  We report measurements of the cyclotron mass in graphene for carrier
concentrations n varying over three orders of magnitude. In contrast to the
single-particle picture, the real spectrum of graphene is profoundly nonlinear
so that the Fermi velocity describing the spectral slope reaches ~3x10^6 m/s at
n <10^10 cm^-2, three times the value commonly used for graphene. The observed
changes are attributed to electron-electron interaction that renormalizes the
Dirac spectrum because of weak screening. Our experiments also put an upper
limit of ~0.1 meV on the possible gap in graphene.
