  The magnetotransport properties of massless Dirac fermions in a gapless HgTe
quantum well are investigated. In samples with narrow channels, a large
negative magnetoresistance with a Lorentzian profile is observed, which is
interpreted as a manifestation of electron viscosity due to electron-electron
interaction. Comparison of experiment with theory yields the shear stress
relaxation time of the Dirac fermions caused by electron-electron scattering.
