  Finding a comprehensive and general description of the collective Lamb shift
and cooperative broadening in a radiatively interacting system is a
long-standing open question. Both energy levels and linewidth of individual
atoms are modified by the exchange of real and virtual photons making up the
dipole-dipole interaction. We introduce a method to theoretically study
weakly-driven, low-excited ensembles of two-level atoms, and obtain an analytic
description of the collective Lamb shift and linewidth via a self-consistent
formalism including infinite order of correlations which stem from only
two-body interactions. We predict the dependency of these quantities, as
measurables, on system parameters: the number density of the ensemble, the
detuning of an external probe field, and the geometry of the sample.
