Measurements of multi-particle correlations and collective flow with the ATLAS detector

The measurement of flow harmonics of charged particles from $v_2$ to $v_7$ in Pb+Pb collisions in the wide range of transverse momentum and pseudorapidity provides not only a way to study the initial state of the nuclear collisions and soft particle collective dynamics, but also provides insight into jet quenching via the measurement of flow harmonics at high transverse momenta. The longitudinal fluctuations of the $v_n$ and event-plane angles $\Psi_n$ are also presented. The longitudinal flow decorrelations have contributions from $v_n$-magnitude fluctuations and event plane twist. A four-particle correlator is used to separate these two effects. Results show both effects have a linear dependence on pseudorapidity separation from $v_2$ to $v_5$, and show a small but measurable variation with collision energy. While collectivity is well established in collisions involving heavy nuclei, its evidence in pp collisions is less clear. In order to assess the collective nature of multi-particle production, the correlation measurements are extended to include azimuthal correlations measured using multi-particle cumulants. The measurements of multi-particle cumulants $c_2\{2\-8\}$ confirm the evidence for collective phenomena in p+Pb and low-multiplicity Pb+Pb collisions. For pp collisions, the measurements of cumulants do not yet provide clear evidence for collectivity as they are susceptible to event-by-event multiplicity fluctuations. A new modified cumulant method, which suppresses both the contribution of multiplicity fluctuations and non-flow effects, is used to address this issue.