### Speaker

### Description

Measurements of four-particle flow cumulants $c_{n}\{4\}=\langle v_n^4\rangle -2\langle v_n^2\rangle^2$ for $n=2$ and 3, and symmetric cumulants $SC(n,m)=\langle v_n^2 v_m^2\rangle-\langle v_n^2\rangle\langle v_m^2\rangle$ for $(n,m)=(2,3)$ and $(2,4)$ are presented in $pp$, $p$+Pb and peripheral Pb+Pb collisions at various collision energies, aiming to probe the long-range collective nature of multi-particle production in small systems. Results are obtained using the standard cumulant method, as well as the two-subevent and three-subevent cumulant methods. Results from the standard method are found to be strongly biased by non-flow correlations as indicated by strong sensitivity to the chosen event class definition. A systematic reduction of non-flow effects is observed when using the two-subevent method and the results become independent of event class definition when the three-subevent method is used. The values of $v_n\{4\}=\sqrt[4]{-c_n\{4\}}$ are found to be constant over the range 40<$N_{ch}$<200 in $pp$ collisions, providing direct evidence that multi-particle collectivity persists to low multiplicity. The measured $SC(n,m)$ shows an anti-correlation between $v_2$ and $v_3$, and a positive correlation between $v_2$ and $v_4$. The magnitude of $SC(n,m)$ is constant with $N_{ch}$ in $pp$ collisions, but increases with $N_{ch}$ in $p$+Pb and Pb+Pb collisions. The normalized symmetric cumulants $SC(n,m)/\langle v_n^2\rangle\langle v_m^2\rangle$ are found to be independent of $p_\mathrm{T}$, suggesting $v_n$-$v_m$ correlations reflect the global properties of the event. These measurements provide further evidence for long-range multi-particle collectivity, and quantify the nature of its event-by-event fluctuations.

Collaboration | ATLAS |
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Content type | Experiment |

Centralised submission by Collaboration | Presenter name already specified |