  The effect of charge-carrier screening on the transport properties of a
neutral graphene sheet is studied by directly probing its electronic structure.
We find that the Fermi velocity, Dirac point velocity, and overall distortion
of the Dirac cone are renormalized due to the screening of the
electron-electron interaction in an unusual way. We also observe an increase of
the electron mean free path due to the screening of charged impurities. These
observations help us to understand the basis for the transport properties of
graphene, as well as the fundamental physics of these interesting
electron-electron interactions at the Dirac point crossing.
