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.
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