Low-energy pion production in high-energy nucleus-nucleus collisions

Doubly differential cross sections for production of pions with 400-MeV/N ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ on C, NaF, Cu, and Pb have been measured for a range of $20 \mathrm{MeV}<{T_{\pi }}^{\mathrm{LAB}}<100\mathrm{} \mathrm{MeV}$ and $30°<{{\theta }_{\pi }}^{\mathrm{LAB}}<150\mathrm{}°$. The results are compared with previous data with 800-MeV/N ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$. The broad maximum at ${\theta }_{\pi }^{\mathrm{c}.\mathrm{m}.\mathrm{}}=90\mathrm{}°$ in the center-of-mass angular distribution of low-energy pions observed at 800 MeV/N is not seen at the present energy. Neither was such a maximum at the central-rapidity region observed in ($p$+ nucleus) reactions at $E_p=730\mathrm{}$ MeV. The 90° maximum seems to be a phenomenon specific to nucleus-nucleus collisions around 1 GeV/N, where the multiplicity of $\Delta$ formation is high and multiple $\Delta$ interaction could influence the pion production. The distribution at higher bombarding energies has yet to be studied.