Pion production below the free nucleon-nucleon threshold in heavy-ion reactions

The results of pion (\pi/sup +or-/) production from the CERN synchrocyclotron with /sup 12/C beams of 60, 75 and 85 A MeV on /sup 7 /Li, /sup 12/C and /sup 208/Pb are investigated theoretically. The energy region is particularly interesting because it lies below the free nucleon-nucleon threshold of 290 MeV for the production of pions. Characteristic features of the data include: a) a smooth fall off of the double differential spectra for all targets, with an absolute cross section approximately A/sup 2/3/; b) a strong beam energy dependence of the absolute\pi/sup +or-/ cross sections; and c) an increase in the\pi^{-}/\pi^{+} ratio from unity for target nuclei with neutron excess. The authors perform detailed calculations on the basis of an independent nucleon-nucleon scattering model, with a view to establish its predictive capabilities.