We discuss an extended particle structure whose decay into ete-may be the source of the correlated electron-positron coincidences in heavy-ion collisions measured at GSI. The general properties of the particle, like its excitation spectrum and lifetime, are calculated. The production of the particle in heavy-ion collisions and its influence on other experiments is studied.

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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. PACS: 14.80.Pb; 12.90. +b; 23.20.Nx This is the characteristics of a two-body decay of a (slowly moving) particle-like object X~ § +e-with mass in the 1.8 MeV region. This has led to a large diversity of models and speculations on the nature of the object X ~ [9]. Meanwhile the situation has become even more complex since several line structures have been found which do not all confirm with the two-body decay scenario [10,11].In this paper we will briefly summarize the experimental situation (Sect. 2) and then concentrate on a particular model for a composite X ~ particle [12,13] which was devised to give a comprehensive description of the data (Sect. 3). In Sect. 4 we investigate the decay modes of X ~ bound to a nucleus. Section 5 confronts the particle hypothesis with recent data from Bhabha scattering experiments.

High-energy electrons have successfully been channelled in various crystals.These electrons lose a large fraction of their energy as bremsstrahlung photons within a fraction of a millimetre of crystal. We argue that this high flux of focused photons offers a promising way to study the production of new heavy particles like, for example the Higgs in the collision of two electron beams channelled in the same crystal.

Recently measured g-electron spectra from elastic U + Pd-collisions at 6.1 MeV/u show for small impact parameters enhancements up to two orders of magnitude relative to coupled-channel calculations which describe g-electron production for larger impact parameters almost perfectly. If these deviations are interpreted as due to modified nuclear trajectories they imply most interesting structures of the internuclear potential at the Coulomb-barrier. We show that an alternative explanation, namely an g-electron enhancement due to strong collective nuclear excitations, e.g. giant resonances, does not reproduce the experimental finding.