  The collective harmonic flow in heavy-ion collisions correlates particles at
all transverse momenta to be emitted preferably some directions. The
factorization breaking coefficient measures the small decorrelation of the flow
harmonics at two different transverse momenta. Using the hydrodynamic model I
study in details the decorrelation of the harmonic flow due to the flow angle
and the flow magnitude decorrelation at two transverse momenta. The effect can
be seen in experiment measuring factorization breaking coefficients for the
square of the harmonic flow vector at two transverse momenta. The hydrodynamic
model predicts that the decorrelation of the flow magnitudes is about one half
of the decorrelation of the overall flow (combining flow angle and flow
magnitude decorrelations). These results are consistent with the principal
component analysis of correlators of flow vectors squared.
