Pionic Fusion of Heavy Ions.

The previously unobserved phenomenon of heavy-ion pionic fusion is the ideal tool for investigation of coherent mechanisms of pion production, since it since it is the only pionic channel open very near absolute threshold, where the contribution from on-shell nucleon-nucleon mechanisms is the smallest. We begin this paper with a discussion of the existing data on heavy-ion pion production at low energies and review the models intended to provide the requisite coherent pion production mechanisms. We then present our measurement of the 12C(12C, 24Mg)π 0 and 12C(12C, 24Na)π + cross sections at E cm = 137 MeV, which is an order of magnitude closer to threshold than the energy of earlier, inclusive heavy-ion measurements. This constitutes the first experimental observation of the pionic fusion of heavy ions and places severe constraints on the pion production mechanisms. We discuss the impact of this finding on the proposed models, which must now incorporate the kinetic energy of the entire 24-nucleon system as well as the binding energy gained in fusion.