Research on the quark-gluon plasma and its probes

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Research was conducted on signals of the quark-gluon plasma formation in ultrarelativistic nucleus-nucleus collision, focusing on the dynamical process of the chiral phase transition and J/ ψ suppression by the onset of Debye screening in the plasma.As a quark-gluon plasma cools down, a phase transition is expected to occur associated with spontaneous break-down of the (approximate) chiral symmetry. We studied the dynamics of the order parameter of this phase transition using the linear sigma model, an effective theory of QCD. We adopted the framework of the functional Schrodinger equation with variational ansatz for the wave-functional. We generalized the classical plane-wave solutions of the non-linear sigma model (the Anselm-Ryskin solutions) to include the effect of the quantum fluctuations by the method. We formulated a mean-field theory of the interacting quantum meson fields based on the Hatree-Bogoliubov method in the many-body problem and showed that our non-covariant functional Schrodinger equations can be transformed to a manifestly covariant form of Klein-Gordon type equations. Our solutions of collective isospin rotating modes of interacting meson fields was also extended to finite temperature using the Gaussian density matrices and the damping time of the modes was estimated by the linear response theory.The problem of J/ ψ suppression as a signal of the quark-gluon plasma was studied focusing on the interplay of a coherent effect such as the plasma screening of the binding potential of heavy quark and its antiparticle and an incoherent effect such as the scattering with individual plasma