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Viscous Leptons in the Quark Gluon Plasma

We investigate the shear viscosity of leptons in a strongly coupled quark gluon plasma (QGP). We find that the shear viscosity due to the lepton-quark scattering is inversely proportional to the ratio of electric conductivity of the QGP to temperature up to the leading logarithmic order of the electromagnetic coupling. The finding implies that the thermal leptons form a more viscous fluid than the quarks by a large ratio. Using the known result for the electrical conductivity of strongly coupled plasmas obtained from gauge/gravity duality, we find that the lepton shear viscosity is suppressed compared with the one from lepton-lepton scattering. Consistently, we find an enhancement of the energy loss of hard leptons in a strongly coupled scenario compared with that in a weakly coupled plasma.