An 'effective-A' dependence of cross sections in ultra-relativistic heavy-ion collisions and its interference with quark-gluon plasma signatures

It is shown that in ultra-relativistic nucleus-nucleus collisions, as a consequence of the geometry of the collision, there is a dependence of the total and differential cross sections on effective atomic mass numbers of the target and beam nuclei. The general expressions of the effective atomic mass numbers are deduced and they are computed for O+Cu, O+U, S+U, Si+Au and U+U collisions. This particular A-dependence of the cross sections can have a strong effect on the production ratio of two particles of different species. As a consequence it might simulate a signature of quark-gluon plasma which is expected to be given by the yields ratio of two different particles. It is shown that this effect can explain quantitatively the K/π ratios measured in Si+Au and S+W collisions as well as the decrease of the J/ψ production relative to Drell-Yan lepton pairs measured in O+Cu, O+U and S+U collisions. The need of precise measurements of the powers α and α(xF, PT) in inclusive nucleon-nucleus interactions for the understanding of particle production in nucleus-nucleus collisions is stressed. Expectations for future experiments are presented. (orig.)