The positron puzzle or what we know about positron production in heavy ion collisions

I will concentrate on the respective EPOS-experiment, which in addition to various positron lines recently also established a correlated electron positron emission with both the electron- and positron-line at comparable energy. The EPOS system, an electron-positron solenoid spectrometer installed at the UNILAC accelerator, combines for positrons and electrons both transported in a solenoidal field to appropriate Si(Li) detectors, the advantages of wide momentum band characteristics, large efficiency and high energy resolution in the Si(Li)-counters. Positrons and electrons are recorded event by event principially in coincidence with the scattered heavy ions. With the installation of positron-sensitive parallel-plate heavy-ion avalanche counters, the spectrometer offers the possibility of determining the collision kinematics from a measure of the ion scattering angles (θ,φ), the difference of their time of flight, and the ion's energy loss. 59 refs., 12 figs. (author)