Diagonal boundary conditions in critical loop models

TL;DR

This paper characterizes diagonal boundary conditions in critical loop models using analytic bootstrap methods, deriving explicit formulas for boundary correlation functions and exploring their spectrum and lattice interpretations.

Contribution

It introduces a new class of diagonal boundary conditions in critical loop models, providing explicit formulas for boundary correlators and analyzing their spectrum and lattice implications.

Findings

Diagonal boundaries are characterized by one complex parameter.

Explicit formulas for disc 1-point and 2-point functions are derived.

Boundary spectrum becomes discrete for specific parameter values.

Abstract

In critical loop models, we define diagonal boundaries as boundaries that couple to diagonal fields only. Using analytic bootstrap methods, we show that diagonal boundaries are characterised by one complex parameter, analogous to the boundary cosmological constant in Liouville theory. We determine disc 1-point functions, and write an explicit formula for disc 2-point functions as infinite combinations of conformal blocks. For a discrete subset of values of the boundary parameter, the boundary spectrum becomes discrete, and made of degenerate representations. In such cases, we check our results by numerically bootstrapping disc 2-point functions. We sketch the interpretation of diagonal boundaries in lattice loop models. In particular, a loop can neither end on a diagonal boundary, nor change weight when it touches it. In bulk-to-boundary OPEs, numbers of legs can be conserved, or increase by even numbers.