The tomographic Fermi liquid (TFL) hypothesis posits starkly different relaxation times for odd and even angular harmonics of electron distribution function in two-dimensional systems, but its experimental verification remains elusive. Traditional electrical transport struggles to discern these lifetimes, as resistivity is largely unaffected by electron scattering. Here, we demonstrate that high-order cyclotron resonance (CR) offers a direct probe: The linewidth of the $m$th CR peak directly reflects the relaxation rate ${\gamma }_m=1/{\tau }_m$ of the corresponding angular harmonic. Combining theory and terahertz photoconductivity measurements in graphene, we show that the third-order CR exhibits a narrower linewidth than the second-order CR, yielding ${\tau }_3>{\tau }_2$. This hierarchy defies conventional impurity or phonon scattering models, instead aligning with TFL predictions where odd harmonics evade relaxation via head-on collisions. Our results provide definitive evidence for the TFL regime and establish high-order CR as a powerful tool to unravel hydrodynamic transport in quantum materials.

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• Electrical conductivity
• Electrical properties
• Fermi surface
• Landau levels
• Magneto-optics
• Magnetotransport
• Photoconductivity
• Plasmonics
• Layered semiconductors
• Narrow band gap systems
• Boltzmann theory
• Fermi liquid theory
• Resistivity measurements
• Terahertz spectroscopy