Perturbative deformations of conformal field theories revisited
  We investigate the moduli space of conformal field theories by setting up a canonical mathematical process for exponentiating perturbations corresponding to critical fields. We apply this process to the free field theory and the Gepner models of the Fermat quintic and quartic. We find algebraic obstructions to exponentiating purely perturbative deformations in the case of the quintic, while in the case of the quartic the obstructions vanish. While this result may seem surprising at first, we find an explanation of these effects via the renormalization analysis of Nemeschansky-Sen. 
Igor Kriz