Longitudinal multiplicity fluctuations and flow decorrelations in Pb+Pb collisions with the ATLAS detector

A comprehensive study of decorrelations of flow harmonics vn is done for Pb+Pb collisions at 2.76 and 5.02 TeV. The decorrelation of vn between η1 and η2 is found to follow a linear dependence on the pseudorapidity separation |η1−η2| for v2−5, and shows a small but a measurable variation with the collision energy. Furthermore, the higher order moments of flow decorrelations are measured. The first measurement of non-linear mode-mixing effects between lower and higher order flow harmonics as a function of pseudorapidity is also presented. These results will help to constrain initial conditions along longitudinal direction and also help to understand the longitudinal evolution of the fireball.