  Plasmons in two-dimensional electron systems with nonparabolic bands, such as
graphene, feature strong dependence on electron-electron interactions. We use a
many-body approach to relate plasmon dispersion at long wavelengths to Landau
Fermi-liquid interactions and quasiparticle velocity. An identical
renormalization is shown to arise for the magnetoplasmon resonance. For a model
with N>>1 fermion species, this approach predicts a power-law dependence for
plasmon frequency vs. carrier concentration, valid in a wide range of doping
densities, both high and low. Gate tunability of plasmons in graphene can be
exploited to directly probe the effects of electron-electron interaction.
