Generalized Kazakov-Migdal-Kontsevich Model: group theory aspects

TL;DR

This paper explores the group-theoretic structure of the generalized Kazakov-Migdal-Kontsevich model, connecting it to character expansions, Toda-lattice tau-functions, and the geometry of the Universal Grassmannian, with implications for 2D Yang-Mills and string models.

Contribution

It introduces a generalized framework for the Kazakov-Migdal-Kontsevich model using group characters and analyzes its relation to integrable systems and Grassmannian elements.

Findings

Model can be expressed as character expansions over GL group.

Partition functions relate to Toda-lattice tau-functions.

Steepest descent leads to complex Grassmannian elements, called 'Kontsevich phase'.

Abstract

The Kazakov-Migdal model, if considered as a functional of external fields, can be always represented as an expansion over characters of $GL$ group. The integration over "matter fields" can be interpreted as going over the {\it model} (the space of all highest weight representations) of $GL$. In the case of compact unitary groups the integrals should be substituted by {\it discrete} sums over weight lattice. The $D=0$ version of the model is the Generalized Kontsevich integral, which in the above-mentioned unitary (discrete) situation coincides with partition function of the $2d$ Yang-Mills theory with the target space of genus $g=0$ and $m=0,1,2$ holes. This particular quantity is always a bilinear combination of characters and appears to be a Toda-lattice $\tau$-function. (This is generalization of the classical statement that individual $GL$ characters are always singular KP $\tau$-functions.) The corresponding element of the Universal Grassmannian is very simple and somewhat similar to the one, arising in investigations of the $c=1$ string models. However, under certain circumstances the formal sum over representations should be evaluated by steepest descent method and this procedure leads to some more complicated elements of Grassmannian. This "Kontsevich phase" as opposed to the simple "character phase" deserves further investigation.