Multi-particle azimuthal correlations in p-Pb and Pb-Pb collisions at the CERN Large Hadron Collider

TL;DR

This study measures multi-particle azimuthal correlations in p-Pb and Pb-Pb collisions at the LHC to investigate the presence of collective flow phenomena, revealing differences and similarities in correlation patterns across systems and multiplicities.

Contribution

It provides a comparative analysis of azimuthal cumulants in p-Pb and Pb-Pb collisions, highlighting evidence for global flow-like correlations in p-Pb at high multiplicities and detailed behavior of harmonic cumulants.

Findings

Two-particle cumulants decrease with multiplicity in p-Pb, but rise with a gap at high multiplicity.

Pb-Pb collisions show higher cumulant values than p-Pb at similar multiplicities.

In Pb-Pb, $v_2\{4\}$ and $v_2\{6\}$ are comparable and non-zero, indicating a Bessel-Gaussian distribution.

Abstract

Measurements of multi-particle azimuthal correlations (cumulants) for charged particles in p-Pb and Pb-Pb collisions are presented. They help address the question of whether there is evidence for global, flow-like, azimuthal correlations in the p-Pb system. Comparisons are made to measurements from the larger Pb-Pb system, where such evidence is established. In particular, the second harmonic two-particle cumulants are found to decrease with multiplicity, characteristic of a dominance of few-particle correlations in p-Pb collisions. However, when a $|\Delta \eta|$ gap is placed to suppress such correlations, the two-particle cumulants begin to rise at high-multiplicity, indicating the presence of global azimuthal correlations. The Pb-Pb values are higher than the p-Pb values at similar multiplicities. In both systems, the second harmonic four-particle cumulants exhibit a transition from positive to negative values when the multiplicity increases. The negative values allow for a measurement of $v_{2}\{4\}$ to be made, which is found to be higher in Pb-Pb collisions at similar multiplicities. The second harmonic six-particle cumulants are also found to be higher in Pb-Pb collisions. In Pb-Pb collisions, we generally find $v_{2}\{4\} \simeq v_{2}\{6\}\neq 0$ which is indicative of a Bessel-Gaussian function for the $v_{2}$ distribution. For very high-multiplicity Pb-Pb collisions, we observe that the four- and six-particle cumulants become consistent with 0. Finally, third harmonic two-particle cumulants in p-Pb and Pb-Pb are measured. These are found to be similar for overlapping multiplicities, when a $|\Delta\eta| > 1.4$ gap is placed.