Large N and double scaling limits in two dimensions

TL;DR

This paper explores double scaling limits in two-dimensional N=2 supersymmetric sigma models, revealing universal continuum theories, dimensional transmutation, and massless non-BPS particles near critical points.

Contribution

It demonstrates the existence and characterization of double scaling limits in 2D N=2 sigma models, extending matrix model techniques to new non-critical string theories.

Findings

Double scaling limits exist and are universal in the model.

Evidence of dimensional transmutation of the string coupling.

Identification of non-BPS particles becoming massless at singularities.

Abstract

Recently, the author has constructed a series of four dimensional non-critical string theories with eight supercharges, dual to theories of light electric and magnetic charges, for which exact formulas for the central charge of the space-time supersymmetry algebra as a function of the world-sheet couplings were obtained. The basic idea was to generalize the old matrix model approach, replacing the simple matrix integrals by the four dimensional matrix path integrals of N=2 supersymmetric Yang-Mills theory, and the Kazakov critical points by the Argyres-Douglas critical points. In the present paper, we study qualitatively similar toy path integrals corresponding to the two dimensional N=2 supersymmetric non-linear sigma model with target space CP^n and twisted mass terms. This theory has some very strong similarities with N=2 super Yang-Mills, including the presence of critical points in the vicinity of which the large n expansion is IR divergent. The model being exactly solvable at large n, we can study non-BPS observables and give full proofs that double scaling limits exist and correspond to universal continuum limits. A complete characterization of the double scaled theories is given. We find evidence for dimensional transmutation of the string coupling in some non-critical string theories. We also identify en passant some non-BPS particles that become massless at the singularities in addition to the usual BPS states.