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Hot and dense matter: from RHIC to LHC: Theoretical overview

Relativistic heavy ion physics studies the phenomena that occur when a very large (in units of QCD scale $Λ_{\rm QCD}$) amount of energy is deposited into a large (in units of $Λ^{-3}_{\rm QCD}$) volume, creating an extended in space and time domain with an energy density that is large in units of $Λ^{4}_{\rm QCD}$. This includes the mechanism by which the energy is deposited (likely a transformation of the colliding Lorentz-contracted "gluon walls" into the strong longitudinal color fields); approach to thermalization; and the static and dynamical properties of the created quark-gluon plasma. Of particular interest is the fate of symmetries (e.g. chiral $SU_L(3) \times SU_R(3)$, scale, and discrete ${\cal P}$ and ${\cal CP}$ invariances) in hot and dense QCD matter. At present, the program at RHIC has entered a stage where new discoveries are enabled by high precision of the measurements; moreover, an array of new capabilities will soon be available due to the numerous and significant upgrades. Very importantly, we will soon have access to unprecedented energies of colliding ions at the LHC. In addition, future RHIC runs at low energies, FAIR at GSI and NICA at JINR will make possible the studies of QCD matter at high baryon density. I will describe the current status of theoretical knowledge about hot QCD, and the ways in which it may be expected to improve in the near future.