We propose a new string model by adding a higher-order gradient term to the rigid string, so that the stiffness can be positive or negative without loosing stability. In the large-D approximation, the model has three phases, one of which with a new type of generalized "antiferromagnetic" orientational correlations. We find an infrared-stable fixed point describing world-sheets with vanishing tension and Hausdorff dimension DH = 2. Crumpling is prevented by the new term which suppresses configurations with rapidly changing extrinsic curvature.PACS: 11.25.Pm 1. So far, fundamental strings [1] can be quantized only in critical spacetime dimensions D c , due to the difficulty of handling the Liouville field, which does not decouple in dimensions different from D c . Strings in four space-time dimensions, however, are of great physical importance since they are supposed to describe the confining phase of non-Abelian gauge theories [2], as confirmed by recent precision numerical tests [3]. As yet, there exists no consistent quantum theory of such strings. Attempts in this direction based on discrete models for the Liouville field failed [4], since these could not be generalized beyond one dimension. Further hope was based on strings with extrinsic curvature stiffness [5], but the infrared irrelevance of stiffness did not prevent the worldsheets from crumpling [6].Recent progress in this field is based on a new type of action. In its local formulation [7], the string action is induced by an antisymmetric tensor field. This action realizes explicitly the necessary zig-zag invariance of confining strings [8]. It can be derived without extra assumptions [9] for the confining phase of compact U (1) gauge theories [10]. The resulting strings are dual versions of the magnetic strings of the Abelian Higgs model [11,12]. Moreover, the action is a specific form of an earlier-proposed class of actions which would generate surfaces with non-trivial elastic properties [13].The characteristic of the effective string action obtained by integrating out the tensor field is a non-local interaction with negative stiffness between world-sheet elements. Such an interaction has been shown [14] to produce the correct high-temperature behavior of confining strings known from large-N QCD calculations [15].Moreover, it has been proposed [16] as a mechanism leading to smooth strings, with "long-range" orientational correlations, a fact confirmed by recent numerical simulations [17].In this letter we report on further progress in this field by considering the simplest possible model in this class of actions, obtained by adding to the rigid string the nextorder gradient term.
The new model is defined in euclidean space by the actionwhere D a are covariant derivatives with respect to the induced metric g ab = ∂ a x µ ∂ b x µ on the surface x (ξ 0 , ξ 1 ) and we have used units c = 1,h = 1. Here, the bracket has to be considered as representing the first few terms in the expansion of the non-local interaction mediated by the original antisymmetric ...