A study of the collective effects in relativistic heavy ion collisions

Motivated by both the collective features observed in relativistic heavy ion reactions and the assumption that QCD vacuum might have properties similar to a type II superconductor we investigate in this thesis a few possible models which may work in an intermediate state corresponding to the vortex line states of the type II superconductor. In these models we assume that several strings from a relativistic heavy ion reaction will form a cluster and then the strings inside such a cluster will interact in a collective way. We argue that with an increasing energy density the hadronic phase may not be directly changed into the QGP phase through a phase transition, but will go through the intermediate state first. Whether the intermediate state can change further into a QGP state in which the strings 'melt' up into an extended flux tube may largely depend on the nature of the collective effects exhibited in this state. The investigations are proved quite successful in describing many experimental data including the high PT enhancement, the low PT enhancement, the production of transverse energy in mid-rapidity, the strangeness production and so on. It also brings up a more fundamental question, can we reach the energy density needed for the formation of a QGP state in the accelerator experiments? The collective effects, like Firecracker, Smokering and color rope which we study in this thesis are of the same character, they will all tend to disperse the energy density obtained in the initial encounter of relativistic heavy ion collisions. This dynamical possibility may actually mean that it will become more difficult to reach the second phase transition, i. e. to 'melt' the vacuum into a quark-gluon plasma. 56 refs