Strong and Electromagnetic Interactions at SPS Energies

Particle production in peripheral Pb+Pb collisions has been measured at a beam energy of 158 GeV per nucleon, corresponding to psNN 17.3 GeV. The measurements provide full double differential coverage in a wide range of longitudinal and transverse momenta, including the central (“mid-rapidity”) area and extending far into the projectile fragmentation region. The resulting analysis shows the heavy ion reaction as a mixture of different processes. In particular, surprising phenomena, like the presence of large and strongly varying structures in the shape of the double differential cross section d2s /dxFd pT , are induced by the final state electromagnetic interaction between produced particles and the charged spectator system. This effect is largest at low transverse momenta, where it results in a deep valley in the xF -dependence of the produced p+/p− ratio. The basic characteristics of the electromagnetic phenomenon described above agree with the results of a theoretical analysis, performed by means of a simple model of the peripheral Pb+Pb reaction. Versatile information on the heavy ion collision mechanism becomes therefore available. In particular, the electromagnetic effect is sensitive to the initial conditions of particle production (time of final state hadron emission, distance of the formation zone from the two spectator systems). As a result, it may provide new information on the space and time evolution of the particle production process.