  A multi-phase transport model is used to understand the origin of long-range
collective azimuthal correlations in small-system collisions. To disentangle
between collectivity associated with initial-state intrinsic momentum
anisotropy and the collectivity arising as a final-state response to the
collision geometry, we studied the development of collectivity in 5.02 TeV
$p$+Pb collisions with both initial-state and final-state effects included. We
find that the initial momentum anisotropy may not be fully isotropized through
parton interactions, and the final-state partonic collectivity in general are
correlated with both the initial momentum anisotropy and the shape of the
collision geometry. The initial momentum anisotropy also influences the event
by event fluctuation of collective flow. Therefore the mere evidence of
geometry response of the collective flow can not rule out the presence of large
contributions from the initial state.
