Recent ATLAS measurements of correlations in Pb+Pb and Xe+Xe collisions

ATLAS measurements of flow harmonics (vn) 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 and Pb+Pb collisions at 2.76 and 5.02 TeV. The vn are measured up to n=6 using the two-particle correlations, multi-particle cumulants, and scalar product methods. The vn 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 vn at high-pT and in peripheral collisions. A universal scaling in the pT dependence of the vn is observed for both systems. Measurements of correlations between the vn 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 v2 and v3 in Xe+Xe and Pb+Pb collisions are also presented and compared with predictions from theoretical calculations. 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.