  This work reports on the real-time detection of internal-state dynamics of
cold Rb$^{87}$ atoms being excited to the $30D_{5/2}$ Rydberg state via
two-photon excitation. A mesoscopic cloud of atoms is overlapped with the mode
volume of a confocal optical cavity and optically pumped by two laser beams
transverse to the cavity axis. The excitation to Rydberg states changes the
collective atom-cavity coupling, which is detected by monitoring the light
transmitted through the cavity while being weakly driven. In addition to the
damped coherent excitation dynamics and the decay back to the ground state, the
data show a superradiant enhancement of the black-body radiation induced
transitions from the $30D_{5/2}$ state to neighboring Rydberg states.
Furthermore, they show a density dependent mitigation of the superradiant decay
which is attributed to long range dipole-dipole interactions between atoms in
the involved Rydberg states. These results contribute to solving a recent
controversy on the interplay between BBR-induced superradiance and Rydberg atom
interactions.
