The goal of this online colloquium is to bring together postdocs and students associated to the Simons Collaboration on Global Categorical Symmetries through talks accessible to an interdisciplinary audience. We hope to create a common vocabulary between mathematicians and physicists working on categorical symmetries, keep up with current developments in the field, and introduce the most important ideas and definitions. The colloquium is also supposed to strengthen the connection between the younger members of the collaboration and give them a place to discuss their (ongoing) research in an informal environment.
We meet biweekly on Mondays, usually at 8 AM Pacific Time (Los Angeles) = 11 AM Eastern Time (New York City) = 5 PM Central European Time (Berlin). Here is the Zoom information:
Link: https://harvard.zoom.us/j/91934644024?pwd=YAcahZLeAqajIreUMvijwBpmIXrMgb.1
Meeting ID: 91934644024
Password: gauss
If you'd like to receive email announcements about the meetings, join our Google Group! Shoot an email to the organizers (see the end of the page) if you'd like to suggest a speaker or a topic.
Happy holidays! See you in 2026.
Monday Dec 15, 2025 -- 9:30 AM PT = 12:30 PM ET = 6:30 PM CET
Speaker: Thibault Décoppet
Title: The Classification of (3+1)d Symmetry Enriched Topological Order
Abstract: I will explain how a 2-categorical version of (de-)equivariantization can be used to classify (3+1)d topological orders with a finite G-symmetry. In particular, I will argue that (3+1)d fermionic topological orders with G-symmetry correspond to 2SVect-enriched G-crossed braided fusion 2-categories. It turns out that the categorical data necessary to define these theories agrees with that arising from a fermionic generalization of the Wang-Wen-Witten construction of bosonic topological theories with G-symmetry saturating an anomaly. More generally, I will explain how 2-categorical (de-)equivariantization yields a classification of all braided fusion 2-categories. This is joint work with Matthew Yu.
Monday Dec 1, 2025 -- 8 AM PT = 11 AM ET = 5 PM CET
Speaker: Siwei Zhong
Title: Twist Defects of Self-Duality Symmetries
Abstract: We investigate self-duality symmetries in the (1+1)d free compact boson and the (3+1)d Maxwell theory, with an emphasis on the associated twist defects. For the free compact boson, we show that a defect ’t Hooft anomaly can be identified by examining the symmetry fusion rules. This anomaly imposes kinematic constraints on the twist defect. For Maxwell theory, we use DCFT techniques to show that the twist defect operators factorize into two sectors: a universal generalized free-field sector, and a chiral current sector analogous to edge modes in Chern-Simons theory.
Monday Nov 10, 2025 -- 8 AM PT = 11 AM ET = 5 PM CET
Speaker: Peter Huston
Title: Algebraic techniques in 3-categories of fusion categories
Abstract: We will explore concrete computational techniques in the Morita 3-category of fusion categories enriched over an MTC A, with attention to applications in studying topological defect networks / twice extended TQFTs with defects in (2+1)D. In particular, we will see that all levels of morphism can be identified and computed without picking representatives of Morita equivalence classes. Such Morita invariant computational strategies enjoy a natural interpretation in terms of localized excitations (such as anyons) and ground-state degeneracy of topological defect networks, and another natural interpretation in terms of Morita-invertible fusion 2-categories.
Monday Oct 24, 2025 -- 8 AM PST = 11 AM EST = 4 PM CET (daylight savings!)
Speaker: Benjamin Moy
Title: Generalized symmetry enriched criticality in (3+1)d
Abstract: Generalized global symmetries are powerful tools for characterizing phases of matter, but their role at phase transitions is much less explored. In this talk, after reviewing the gapped phases of pure SU(N) and PSU(N) gauge theories in (3+1)d, I will couple to matter in the adjoint representation and construct two classes of quantum critical points between gapped phases that break distinct generalized global symmetries. Specifically, I consider PSU(N) gauge theory coupled to N_f odd flavors of Majorana fermions in the adjoint representation. For N even and sufficiently large N_f, upon imposing time-reversal symmetry and an SO(N_f) flavor symmetry, I will show that the massless theory realizes a quantum critical point with only one relevant operator that can tune a transition between a phase in which a Z_N one-form symmetry is completely broken and another where it is broken to Z_2, leading to Z_{N/2} topological order. The critical point has an enhanced symmetry, which includes non-invertible analogues of time-reversal symmetry. For N and N_f both odd, I will demonstrate that restricting instead only to deformations of the critical point that respect a non-invertible time-reversal symmetry and the SO(N_f) flavor symmetry leads to a transition between different gapped phases—one that spontaneously breaks an invertible one-form symmetry and another that spontaneously breaks the non-invertible time-reversal symmetry, furnishing an analogue of deconfined quantum criticality for generalized symmetries.
