Ridges in p–A (and pp) collisions

Correlations between particles separated by several units of pseudorapidity were discovered in high-multiplicity pp and p–Pb collisions at the LHC. These long- range structures observed in two-particle correlation functions are reminiscent of features seen in Pb–Pb collisions, where they are often viewed as a signature of collective behavior and the formation of a quark-gluon plasma (QGP). Therefore, the discovery of these 'ridges' in small systems has implications for the study of collectivity in small systems as well as in heavy-ion collisions. The ridges in pp and p–Pb collisions have been studied in the ALICE, ATLAS, CMS, and LHCb experiments to characterize the $p_{T^-}, $\eta^-$, and multiplicity-dependences of the ridge yield, as well as its particle composition.