%0 Journal Article %J Physical Review Letters %D 2016 %T Anomalous broadening in driven dissipative Rydberg systems %A E. A. Goldschmidt %A T. Boulier %A R. C. Brown %A S. B. Koller %A J. T. Young %A Alexey V. Gorshkov %A S. L. Rolston %A J. V. Porto %X We observe interaction-induced broadening of the two-photon 5s-18s transition in 87Rb atoms trapped in a 3D optical lattice. The measured linewidth increases by nearly two orders of magnitude with increasing atomic density and excitation strength, with corresponding suppression of resonant scattering and enhancement of off-resonant scattering. We attribute the increased linewidth to resonant dipole-dipole interactions of 18s atoms with spontaneously created populations of nearby np states. Over a range of initial atomic densities and excitation strengths, the transition width is described by a single function of the steady-state density of Rydberg atoms, and the observed resonant excitation rate corresponds to that of a two-level system with the measured, rather than natural, linewidth. The broadening mechanism observed here is likely to have negative implications for many proposals with coherently interacting Rydberg atoms. %B Physical Review Letters %V 116 %P 113001 %8 2016/03/16 %G eng %U http://arxiv.org/abs/1510.08710 %N 11 %R 10.1103/PhysRevLett.116.113001