  In this work, we investigate the collective flow development in high energy
proton proton (pp) collisions with a multiphase transport model (AMPT) based on
PYTHIA8 initial conditions with a sub-nucleon structure. It is found that the
PYTHIA8 based AMPT model can reasonably describe both the charged hadron
productions and elliptic flow experimental data measured in pp collisions at
$\sqrt{s}=13$ TeV. By turning on the parton and hadron rescatterings in AMPT
separately, we find that the observed collective flow in pp collisions is
largely developed during the parton evolutions, while no significant flow
effect can be generated with the pure hadronic rescatterings. It is also shown
that the parton escape mechanism is important for describing both the magnitude
of the two-particle cumulant and the sign of the four-particle cumulants. We
emphasize that the strong mass ordering of the elliptic flow results from the
coalescence process in the transport model can thus be regarded as unique
evidence related to the creation of deconfined parton matter in high energy pp
collisions.
