  A measurement of event-plane correlations involving two or three event planes
of different order is presented as a function of centrality for 7 ub-1 Pb+Pb
collision data at sqrt(s_NN)=2.76 TeV, recorded by the ATLAS experiment at the
LHC. Fourteen correlators are measured using a standard event-plane method and
a scalar-product method, and the latter method is found to give a
systematically larger correlation signal. Several different trends in the
centrality dependence of these correlators are observed. These trends are not
reproduced by predictions based on the Glauber model, which includes only the
correlations from the collision geometry in the initial state. Calculations
that include the final-state collective dynamics are able to describe
qualitatively, and in some cases also quantitatively, the centrality dependence
of the measured correlators. These observations suggest that both the
fluctuations in the initial geometry and non-linear mixing between different
harmonics in the final state are important for creating these correlations in
momentum space.
