An Improved Mean Field Approximation on the Worldsheet for Planar phi^3   Theory
  We present an improved version of our earlier work on summing the planar graphs in phi^3 field theory. The present treatment is also based on our world sheet formalism and the mean field approximation, but it makes use of no further approximations. We derive a set of equations between the expectation values of the world sheet fields, and we investigate them in certain limits. We show that the equations can give rise to (metastable) string forming solutions. 
Korkut Bardakci and Charles B. Thorn