Actions for QCD-like strings
  We introduce a random lattice corresponding to ordinary Feynman diagrams, with 1/p-squared propagators instead of the Gaussians used in the usual strings. The continuum limit defines a new type of string action with two worldsheet metrics, one Minkowskian and one Euclidean. The propagators correspond to curved lightlike paths with respect to the Minkowskian worldsheet metric. Spacetime dimensionality of four is implied not only as the usual critical dimension of renormalizable quantum field theory, but also from T-duality. 
W. Siegel