The experiment was performed at the PH2 beam-line of the Heavy Ion Medical Accelerator in Chiba (HIMAC) in the National Institute of Radiological Science (NIRS). A range counter consisted of 13 layers of plastic scintillator was constructed and used for the measurement of charged pions, $\pi^+$ and $\pi^-$. It was found that both $\pi^+$ and $\pi^-$ are emitted isotropically from the common single moving source which velocity is close to the center-of-mass velocity of the entire reaction system. Such slow velocity can be explained by neither a sharp-geometry overlap model nor a transport model (JQMD). Though the total charged pion cross sections and their ratio are reasonably accounted by the default JQMD calculation, the same model fails to reproduce each doubly differential cross section, i.e. the angular dependence of spectra. These facts clearly indicate that significant improvements are required for the commonly used transport model, like JQMD, in order to reliably describe the pion production in nucleus-nucleus collisions at several hundreds MeV/nucleon region before applying it to the investigation of nuclear symmetry energy.