We employ radio-frequency spectroscopy to investigate a polarized spin mixture of ultracold 6 Li atoms close to a broad Feshbach scattering resonance. Focusing on the regime of strong repulsive interactions, we observe well-defined coherent quasiparticles even for unitarity-limited interactions. We characterize the many-body system by extracting the key properties of repulsive Fermi polarons: the energy E + , the effective mass m * , the residue Z , and the decay rate G . Above a critical interaction, E + is found to exceed the Fermi energy of the bath, while m * diverges and even turns negative, thereby indicating that the repulsive Fermi liquid state becomes energetically and thermodynamically unstable.

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