Subthreshold Pion Production in Heavy-Ion Collisions at85AMeV

The ${\pi }^{+}$ production in $85A$-MeV C+C and C+Au reactions has been studied for $20 \mathrm{MeV}<~E_{\pi }<~80 \mathrm{MeV}$ and ${\theta }_{\mathrm{lab}}>55\mathrm{}°$. The cross sections, $\frac{d^2\sigma }{d\Omega \mathrm{dE}}$, fall exponentially with $E_{\pi }$ and are 9 times larger for C+Au than for C+C. The angular distribution is backward peaked for C+Au in the nucleon-nucleon c.m. system, while it is weakly forward peaked in the nucleus-nucleus c.m. system. An independent nucleon-nucleon scattering model can explain the data only if diffuse internal momentum distributions are introduced.