The phenomenon of narrow positron line emission in very heavy ion collisions

Narrow positron lines have been found in very heavy ion collisions which recently could also be connected with a correlated emission of electron lines of comparable energies. The experimental signature of these lines closely resembles the process of a back-to-back emission of an electron-positron pair. Nuclear internal pair conversion and the quasiatomic spontaneous emission of positrons are excluded as a conceivable explanation. The hypothesis of the existence of a light neutral object decaying into electron-positron pairs, widely discussed theoretically, has difficulties to explain the observed production cross sections and is not supported by subsequent experiments