  Dirac Fermion, which is the low energy collective excitation near the Dirac
cone in monolayer graphene, have gained great attention by low energy Terahertz
probe. In the case of undoped graphene, it has been generally understood that
the ultrafast terahertz thermal relaxation is mostly driven by the
electron-phonon coupling (EOP), which can be prolonged to tens and hundreds of
picoseconds. However, for the high doped graphene, which manifests the negative
photoinduced terahertz conductivity, there is still no consensus on the
dominant aspects of the cooling process on a time scale of a few picoseconds.
Here, the competition between the disorders assisted defect scattering and the
electron-phonon coupling process in the cooling process of the graphene
terahertz dynamics is systematically studied and disentangled. We verify
experimentally that the ultrafast disorder assisted lattice-phonon interaction,
rather than the electron-phonon coupling process, would play the key role in
the ultrafast thermal relaxation of the terahertz dynamics. Furthermore, the
cooling process features robustness which is independent on the pump wavelength
and external temperature. Our finding is expected to propose a considerable
possible cooling channel in CVD-graphene and to increase the hot electron
extracting efficiency for the design of graphene-based photoconversion devices.
