Pion production: A probe for coherence in medium-energy heavy-ion collisions

The production of neutral pions has been studied in reactions of 35 MeV/nucleon $^{14}$N${+\mathrm{}}^{27}$Al,Ni,W and 25 MeV/nucleon $^{16}$O${+\mathrm{}}^{27}$Al,Ni. Inclusive pion differential distributions d\ensuremath{\sigma}/${\mathrm{dT}}_{\ensuremath{\pi}}$, d\ensuremath{\sigma}/d\ensuremath{\Omega}, d\ensuremath{\sigma}/dy, d\ensuremath{\sigma}/${\mathrm{dp}}_{\ensuremath{\perp}}$, and ${d}^{2}$\ensuremath{\sigma}/dy ${\mathrm{dp}}_{\ensuremath{\perp}}$ have been measured by detecting the two pion-decay \ensuremath{\gamma} rays in a setup of 20 lead glass C\ifmmode \check{}\else \v{}\fi{}erenkov detector telescopes. Special care was taken to understand and suppress background events. Effects of pion reabsorption are discussed and it is found that the cross sections presented here are substantially affected by such final state interactions. The comparatively large experimental cross sections and the shape of the spectral distributions cannot be accounted for in single nucleon-nucleon collision or statistical models; they rather call for a coherent pion production mechanism.