Study of pion emission processes in Pb + Pb collisions at 17.3 AGeV energy and in Au + Au collisions at 200 AGeV, through the optimization of the silicon strip detector in the STAR experiment at RHIC

Pions are the most abundant particle species produced in the ultra-relativistic heavy-ion collisions. They may carry informations about the formation of a quark gluon plasma (QGP). We have studied charged pions produced in the Pb+Pb collisions at 17.3 AGeV, using the WA98 charged particle spectrometers. We have built the ratio of the transverse mass distribution of the π^- and π⁺, and constructed the correlation function of the π⁺-π^- pair. The ratio provides information about the coulomb interaction of the charged pions with a central charge which is directly related to the freeze out size of the system. The correlation function is sensitive to the relative distance between pions at freeze out. Comparing the experimental results predictions of two models, RQMD and NEXUS, we conclude that few pions are emitted in the early stage of the collision. Such a behavior is hardly compatible with the evolution of a system described only with hadronic degree of freedom. The Relativistic Heavy Ion Collider allows the study of gold nuclei collisions at 200 AGeV which may offer the opportunity of observing signatures of the QGP. If it is formed, its lifetime may be large enough to have a strong impact on many observables. The ones related to charged pions can be studied by the STAR experiment. The Silicon Strip Detector (SSD), the fourth layer of the STAR vertex tracker, will greatly help reconstructing the low transverse momentum pions. In order to extract physical signals, a precise understanding of the detector response is required. A 120 GeV/c pion beam was used to probe the behaviour of the SSD detection modules. Based on this analysis, a detailed simulation of the detector was developed.