Measurements of long-range two-particle correlation over a wide pseudorapidity range in p$-$Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV

TL;DR

This study extends measurements of long-range particle correlations in p-Pb collisions at 5.02 TeV over a wide pseudorapidity range, revealing persistent collective flow-like behavior and providing insights into the origin of these correlations in small systems.

Contribution

It presents the first observation of the ridge phenomenon extending up to a pseudorapidity gap of 8 in p-Pb collisions, and analyzes the pseudorapidity dependence of flow coefficients across various centralities.

Findings

The ridge persists up to Δη ≈ 8 in p-Pb collisions.

The pseudorapidity dependence of v2(η) is measured over -3.1 < η < 4.8.

Hydrodynamic and transport models suggest final-state interactions dominate flow development.

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 flow) of the produced particles. Extending these measurements over a wider range in pseudorapidity and final-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 $\sqrt{s_{\rm NN}} = 5.02$ TeV are extended to a pseudorapidity gap of $\Delta\eta \sim 8$ between particles using the ALICE, forward multiplicity detectors. After suppressing non-flow correlations, e.g., from jet and resonance decays, the ridge structure is observed to persist up to a very large gap of $\Delta\eta \sim 8$ for the first time in p$-$Pb collisions. This shows that the collective flow-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 flow coefficient, $v_{2}({\eta})$, is extracted from the long-range correlations. The $v_{2}(\eta)$ results are presented for a wide pseudorapidity range of $-3.1 < \eta < 4.8$ in various centrality classes in p$-$Pb collisions. To gain a comprehensive understanding of the source of anisotropic flow in small-collision systems, the $v_{2}(\eta)$ measurements are compared to hydrodynamic and transport model calculations. The comparison suggests that the final-state interactions play a dominant role in developing the anisotropic flow in small-collision systems.