# Measurement of multi-particle azimuthal correlations in $pp$, $p$+Pb and low-multiplicity Pb+Pb collisions with the ATLAS detector

## TL;DR

This paper measures multi-particle azimuthal correlations in different collision systems to investigate collective behavior, finding evidence in p+Pb and Pb+Pb but not in pp collisions, with results consistent across energies.

## Contribution

It provides a comparative analysis of multi-particle cumulants across pp, p+Pb, and Pb+Pb collisions, highlighting the presence or absence of collective phenomena.

## Key findings

- Collective phenomena confirmed in p+Pb and Pb+Pb collisions.
- No evidence of collective behavior in pp collisions for four-particle cumulants.
- Fourier harmonics depend on the collision system and multiplicity, with the largest in Pb+Pb.

## Abstract

Multi-particle cumulants and corresponding Fourier harmonics are measured for azimuthal angle distributions of charged particles in $pp$ collisions at $\sqrt{s}$ = 5.02 and 13 TeV and in $p$+Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV, and compared to the results obtained for low-multiplicity Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV. These measurements aim to assess the collective nature of particle production. The measurements of multi-particle cumulants confirm the evidence for collective phenomena in $p$+Pb and low-multiplicity Pb+Pb collisions. On the other hand, the $pp$ results for four-particle cumulants do not demonstrate collective behaviour, indicating that they may be biased by contributions from non-flow correlations. A comparison of multi-particle cumulants and derived Fourier harmonics across different collision systems is presented as a function of the charged-particle multiplicity. For a given multiplicity, the measured Fourier harmonics are largest in Pb+Pb, smaller in $p$+Pb and smallest in $pp$ collisions. The $pp$ results show no dependence on the collision energy, nor on the multiplicity.