(upper panels) Elliptic flow relative to the spectator plane, \vtsp, and to the participant plane, \vtf, as a function of transverse momentum in different centrality classes for \PbPb collisions The linear fits to the ratio \vtspvtf are shown as red lines for the individual centrality classes The error bars (open boxes) indicate statistical (systematic) uncertainties The correlated uncertainties, related to the ZDC, and bin-to-bin uncorrelated ones are combined for \vtsp (upper panels) For the ratio \vtspvtf (lower panels), the correlated uncertainty is shown by the grey band at unity.