Research The Early Stage Of High Energy Nuclear Collisions By Classical Approach

Being the fundamental theory of strong interactions between quarks and gluons,Quantum Chromodynamics(QCD)proves itself to be a victory of quantum field theory with simple but elegant mathematical structures and successful explanations and predictions in experiments.QCD predicts a variety of phase structures of strong-interacting matter on temperature-density plane.At low-temperature and low-density regions of the phase diagram,quarks and gluons with colour charges are confined in colourless hadrons.While at high-temperature or high-density regions,quarks are no longer confined,but have the propagation ranges far beyond the typical scale of hadrons,thus endows the colour deconfinement nature as the typical feature of this QCD phase region.The fundamental degrees of freedom in the deconfinement region are quarks and gluons,corresponds to a specific state of QCD matter called Quark-Gluon Plasma(QGP).We expect to find QGP in 3 places: the early universe,the cores of compact stars and the high energy nuclear collisions.So far the high energy nuclear collision experiments are most suitable approach to investigate the quark gluon plasma,considering its maneuverability.The thesis focuses on the physical picture of the initial stage in high energy nuclear collisions.To investigate the related problems,we adopted the classical approach: the Yang-Mills fields,and developed the related simulation program based on the representation of SU(2)group,with which we performed a systematic study of the energy density,pressure and the correlation functions in the initial stage of the collisions,improved our understanding of the formation and the breaking of the colour strings of the collision system in the stage.Besides,we are also interested in the probes of the collision systems.In high energy nuclear collisions,heavy quarks are produced at very early stages due to their large masses.Together with their little perturbation to the system,heavy quarks become perfect probes in high energy nuclear collisions.We calculated the equation of motion of heavy quarks in the classical Yang-Mills fields in the initial stage with the program we developed,and we find that the impact of the system on the heavy quarks at the initial stage is to broaden the distributions in the momentum space.And we provided an good explanation to the experimental data of the nuclear modification factors through a systematic study of the broadening.We on the one hand proposed a reliable theoretical explanation to the experiments,and on the other hand,the validity of classical Yang-Mills field approach of the high energy nuclear collisions at the initial stage is identified.