Approaching the Self-Dual Point of the Sinh-Gordon model

TL;DR

This paper investigates the sinh-Gordon model's self-duality at the coupling $b=1$, revealing limitations of truncated spectrum methods near this point and proposing strategies to better understand the model's behavior, including its massless phase.

Contribution

The study develops and assesses truncated spectrum methods for the sinh-Gordon model at finite volume, especially near the self-dual point, and introduces new approaches to overcome computational limitations.

Findings

TSM results agree with expectations for $b eq 1$

Breakdown of TSM near $b=1$ with strong cutoff dependence

Evidence suggesting the $b>1$ phase may be massless

Abstract

One of the most striking but mysterious properties of the sinh-Gordon model (ShG) is the $b \rightarrow 1/b$ self-duality of its $S$-matrix, of which there is no trace in its Lagrangian formulation. Here $b$ is the coupling appearing in the model's eponymous hyperbolic cosine present in its Lagrangian, $\cosh(b\phi)$. In this paper we develop truncated spectrum methods (TSMs) for studying the sinh-Gordon model at a finite volume as we vary the coupling constant. We obtain the expected results for $b \ll 1$ and intermediate values of $b$, but as the self-dual point $b=1$ is approached, the basic application of the TSM to the ShG breaks down. We find that the TSM gives results with a strong cutoff $E_c$ dependence, which disappears according only to a very slow power law in $E_c$. Standard renormalization group strategies -- whether they be numerical or analytic -- also fail to improve upon matters here. We thus explore three strategies to address the basic limitations of the TSM in the vicinity of $b=1$. In the first, we focus on the small-volume spectrum. We attempt to understand how much of the physics of the ShG is encoded in the zero mode part of its Hamiltonian, in essence how `quantum mechanical' vs `quantum field theoretic' the problem is. In the second, we identify the divergencies present in perturbation theory and perform their resummation using a supra-Borel approximate. In the third approach, we use the exact form factors of the model to treat the ShG at one value of $b$ as a perturbation of a ShG at a different coupling. In the light of this work, we argue that the strong coupling phase $b > 1$ of the Lagrangian formulation of model may be different from what is na\"ively inferred from its $S$-matrix. In particular, we present an argument that the theory is massless for $b>1$.