13-19 May 2018
Venice, Italy
Europe/Zurich timezone
The organisers warmly thank all participants for such a lively QM2018! See you in China in 2019!

Measurement of four-particle cumulants and symmetric cumulants with subevent methods in small collision systems with the ATLAS detector

15 May 2018, 11:50
Sala Mosaici-2, 3rd Floor (Palazzo del Casinò)

Sala Mosaici-2, 3rd Floor

Palazzo del Casinò

Parallel Talk Collectivity in small systems Collectivity in small systems


Dominik Karol Derendarz (Polish Academy of Sciences (PL))


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
Content type Experiment
Centralised submission by Collaboration Presenter name already specified

Primary author

Barbara Krystyna Wosiek (Institute of Nuclear Physics Polish Academy of Sciences (PL))

Presentation Materials