This thesis for "Habilitation à Diriger des Recherches" consists of six chapters. Chapter I provides an overview of my scientific career, including a summary of my research tasks, professional experiences, and scientific output, list of my oral presentations at international conferences and my publications history. Chapter II introduces the background and goals of research in relativistic heavy ion physics, the main axis of research at the RHIC collider facility. Chapter III describes the context of Quarks-Gluon Plasma (QGP) physics including theoretical aspects, experimental aspects, the signatures of deconfinement and the evolution of QGP physics from fixed-target (SPS) to collider (RHIC) beam energies. Chapter IV details my personal contribution to the construction, assembly, installation, operation, evaluation of the signals and the maintenance of (i) the silicon pixel detectors used for the measurements of the charged particles multiplicity in PHOBOS experiment, and also (ii) the silicon vertex tracker (VTX) in PHENIX, with the main goal being to differentiate measurements of the heavy quarks charm and beauty. Chapter V presents my analysis work using the "hit-counting" (4휋) method which allows measurement of the pseudorapidity density distributions of charged particles in PHOBOS at several RHIC energies. This chapter also illustrates my predictions for the LHC as well my publications as principal author and my responsibilities as "Co-convenor" of the multiplicity group in PHOBOS. Finally, chapter VI presents the highlights of the RHIC results: "Nearly Perfect Fluid of Quarks and Gluons". This chapter illustrates a great wealth of scientific discoveries, and some great surprises encountered in the RHIC era which provided new perspectives in quantum chromodynamics (QCD) calculations. At the end of this chapter, I concluded while answering the question: What did we learn and where are we?
