Origin of long-range azimuthal correlations in hadronic collisions

I review the models suggested to date as an explanation for the so-called ridge phenomenon: an elongation in rapidity of two-particle correlations seen at energies of the BNL Relativistic Heavy Ion Collider and the CERN Large Hadron Collider. I argue that these models can be divided into two phenomenologically distinct classes: Hotspot + flow-driven correlations, where initial-state correlations created by structures local in configuration space are collimated by transverse flow, and models where the azimuthal correlation is created through local partonic interactions in a high-gluon-density initial state. I argue that the measurement of a strong double ridge in pA and dA collisions allows a good opportunity to understand the ridge's origin because it allows us to see if a common Knudsen number scaling, which is expected if the ridge has a hydrodynamic origin, can be used to understand all data. I show that current data present evidence that this scaling is lacking, which presents a challenge to the hydrodynamic models. On the other hand, particle-identified correlations are a particularly promising way of testing the assumption that distinguishes the two models; namely, of whether the correlation is formed initially in the partonic phase, or as a final-state effect. Assuming fragmentation occurs as in vacuum can be used to predict scaling trends which are generally broken by models, such as hydrodynamics, where the ridge is created as a final-state effect. While evidence is again not fully conclusive, data do seem to follow a scaling compatible with hydrodynamics [Phys. Rev. Lett. 111, 172303 (2013)]. I close by discussing experimental observables capable of clarifying the situation.