Constraints from hadron spectra on thermal electromagnetic emission in heavy-ion collisions at the CERN SPS

Abstract

In this thesis the space-time evolution of matter produced in an ultrarelativistic heavy-ion collision is studied using a one-fluid hydrodynamical model. The initial state of the evolution is constrained to reproduce the observed hadron spectra in S+Au and Pb+Pb collisions at the CERN-SPS energy. Simultaneously the photon and electron pair emission are calculated and their dependence on an equation of state (EoS) and initial density distribution are studied. The initial state of a hydrodynamic evolution is difficult to determine. To reduce the arbitrariness of choosing the initial conditions for different collisions, a parametrisation of the initial state based on baryon stopping in terms of the thickness of colliding nuclei was developed. This parametrization is applicable to all nucleus-nucleus collisions measured at the CERN SPS. With only a few parameters the main features of the hadronic spectra can be reproduced in all of them. If the evolution is constrained to reproduce the observed hadronic spectra, the remaining freedom in the equation of state and initial distributions affects different electromagnetic observables in different ways. The photon yield depends clearly on the phase transition temperature but is sensitive to the maximum temperature only at very high values of pT. Intermediate mass lepton pairs reflect the maximum value of temperature during the evolution and are therefore much more sensitive to the initial distributions than to the EoS. On the other hand the mass spectrum of low mass electron pairs is almost insensitive to both EoS and initial state but the shape of electron pair rapidity spectrum depends strongly on the flow and therefore on the EoS.