  We study the phase diagram of strongly interacting matter with light quarks
using a recently proposed, small parameter approach to infrared QCD in the
Landau gauge. This is based on an expansion with respect to both the inverse
number of colors and the pure Yang-Mills coupling in the presence of a
Curci-Ferrari mass term. At leading order, this leads to the well-known rainbow
equation for the quark propagator with a massive gluon propagator. We solve the
latter at nonzero temperature and chemical potential using a simple
semi-analytic approximation known to capture the essence of chiral symmetry
breaking in the vacuum. In the chiral limit, we find a tricritical point which
becomes a critical endpoint in the presence of a nonzero bare quark mass, in
agreement with the results of nonperturbative functional methods and model
calculations. This supports the view that the present approach allows for a
systematic study of the QCD phase diagram in a controlled expansion scheme.
