We report direct spectroscopic measurements of the dynamic effective mass in high‐mobility GaAs/AlGaAs two‐dimensional electron systems (2DESs) via disk‐geometry plasma resonances. From the zero‐field plasmon frequency and independently measured electron density, we extract the dynamic quasiparticle effective mass using the standard two-dimensional plasmon relation. It is established that with decreasing electron density (increasing parameter $r_s$ up to 4) the effective mass of electrons increases by 37% compared to the cyclotron electron mass. The comparison to previous experimental works proves that observed phenomenon can be ascribed to the renormalization of the Fermi-liquid quasiparticle mass at the Fermi surface. Our results demonstrate that collective‐mode spectroscopy provides a direct and complementary method to study many-body effects in 2DESs.

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• Charge density waves
• Plasma oscillations
• Plasmons
• Two-dimensional electron system
• Fermi liquid theory
• Microwave techniques