Graph zeta functions and Wilson loops in a Kazakov–Migdal model

In this paper, we consider an extended Kazakov–Migdal model defined on an arbitrary graph. The partition function of the model, which is expressed as the summation of all Wilson loops on the graph, turns out to be represented by the Bartholdi zeta function weighted by unitary matrices on the edges of the graph. The partition function on the cycle graph at finite N is expressed by the generating function of the generalized Catalan numbers. The partition function on an arbitrary graph can be exactly evaluated at large N, which is expressed as an infinite product of a kind of deformed Ihara zeta function. The non-zero-area Wilson loops do not contribute to the leading part of the 1/N expansion of the free energy but to the next leading. The semi-circle distribution of the eigenvalues of the scalar fields is still an exact solution of the model at large N on an arbitrary regular graph, but it reflects only zero-area Wilson loops.