  A number of interesting properties of graphene and graphite are postulated to
derive from the peculiar bandstructure of graphene. This bandstructure consists
of conical electron and hole pockets that meet at a single point in momentum
(k) space--the Dirac crossing, at energy $E_{D} = \hbar \omega_{D}$. Direct
investigations of the accuracy of this bandstructure, the validity of the
quasiparticle picture, and the influence of many-body interactions on the
electronic structure have not been addressed for pure graphene by experiment to
date. Using angle resolved photoelectron spectroscopy (ARPES), we find that the
expected conical bands are distorted by strong electron-electron,
electron-phonon, and electron-plasmon coupling effects. The band velocity at
$E_{F}$ and the Dirac crossing energy $E_{D}$ are both renormalized by these
many-body interactions, in analogy with mass renormalization by electron-boson
coupling in ordinary metals. These results are of importance not only for
graphene but also graphite and carbon nanotubes which have similar
bandstructures.
