Pion Production in Heavy-Ion Collisions at ElabA=35 MeV

Pion production was studied at $\frac{{E}_{\mathrm{lab}}}{A}=35$ MeV in the reactions $^{14}\mathrm{N}$ + Al, Ni, $\mathrm{W}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}+X$. Neutral pions were measured by detecting their two decay $\ensuremath{\gamma}$ rays in coincidence in an array of ten Pb-glass Cherenkov-detector telescopes. The measured cross sections are much larger than predicted within the framework of independent nucleon-nucleon collisions and indicate that, at this energy, pion production is the result of cooperative action among many nucleons.