Charge dependence of pion production in heavy ion collisions

Abstract

Benenson et al.¹ measured the pion production cross-section in nuclear heavy ion collisions in the energy range 100–400 MeV per nucleon with the aim of determining whether collective effects associated with the larger number of nucleons were present in the pion field. An instability in the pion field, such as the putative pion condensation phase transition², would give rise to an enhanced number of physical pions. The expected production cross-section, in the absence of collective effects, is found from two simple models. One of these is a statistical model, in which the number of pions is calculated from a statistical equilibrium. At the other extreme, the nucleons in the target and projectile are assumed to be free and independent, except for their Fermi momentum, and create pions by two-body collisions. The experimental cross-section at the lowest energy was found to be larger than either model by at least an order of magnitude³, suggesting that collective effects are indeed present. Interestingly, the cross-section had unexpectedly strong dependence on the charge of the pion produced. At the highest energy studied, 400 MeV per nucleon, many more π⁻ than π⁺ were produced in the forward direction. At the lowest energy studied, 100 MeV per nucleon, the relative yields are reversed, with more π⁺ produced. The experimental ratios of π⁻ to π⁺, R_−/+ = dσ_π⁻d³p/dσ_π⁺d³p are quoted in Table 1. Benenson et al.¹ explain the π⁻ enhancement in terms of the Coulomb distortions of the pion wavefunctions. Here I include in the same framework an explanation of the low energy result.