Particle correlations from the initial state

Abstract:

The observation in small size collision systems, pp and pA, of strong correlations with long range in rapidity and a characteristics structure in azimuth, the ridge phenomenon, is one of the most interesting results obtained at the LHC. Earlier observations of these correlations in heavy ion collisions at RHIC are standardly attributed to a collective flow due to strong final state interactions, described in the framework of viscous hydrodynamics. Even though  data for small size systems is well described in this framework, the applicability of hydrodynamics is less well grounded and initial state based mechanisms that have been suggested to explain particle correlations. In this talk, we discuss particle correlations from the initial state point of view, with focus on the most recent  theoretical developments.