According to the quantum chromodynamics (QCD), the theory of strong interaction, the nuclear matter at high energies is considered to melt down to the state of deconfined quarks and gluons called quark gluon plasma (QGP). It is expected that the required high temperatures and densities can be accomplished by colliding nuclei at relativistic energies where a large fraction of the kinetic energy of the beam will be converted to thermal energy. The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory and the Large Hadron Collider (LHC) at Geneva can provide nuclear beams for the production of QGP, the primordial fluid that permeated the universe when it was a few microsecond old. Therefore, the study of QGP is very important for early universe QGP. This is an important issue for astrophysics also because the core of the compact astrophysical objects like neutron star may contain QGP.

While at the existing facilities like RHIC and LHC, the QGP may be produced with properties similar to that existed in the microsecond old universe, at the future facilities like FAIR (Germany) the same may be created with characteristics similar to the one that may exist in the core of the neutron star.

Currently, there are rigorous international experimental and theoretical efforts to create and study QGP. A large number of young researchers are working in this exciting field of research. Keeping these factors in mind, we intend to publish a special issue on this field—the nuclear collisions at ultrarelativistic energies.

But the editors would prefer to emphasize issues more on the particle physics aspects than on astrophysical implications, and we would welcome original research articles as well as review articles from both the theorists and experimentalists. Potential topics include, but are not limited to:

• QGP: past developments (origin, QGP diagnostics and the evolutions)
• QGP: present status (vis-à-vis the reportings on the RHIC discoveries)
• QGP: future prospects (with LHC experiments and the continuing RHIC studies)

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