Nuclear Matter Under Extreme Conditions: from Quark-Gluon Plasma to Neutron Stars

Most of the mass in the Universe is dynamically generated by the strong interaction that can be described by the Quantum Chromodynamics (QCD) in the present Standard Model. However, QCD works only in the limit of asymptotic freedom with small coupling constant at high energies. When the two heavy ions collide each other at high energies, very dense quark-gluon matter (the so-called quark-gluon plasma or QGP) is formed, and QCD cannot properly describe the dynamics in QGP at present. Another major trend in the contemporary QCD research is to explore the neutron-rich nuclear matter produced by the radioactive ion beams. Such an abnormal state of the strongly interacting matter is conjectured to be the core of the neutron star. The dense neutron-rich matter is expected to be the ground of the various exotic phenomena like kaon condensation. Understanding the structure and the dynamics of the neutron matter is one of the major research topics in not only nuclear physics but also astrophysics. In this review, the status and future perspectives of the research on nuclear matter under extreme conditions are briefly summarized.