The observation of a long-range, near-side two-particle angular correlation in very high multiplicity proton-proton (pp) and proton-lead (pA) collisions at the LHC has changed our view of multi-particle production in these systems.
This phenomenon, known as the "Ridge", was first seen in high energy nucleus-nucleus (AA) collisions at RHIC. By now, it has been well established that the ridge in AA is related to the anisotropic flow phenomena from collective evolution of a strongly interacting QCD matter. In this talk, I will first review the history of ridge studies in AA collisions and unexpected discovery of the ridge correlations in pp and pA systems. Focus will then be given to the most recent results of pA collisions from 2013 LHC run, including identified particle spectra, two- and multi-particle correlations. I will discuss the new insights these results provide us toward disentangling different theoretical interpretations to the physical origin of the ridge phenomenon in small systems of pp and pA.
