Pion production was studied at $\frac{E_{\mathrm{lab}}}{A}=35\mathrm{}$ MeV in the reactions ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ + Al, Ni, $\mathrm{W}\to {\pi }^0+X$. Neutral pions were measured by detecting their two decay $\gamma$ 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.

• Received 24 October 1983

DOI:https://doi.org/10.1103/PhysRevLett.52.255