  Calculations of the single particle density of states (SPDOS) of electron
liquids have long predicted that there exist two distinct charged excitations:
the usual quasiparticle consisting of an electron or hole screened by a
correlation hole, and a "plasmaron" consisting of a hole resonantly bound to
real plasmons in the Fermi sea(1,2). Using tunneling spectroscopy to measure
the SPDOS of a two-dimensional electronic system, we demonstrate the first
detection of the plasmaron in a system in which electrons have mass. We monitor
the evolution of the plasmaron with applied magnetic field and discover
unpredicted "magnetoplasmarons" which appear in spectra as negative index
Landau levels. These sharp features corresponding to long-lived quasiparticles
appear at high energies where SPDOS structure is ordinarily broadened by
electron-electron interactions.
