A factorization theorem for the invariant mass distribution of top
quarks in the resonance region is derived that allows the systematic
computation and parametrization of perturbative and nonperturbative
effects and that can incorporate different invariant mass
prescriptions and features of jet algorithms. The factorization is in
a first step derived for e+e- collisions and uses a sequence of
effective theories that connect the production energy, mass, and width
scales. The factorization theorem can be used to determine a short
distance top quark mass with accuracy better than Lambda_QCD from top
quark mass recontruction.