Measurements of longitudinal flow correlations
are presented for charged particles in the pseudorapidity
range |η| < 2.4 using 7 µb−1 and 470 µb−1 of Pb+Pb collisions
at √sNN = 2.76 and 5.02 TeV, respectively, recorded
by the ATLAS detector at the LHC. It is found that the correlation
between the harmonic flow coefficients vn measured
in two separated η intervals does not factorise into the product
of single-particle coefficients, and this breaking of factorisation,
or flow decorrelation, increases linearly with the
η separation between the intervals. The flow decorrelation is
stronger at 2.76 TeV than at 5.02 TeV. Higher-order moments
of the correlations are also measured, and the corresponding
linear coefficients for the kth-moment of the vn are found to
be proportional to k for v3, but not for v2. The decorrelation
effect is separated into contributions from the magnitude of
vn and the event-plane orientation, each as a function of η.
These two contributions are found to be comparable. The
longitudinal flow correlations are also measured between vn
of different order in n. The decorrelations of v2 and v3 are
found to be independent of each other, while the decorrelations
of v4 and v5 are found to be driven by the nonlinear
contribution from v2
2 and v2v3, respectively.