Can extended objects explain the GSI e+ e− lines?

The anomalous electron-positron coincidences observed in heavy-ion collisions have been interpreted as signal for the pair decay of hitherto unknown neutral objects with masses around 1.8 MeV. We discuss the decay modes of such extended composite particles when they are bound to a nucleus. In particular we investigate the angular correlation of the emitted pair and the competing single-photon decay channel. We confront the particle hypothesis with recent negative results from experiments searching for resonances in Bhabha scattering. The induced pair decay of a metastable 1⁺⁺ state in secondary collisions with target atoms is discussed as a possible explanation.