Apex and the e+/e− Puzzle: Recent Results

Narrow structures first reported in positron singles energy spectra [1, 2, 3, 4, 5, 6, 7] and later in electron-positron energy distributions [5, 8, 9, 10, 11, 12] associated with collisions of very heavy ions near the Coulomb barrier have been an outstanding puzzle in nuclear physics for nearly fifteen years. In a series of positron and positron/electron experiments by three different groups (EPOS, ORANGE and TORI) ([5] and citations therein) at the GSI UNILAC at Darmstadt, Germany narrow structures (‘lines’) were observed by EPOS and ORANGE for several heavy-ion collision systems, with some features such as the line energies and widths having apparently similar values for the different collision systems.