Positron line emission in very heavy ion collisions. A status report on experimental results

The detection of narrow lines first in positron singles- and later also in the electron positron sum-energy spectra measured after heavy-ion collisions at the UNILAC accelerator of GSI, Darmstadt has generated increasing interest. Whereas the line phenomenon itself appears to be experimentally well established, no conceivable explanation is given until today although many different attempts have been made. These reach from conventional processes to newly proposed mechanisms, including spontaneous positron emission in long-living quasiatomic heavy-ion formations as well as the production of light neutral fundamental particles. This article, intends to summarize some of the main features of the positron lines including the recent experimental progress. This article covers results from the EPOS and ORANGE experiments and is consequently subdivided into two major parts