ATLAS measurements of transverse and longitudinal flow fluctuations

ATLAS measurements of flow harmonics ($v_n$) and their fluctuations in Pb+Pb and Xe+Xe collisions covering a wide range of transverse momenta, pseudorapidity and collision centrality are presented. The measurements are performed using data from Xe+Xe collisions at 5.44 TeV, Pb+Pb collisions at 5.02 TeV, and $p$+Pb collisions at 5.02 and 8.16 TeV. The $v_n$ are measured up to n=6 using the two-particle correlations, multi-particle cumulants, and scalar product methods. The $v_n$ values are also performed using a non-flow subtraction technique that was developed for flow measurements in $pp$ and $p$+Pb collisions. This non-flow subtraction is found to have a significant effect on the measured $v_n$ at high-$p_T$ and in peripheral collisions. A universal scaling in the $p_T$ dependence of the $v_n$ is observed for both systems. Measurements of correlations between the $v_n$ for different order n, studied with three- and four-particle mixed-harmonic cumulants, are also presented, and contributions to these correlations from ``centrality fluctuations'' are also discussed. Measurements of longitudinal flow decorrelations involving two- and four-particle correlations for $v_2$ and $v_3$ in Xe+Xe and $p$+Pb collisions are also presented and compared with the corresponding measurements in Pb+Pb collisions. The four-particle decorrelation is found to not factorize as a product of two-particle decorrelations. The ability of such measurements to distinguish between different models of initial geometry and to reduce the uncertainty in determining the effective shear-viscosity to entropy-density ratio of the QGP is demonstrated.