Perturbative calculation of non-perturbative effects in quantum field theory

We define an effective action for local composite operators which is renormalizable to all orders. The perturbation series has no infrared renormalons and calculation of non-perturbative effects becomes possible. For the massgap of the U(N) Gross-Neveu model, a one loop calculation gives the exact N → f8 limit, and for small N values, gives results correct to within 5 à 10 %.