  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.

High Energy Physics - Theory