Investigation of (e+e−) Correlations in Heavy Ion Collisions with the Double Orange Spectrometer
About 10 years ago positron spectra obtained from heavy-ion (HI) collisions of U+U and U+Cm ions at the Coulomb barrier showed a narrow ( ~ 80 keV ) unexpected line. In subsequent investigations, performed mainly by two groups at GSI, the ORANGE and the EPOS collaboration, positron lines were observed for a variety of HI systems including the so-called subcritical systems (Z target + Z proj < 173) and, thus, excluding spontaneous positron emission as the origin of these lines. Within the experimental resolution of several 10 keV the line energies seemed to be independent of the HI system. This triggered the idea that the ”monoenergetic” positrons could originate from the two-body decay of a neutral particle into an (e+e−) pair. A first pioneering experiment of the EPOS collaboration investigating (e+e− coincidences gave indeed a result consistent with the assumption of a two-body decay. Since then various systems were investigated, and more complex patterns of angular and energy correlations of the (e+e−) pair were observed. This report describes the status of the experiments performed by the ORANGE collaboration. The results obtained by the EPOS collaboration are summarized in the contribution of H. Bokemeyer to this conference. In the following solely experimental results will be presented and discussed. No attempt will be made to describe the various theoretical models and speculations which can be found elsewhere 1.
KeywordsCoulomb Barrier Incident Beam Energy Heavy Partner Positron Line Deep Inelastic Event
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