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Planar UST branches and $c=-2$ degenerate boundary correlations

Alex Karrila, Augustin Lafay, Eveliina Peltola and Julien Roussillon

Vol. 6 (2025), No. 4, 1443–1506
Abstract

We provide a conformal field theory (CFT) description of the probabilistic model of boundary effects in the wired uniform spanning tree (UST) and its algebraic content, concerning the entire first row of the Kac table with central charge c = 2. Namely, we prove that all boundary-to-boundary connection probabilities for (potentially fused) branches in the wired UST converge in the scaling limit to explicit CFT quantities, expressed in terms of determinants, which can also be viewed as conformal blocks of degenerate primary fields in a boundary CFT with central charge c = 2.

Moreover, we verify that the Belavin–Polyakov–Zamolodchikov (BPZ) PDEs (i.e., Virasoro degeneracies) of arbitrary orders hold, and we also reveal an underlying valenced Temperley–Lieb algebra action on the space of boundary correlation functions of primary fields in this model. To prove these results, we combine probabilistic techniques with representation theory.

Keywords
conformal field theory, uniform spanning tree, connection probability, primary field, Fomin formula
Mathematical Subject Classification
Primary: 35C05, 60J67, 81T40, 82B20
Secondary: 20C08, 82B27
Milestones
Received: 13 November 2024
Revised: 4 August 2025
Accepted: 14 August 2025
Published: 12 October 2025
Authors
Alex Karrila
Åbo Akademi Matematik
20500 Åbo
Finland
Augustin Lafay
Department of Mathematics and Systems Analysis
Aalto University
00076 Espoo
Finland
Eveliina Peltola
Department of Mathematics and Systems Analysis
Aalto University
00076 Espoo
Finland
Institute for Applied Mathematics
University of Bonn
53115 Bonn
Germany
Julien Roussillon
Department of Mathematics and Systems Analysis
Aalto University
00076 Espoo
Finland