# Investigations of anisotropic flow using multi-particle azimuthal correlations in pp, p-Pb, Xe-Xe, and Pb-Pb collisions at the LHC

## TL;DR

This study measures anisotropic flow coefficients and their correlations in various collision systems at the LHC, revealing collective effects in small systems that challenge existing models and show similarities to larger nucleus-nucleus collisions.

## Contribution

It introduces a novel subevent method for flow measurement and provides detailed flow correlation data across different collision systems and multiplicities.

## Key findings

- Ordered flow coefficients $v_2 > v_3 > v_4$ in small systems.
- Positive symmetric cumulants for $v_2$ and $v_4$, negative for $v_2$ and $v_3$ at low multiplicity.
- Models like PYTHIA 8 and IP-Glasma+MUSIC+UrQMD do not reproduce the observed correlations.

## Abstract

Measurements of anisotropic flow coefficients ($v_n$) and their cross-correlations using two- and multi-particle cumulant methods are reported in collisions of pp at $\sqrt{s} = 13$ TeV, p-Pb at $\sqrt{s_{_{\rm NN}}} = 5.02$ TeV, Xe-Xe at $\sqrt{s_{_{\rm NN}}} = 5.44$ TeV, and Pb-Pb at $\sqrt{s_{_{\rm NN}}} = 5.02$ TeV recorded with the ALICE detector. The multiplicity dependence of $v_n$ is studied in a very wide range from 20 to 3000 particles produced in the mid-rapidity region $|\eta|<0.8$ for the transverse momentum range $0.2 < p_{\rm T} < 3.0$ GeV/$c$. An ordering of the coefficients $v_2 > v_3 > v_4$ is found in pp and p-Pb collisions, similar to that seen in large collision systems, while a weak $v_2$ multiplicity dependence is observed relative to nucleus-nucleus collisions in the same multiplicity range. Using a novel subevent method, $v_{2}$ measured with four-particle cumulants is found to be compatible with that from six-particle cumulants in pp and p-Pb collisions. The magnitude of the correlation between $v_n^2$ and $v_m^2$, evaluated with the symmetric cumulants SC$(m,n)$ is observed to be positive at all multiplicities for $v_2$ and $v_4$, while for $v_2$ and $v_3$ it is negative and changes sign for multiplicities below 100, which may indicate a different $v_n$ fluctuation pattern in this multiplicity range. The observed long-range multi-particle azimuthal correlations in high multiplicity pp and p-Pb collisions can neither be described by PYTHIA 8 nor by IP-Glasma+MUSIC+UrQMD model calculations, and hence provide new insights into the understanding of collective effects in small collision systems.