Triangle Seminars

Abstract:
Phases of matter and the transitions between them are famously characterized within the Landau paradigm and Landau–Ginzburg theory: phases are labeled by broken or unbroken symmetries, while phase transitions are described in terms of scalar order parameters. There are, however, phases beyond the standard Landau paradigm that are distinguished not by the realization of ordinary symmetries, but by that of generalized global symmetries. Among these, topologically ordered phases in (2+1)d possess non-invertible 1-form symmetries described by anyon lines. In this talk, I will present a framework for describing transitions between topologically ordered phases related by anyon proliferation, which can be viewed as a non-invertible generalization of the choice of the unbroken symmetry. The resulting theories involve scalar fields coupled to gauge fields. A crucial ingredient in this construction is the (3+1)d Symmetry Topological Field Theory (SymTFT), which allows to separate symmetry from dynamics and will be employed in a rather unconventional way.

Abstract:
I will discuss tree-level wavefunction coefficients in scalar QED in four-dimensional de Sitter space, focusing on a charged scalar pair coupled to an arbitrary number of positive-helicity photons. We find a remarkably simple all-multiplicity structure for these coefficients. I will present the resulting compact formula and explain how its analytic structure, Abelian Ward identities, and conformal symmetry lead to an exact recursion.

Abstract:
The answer is no. Landau's paradigm characterizes phases of matter by their pattern of spontaneous symmetry breaking, traditionally for groups, and has more recently been extended to so-called non-invertible or categorical symmetries. In this colloquium we will see how recent insights into these new symmetries teach us even new things about ordinary group symmetries.

Central to this endeavour is the symmetry topological field theory (SymTFT), which conveniently separates symmetry from dynamics. Gapped phases correspond to topological boundary conditions of the SymTFT, their order parameters, local operators as well as string-operators, to the topological lines in the SymTFT, and phase transitions to condensable algebras.

Many transitions that appear to be beyond Landau, such as transitions between symmetry-protected topological (SPT) phases and certain deconfined quantum critical points in 1+1d, become symmetry-breaking transitions, i.e. of Landau type, after gauging. This may hint at the possibility that all transitions are, after suitable gauging and other manipulations, of Landau symmetry-breaking type. I will explain how, with the help of the SymTFT, we can uncover phase transitions which do not admit any dual frame that would make them of Landau type. Such so-called twin phase transitions occur already for finite groups. This brings us full circle: even for group symmetries, we can learn something new from the recent developments in generalized and categorical symmetries.

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Colloquium at 3 PM, 9 Oct 2026
https://lims.ac.uk/event/are-all-phase-transitions-of-landau-type-in-disguise/

Abstract:
QFT in anti-de Sitter space (AdS) provides a useful setting in which problems in strongly coupled QFT can be reformulated in terms of discrete spectral data and boundary correlation functions. In this informal discussion, I will give an elementary introduction to this point of view, aimed at people who are interested in learning what QFT in AdS is useful for and what kinds of problems are currently being studied.

I will begin with the basic motivation for placing an ordinary QFT on a fixed AdS background, and explain how the AdS curvature introduces a useful infrared scale while preserving a large symmetry group. I will then discuss some of the main quantities one would like to determine, such as the energy spectrum, boundary OPE data, bulk observables, and their relation to the flat-space S-matrix.

I will try to cover several approaches to these questions, focusing in particular on conformal bootstrap ideas, Hamiltonian truncation, and a recent ODE-based approach in which QFT data are evolved under deformations of the theory. If time permits, I will also comment on bulk locality and on the flat-space limit. The goal is not to give a comprehensive review, but rather to explain the basic ideas and some open questions to people who would like to get a first introduction to this topic.