  We investigate the ultrafast relaxation dynamics of hot Dirac fermionic
quasiparticles in multilayer epitaxial graphene using ultrafast optical
differential transmission spectroscopy. We observe DT spectra which are well
described by interband transitions with no electron-hole interaction. Following
the initial thermalization and emission of high-energy phonons, the electron
cooling is determined by electron-acoustic phonon scattering, found to occur on
the time scale of 1 ps for highly doped layers, and 4-11 ps in undoped layers.
The spectra also provide strong evidence for the multilayer stucture and doping
profile of thermally grown epitaxial graphene on SiC.
