On the origin of long-range azimuthal correlations in hadronic collisions

Abstract

We review the models suggested, to date, as an explanation for the so called "ridge" phenomenon, an elongation in rapidity of 2-particle correlations seen at RHIC and LHC energies. We 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. We further argue that the key to distinguish these two scenarios is to experimentally examine the role flow plays in the ridge. The measurement of a strong double ridge in $pA$ and $dA$ collisions allows a good opportunity to understand the ridge's origin. In this sense, particle-identified correlations are particularly promising way of testing the assumptions at the base of either of the two models above. The successful fit of the pA ridge by hydrodynamic models \cite{us}, therefore, suggests that the ridge is primarily a "final state effect" generated by radial expansion, although its scaling across system sizes is non-trivial to reconcile with hydrodynamic expectations. We close by discussing experimental observables capable of clarifying the situation.

Accepted for publication, Phys.Rev.C

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Cited by in corpus (10)

• Mass ordering of spectra from fragmentation of saturated gluon states in high multiplicity proton-proton collisions
• Sensitivity of flow harmonics to sub-nucleon scale fluctuations in heavy ion collisions
• Flow harmonics within an analytically solvable viscous hydrodynamic model
• Energy loss as the origin of an universal scaling law of the elliptic flow
• Hadronization Scheme Dependence of Long-Range Azimuthal Harmonics in High Energy p+A Reactions
• Color reconnection effects on resonance production
• Universal geometrical scaling of the elliptic flow
• Azimuthal instabilities of the Gribov-Levin-Ryskin equation
• INFN What Next: Ultra-relativistic Heavy-Ion Collisions
• Scaling of flow in heavy ion collisions and the low-energy frontier