The fishnet as anti-ferromagnetic phase of worldsheet Ising spins

We identify the strong coupling fishnet diagram with a certain Ising spin configuration in the lightcone worldsheet description of planar Trphi(3) field theory. Then, using a mean field formalism, we take the remaining planar diagrams into account in an average way. Since the fishnet spin configuration is regular but non-uniform, we introduce two mean fields phi, phi' where the fishnet diagram is the case phi = 1, phi' = 0. For general values of these fields, the system is then approximated as a light-cone quantized string with a field dependent effective string tension T-eff (phi, phi'). We also calculate the worldsheet energy density epsilon(phi, phi'), and find the field values that minimize it in the presence of a transverse space infra-red cutoff epsilon > 0. The criterion for string formation is that the tension in this minimum energy state remains non-zero as epsilon --> 0. In the most simple-minded implementation of the mean field method, which neglects all short range correlations of the Ising spins, we find, in this limit, that the tension vanishes for weak and moderate coupling, but for very large coupling does indeed stay non-zero. However, a more elaborate treatment, taking temporal correlations into account (but still neglecting spatial correlations), removes this "phase transition" and the string tension of the minimum energy state vanishes for all values of the coupling when epsilon --> 0. Our mean field analysis thus suggests that the "fishnet phase" of Trphi(3) theory is unstable, and there is no string formation for any value of the coupling. This is probably a reasonable outcome given the instability of the underlying theory. It is encouraging for our method, that an approach designed for a string description can predict, where appropriate, the absence of string formation within an intuitive and simple approximation. (C) 2003 Elsevier B.V. All rights reserved.