  We experimentally investigate the nonlinear transmission spectrum of coherent
light fields propagating through a Rydberg-EIT medium with strong atomic
interactions. In contrast to previous investigations, which have largely
focused on resonant control fields, we explore here the full two-dimensional
spectral response of the Rydberg gas. Our measurements confirm previously
observed spectral features for a vanishing control-field detuning that are
explainable by existing theories, but also reveal significant differences on
two-photon resonance. In particular, we find qualitative deficiencies of
mean-field models and rate-equation simulations in describing the nonlinear
probe-field response under EIT conditions, suggesting spectral signatures of an
interaction-induced resonance with laser-dressed entangled pair states. While
this effect is captured by the third-order nonlinear susceptibility that
accounts for pair-wise interaction effects, the experiments show that many-body
processes beyond such two-body effects play a significant role already at
surprisingly low probe-field intensities. These results suggest that a more
complete understanding of Rydberg-EIT and emerging photon interactions requires
to go beyond existing simplified models as well as few-photon theories.
