Strong and electromagnetic effects in peripheral nucleus-nucleus collisions at SPS energies

Abstract

This paper deals with low momentum transfer (“soft”) hadronic interactions which build up most of the total cross section in high energy collisions of hadrons or atomic nuclei. The quantitative features of these interactions cannot be predicted on the basis of the present theory of strong interactions, Quantum Chromodynamics. The understanding of these processes relies therefore mostly on experimental knowledge. The main emphasis in this work is put on an attempt to extract new, quantitative information on the process of particle production in the reaction center-of-mass energy regime of several GeV per incoming hadron. This specifically concerns two issues: 1) the change induced in this process by transition from the elementary nucleon-nucleon collision to the nuclear heavy ion reaction, and 2) the possibility of obtaining novel information on the strong interaction on the basis of specific electromagnetic processes. Both issues are studied by means of a new, high quality experimental dataset on π meson production in peripheral lead-lead collisions. This dataset is compared with equally precise reference data on elementary interactions, allowing for the disentanglement of several effects present in the nuclear collision. One of these effects, the electromagnetic interaction between produced particles and the highly charged nuclear remnant (“spectator system”) is subsequently subject to a phenomenological analysis. The results of this analysis suggest that the latter electromagnetic effect can provide new information on the space-time evolution of the reaction.