  Recent scattering-type scanning near-field optical spectroscopy (s-SNOM)
experiments on single-layer graphene have reported Dirac plasmon lifetimes that
are substantially shorter than the dc transport scattering time \tau_{tr}. We
highlight that the plasmon lifetime is fundamentally different from \tau_{tr}
since it is controlled by the imaginary part of the current-current linear
response function at finite momentum and frequency. We first present the
minimal theory of the extrinsic lifetime of Dirac plasmons due to scattering
against impurities. We then show that a very reasonable concentration of
charged impurities yields a plasmon damping rate which is in good agreement
with s-SNOM experimental results.
