A Mean Field Approximation to the Worldsheet Model of Planar phi^3 Field   Theory
  We develop an approximation scheme for our worldsheet model of the sum of planar diagrams based on mean field theory. At finite coupling the mean field equations show a weak coupling solution that resembles the perturbative diagrams and a strong coupling solution that seems to represent a tensionless soup of field quanta. With a certain amount of fine-tuning, we find a solution of the mean field equations that seems to support string formation. 
Korkut Bardakci and Charles B. Thorn