  We develop a theory for the long-wavelength plasma oscillation of a
collection of charged massless Dirac particles in a solid, as occurring for
example in doped graphene layers, interacting via the long-range Coulomb
interaction. We find that the long-wavelength plasmon frequency in such a doped
massless Dirac plasma is explicitly non-classical in all dimensions with the
plasma frequency being proportional to \hbar^{-1/2}. We also show that the long
wavelength plasma frequency of the D-dimensional superlattice made from such a
plasma does not agree with the corresponding (D + 1)-dimensional bulk plasmon
frequency. We compare and contrast such Dirac plasmons with the well-studied
regular palsmons in metals and doped semiconductors which manifest the usual
classical long wavelength plasma oscillation.
