Electromagnetic production of spinless neutral particles in heavy-ion collisions

Abstract

Recently observed electron and positron kinetic-energy peaks at 336 keV with a width 80 keV in several colliding heavy-ion systems have suggested the possible existence of a particle G decaying into $e^{+}$ $e^{\mathrm{-}}$. We study in detail the particle production by the electromagnetic field of the ions for an effective scalar ($E^2$-$B^2$) or pseudoscalar (E⋅B) coupling. Use of an extended nuclear charge distribution yields results consistent with previous calculations using (E⋅B) coupled to point charges, and shows that ($E^2$-$B^2$) production is much stronger than (E⋅B) for the same value of the effective coupling constant g. Nevertheless, comparison with experimental data requires a value for g which is much larger than what one would expect from general considerations about the energy scale of the interaction. Particle production by higher harmonics of the electromagnetic field of orbiting ions is suppressed. We briefly discuss another, nonelectromagnetic, production mechanism.