May 20, 2024 -- 2 PM UTC
Topic: Defect CFTs and Tensor Networks
Speaker 1: Avia Raviv
Title 1: Line Defects in CFTs: from Magnets and Impurities to Wilson Lines
Speaker 2: Pranay Gorantla
Title 2: Non-invertible duality symmetries using tensor networks
Apr 15, 2024 -- 1 PM UTC
Topic: LSM anomalies and gauging on the lattice
Speaker 1: Ömer Mert Aksoy
Title 1: From Lieb-Schultz-Mattis Theorems to Anomalies with Crystaline Symmetries
Speaker 2: Sahand Seifnashri,
Title 2: Non-invertible lattice translation symmetries and LSM anomalies
Mar 18, 2024 -- 3PM UTC
Speaker 1: Xingyang Yu
Title 1: Stringy Approach to Categorical Symmetries
Speaker 2: Giovanni Galati
Title 2: Exploring Duality Symmetries, Multicriticality, and RG Flows at c=2
Feb 19, 2024 -- 3 PM UTC
Topic: Non-semisimple fully extended skein TQFTs
Speaker 1: Lukas Müller
Title 1: An introduction to Skein Theory
Speaker 2: Luuk Stehouwer
Title 2: The Cobordism Hypothesis: A Computational Approach to TQFTs
Speaker 3: Benjamin Haioun
Title 3: Non-semisimple fully extended skein TQFTs
Dec 18, 2023 -- 12 PM UTC
Topic: Higgs branches and their categorical properties
Speaker 1: Zhenghao Zhong
Title 1: Magnetic Quivers and the Radioactive Higgs Mechanism
Speaker 2: Veronica Pasquarella
Title 2: Moore-Tachikawa Varieties: Beyond Duality
Nov 20, 2023 -- 3 PM UTC
Topic: Defects in TQFTs
Speaker 1: Jenny Brown
Title 1: Defects in Skein Theory
Speaker 2: Wenjun Niu
Title 2: Kazhdan-Lusztig correspondence for a class of Lie superalgebras
May 20, 2024 -- 2 PM UTC
Speaker 1: Andrea Antinucci & Giovanni Rizi
Title 1: An introduction to holographic approach to symmetry
Speaker 2: Azeem Hasan
Title 2: Class S and geometric origin of its non-invertible symmetries
May 20, 2024 -- 2 PM UTC
Speaker 1: Araminta Amabel
Title 1: Factorization Algebras of Observables
Speaker 2: Chris Elliott
Title 2: Framing Anomalies and Kapustin-Witten Theory
May 20, 2024 -- 2 PM UTC
Speaker 1: Jackson Van Dyke & Will Stewart
Title 1: Topological symmetries and the Drinfeld center
Speaker 2: Andrea Grigoletto
Title 2: Representation theory for categorical symmetries
Mar 20, 2023 -- 3 PM UTC
Speaker 1: Héctor Martín Peña Pollastri
Title 1: Hopf algebras, quantum groups, an approach to classification and new examples
Speaker 2: Iordanis Romaidis
Title 2: Mapping class group actions and rational CFT
Feb 27, 2023 -- 3 PM UTC
Speaker 1: Anuj Apte
Title 1: Non-invertible Symmetry Enforced Gaplessness
Speaker 2: Po-Shen Hsin
Title 2: Higher group symmetry and finite group gauge theory
Jan 23, 2023 -- 3 PM UTC
Topic: Anomalies
Speaker 1: Arun Debray & Matthew Yu
Title 1: Background talk on anomalies
Speaker 2: Diego Delmastro
Title 2: Anomalies and all that stuff
Dec 12, 2022 -- 3 PM UTC
Speaker 1: Saghar Sophie Hosseini
Title 1: Geometric engineering of quantum field theories and symmetries from string theory
Speaker 2: Sunghyuk Park
Title 2: 3-manifolds and q-series
Nov 14, 2022 -- 3 PM UTC
Topic: Morita Categories
Speaker 1: Thibault Décoppet & Sean Sanford
Title 1: Morita Theory of (Higher) Fusion Categories: Intro and Examples
Speaker 2: Eilind Karlsson
Title 2: Higher Morita categories and dualizability
Organizers for the academic year 2025-2026: Pranay Gorantla (gorantla@uchicago.edu), Lorenzo Riva (lorenzo@cmsa.fas.harvard.edu), and Zhengdi Sun (zdsun@physics.ucla.edu).
Past organizers:
2024-2025: Sunghyuk Park, Konstantinos Roumpedakis, Pelle Steffens
2023-2024: Andrea Grigoletto, Shani Meynet, Iordanis Romaidis
2022-2023: T. Daniel Brennan, Colleen Delaney, Lukas Müller