Measurements of long-range two-particle correlation over a wide pseudorapidity range in p–Pb collisions at √sNN = 5.02 TeV

Abstract

Correlations in azimuthal angle extending over a long range in pseudorapidity between particles, usually called the “ridge” phenomenon, were discovered in heavy-ion collisions, and later found in pp and p–Pb collisions. In large systems, they are thought to arise from the expansion (collective fow) of the produced particles. Extending these measurements over a wider range in pseudorapidity and fnal-state particle multiplicity is important to understand better the origin of these long-range correlations in small collision systems. In this Letter, measurements of the long-range correlations in p–Pb collisions at √ sNN = 5.02 TeV are extended to a pseudorapidity gap of ∆η ∼ 8 between particles using the ALICE forward multiplicity detectors. After suppressing non-fow correlations, e.g., from jet and resonance decays, the ridge structure is observed to persist up to a very large gap of ∆η ∼ 8 for the frst time in p–Pb collisions. This shows that the collective fow-like correlations extend over an extensive pseudorapidity range also in small collision systems such as p–Pb collisions. The pseudorapidity dependence of the second-order anisotropic fow coefcient, v2(η), is extracted from the long-range correlations. The v2(η) results are presented for a wide pseudorapidity range of −3.1 < η < 4.8 in various centrality classes in p–Pb collisions. To gain a comprehensive understanding of the source of anisotropic fow in small collision systems, the v2(η) measurements are compared with hydrodynamic and transport model calculations. The comparison suggests that the fnal-state interactions play a dominant role in developing the anisotropic fow in small collision systems.