Induced decay of composite ${\mathit{J}}^{\mathit{P}\mathit{C}}$=$1^{++}$ particles in atomic Coulomb fields

The electron-positron pairs observed in heavy-ion collisions at Gesellschaft für Schwerionen-forschung Darmstadt mbH have been interpreted as the decay products of yet unknown particles with masses around 1.8 MeV. The negative results of resonant Bhabha scattering experiments, however, do not support such an interpretation. Therefore we focus on a more complex decay scenario, where the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ lines result from a two-collision process. We discuss the induced decay of a metastable $1^{++}$ state into ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ pairs. For most realizations of a $1^{++}$ state such a decay in leading order can only take place in the Coulomb field of a target atom. This fact has the attractive consequence that for such a state the Bhabha bounds are no longer valid. However, the absolute value of the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ production cross section turns out to be unacceptably small.