Quantum chromodynamics (QCD) is one part of the modern
quantum field theory of particle physics which describes the interactions among quarks and gluons. At sufficiently high densities and/or temperatures the quarks and gluons confined inside hadrons undergo a
phase transition to a plasma of quarks and gluons and manifested into new phases of QCD, viz. Quark-gluon Plasma (QGP), Color-superconductivity etc. The last three decades of high energy nuclear and particle physics activity has been directed towards the production of this new state of matter through relativistic heavy ion collisions. This has led to experiments at BNL AGS and CERN SPS and to the building of the BNL Relativistic Heavy Ion Collider and the ALICE experiment at the CERN Large Hadron Collider, Geneva. With the reported confirmations of the quark-hadron phase transition at the relativistic heavy ion collision experiments, the first step in the search for QGP, which pervaded the early universe, microseconds after the big bang and which may be present in the core of neutron stars, is complete. QGP is created in the early stage of the collision, it cools rapidly by expanding and emitting various radiation to a hadron
gas and then freeze-out, when the particles leave the fireball and reach the detectors. As the lifetime of QGP is very small, a direct detection of QGP in experiments is not possible. In my talk, we will discuss different perspectives of this new matter:
how is this new state of matter formed, how can this new matter be diagnosed, how does this phase transition affect big bang nucleosysthesis
etc.