  We resum distributions that are singular at partonic threshold (the elastic
limit) in heavy quark production, in terms of logarithmic behavior in moment
space. The method may be applied to a variety of cross sections sensitive to
the edge of phase space, including transverse momentum distributions. Beyond
leading logarithm, dependence on the moment variable is controlled by a matrix
renormalization group equation, reflecting the evolution of composite operators
that represent the color structure of the underlying hard scattering. At
next-to-leading logarithmic accuracy, these evolution equations may be
diagonalized, and moment dependence in the cross section is a sum of
exponentials. Beyond next-to-leading logarithm, resummation involves
matrix-ordering. We give a detailed analysis for the case of heavy quark
production by light quark annihilation and gluon fusion.
