  We present first-principles calculations of the linewidths of low-energy
quasiparticles in n-doped graphene arising from both the electron-electron and
the electron-phonon interactions. The contribution to the electron linewidth
arising from the electron-electron interactions vary significantly with
wavevector at fixed energy; in contrast, the electron-phonon contribution is
virtually wavevector-independent. These two contributions are comparable in
magnitude at a binding energy of ~0.2 eV, corresponding to the optical phonon
energy. The calculated linewidths, with both electron-electron and
electron-phonon interactions included, explain to a large extent the linewidths
seen in recent photoemission experiments.
