We discuss dense states of QCD matter formed in high-energy hadronic and heavy-ion collisions from the point of view of statistical physics of nonequilibrium processes. For this sake, we first propose a formulation of the dynamical entropy of dense QCD states in the “saturation regime” leading to a color glass condensate. The statistical physics description amounts to describing the modification of the color correlation length with energy as a compression process for which nonequilibrium thermodynamic properties are applicable. We derive an expression of the dynamical entropy in terms of the rapidity evolution of the unintegrated gluon distributions in the colliding nuclei, verifying suitable positivity and irreversibility properties. We extend this approach to the initial preequilibrium (glasma) state of a heavy-ion collision. It allows for a definition of the initial entropy before the evolution towards the hydrodynamic regime as a function of the glasma correlation length and an overlap parameter characterizing the low-momentum spectrum of the glasma state. This initial entropy, by extension to the $\mathcal{N}=4$ Super Yang-Mills theory, is then matched as the key input parameter to the strong coupling evaluation of thermalization towards the hydrodynamic regime based on the AdS/CFT correspondence. It thus allows one to cast a bridge between the weak and strong coupling phases of a heavy-ion reaction.

• Received 8 January 2013

DOI:https://doi.org/10.1103/PhysRevD.87.034042