PION PRODUCTION – A PROBE FOR COHERENCE IN MEDIUM ENERGY HEAVY ION COLLISIONS

Abstract

This chapter provides an overview of the pion production. In an experiment described in the chapter, neutral pion production was observed in heavy-ion collisions at beam energies as low as 25 MeV/u, where this process consumes a major portion of the total center of mass energy available. At these low beam energies, single nucleon–nucleon collision models and also models that incorporate the cooperative sharing of the beam energy of several nucleons do not reproduce the data. At low beam energies, multiple Coulomb excitation is known as a coherent mechanism to study collective modes of the projectile or target nucleus. The pion kinetic energy spectra, in the laboratory system, are typically peaked at low energies of 10–15 MeV and fall off exponentially toward higher kinetic energies. The inverse slope constants are surprisingly large even at these low beam energies. Single nucleon–nucleon collision mechanisms were found to be successful in explaining pion production data even below the free nucleon–nucleon threshold for pion production.