Soutenance de thèse d’habilitation : The Color Glass Condensate (CGC) is a high density form of hadronic matter which controls the interactions in QCD at high energies, or small values of Bjorken x. Predicted by theoretical considerations, this matter might have been seen in the recent experiments at HERA and RHIC, but the experimental conditions should be even more favorable for its observation at LHC. The CGC is `colored’ since made of gluons which carry the `color’ charge of QCD; it is a `glass’ since its internal dynamics is frozen over the natural time scales for high energy scattering; it is finally a `condensate’ since it is characterized by high occupation numbers, or strong classical color fields. High density together with asymptotic freedom imply that the CGC is weakly coupled. Based on this observation, an effective theory has been constructed within perturbative QCD, which describes the evolution of a hadronic wavefunction with increasing energy in the presence of the non-linear effects associated with the high gluon density. This theory predicts the saturation of the gluon distribution for transverse momenta below some characteristic `saturation scale’ which grows rapidly with the energy. It further implies the unitarization of the scattering amplitudes at high energy and provides a natural explanation for some remarkable regularities seen in the experimental data, like the `geometric scaling’ in the HERA data for deep inelastic scattering at small x, or the `high-p_T suppression’ in the deuteron-gold collisions at RHIC.

CEA/Saclay/DSM/SPhT et CNRS