Effective Theories for Nonperturbative Physics 2026

24 Aug 2026, 09:00 → 4 Sept 2026, 17:00 Europe/London

IPPP, Durham University

Carlos Tamarit (Durham IPPP), Felix Yu (JGU Mainz), James Ingoldby (Durham U, IPPP), Rachel Houtz (IPPP Durham), Rodrigo Alonso (Durham), Valentin V Khoze (IPPP)

This workshop, hosted at the IPPP in Durham, continues a series of meetings on Effective Theories for Nonperturbative Physics previously held at CERN and MITP, Mainz. Its goal is to further develop a growing and interdisciplinary community focused on constructing and exploiting effective descriptions of nonperturbative dynamics. A central theme is the interplay between different frameworks, and the extent to which they offer complementary insights into regimes where conventional perturbative expansions break down. The workshop will place particular emphasis on fostering new connections across subfields, including among early career researchers.

Strong coupling and other nonperturbative regimes arise generically in quantum field theory, yet remain comparatively less well understood than their perturbative counterparts. Developing controlled and quantitative descriptions of nonperturbative phenomena is therefore an important objective, both in its own right and for its implications across a wide range of areas. The workshop will highlight the wide variety of qualitative behaviours that emerge in such regimes, and the extent to which they can be captured and organised using effective theoretical frameworks.

We envision the thematic focus of talks and discussions to include:

• Perturbation theory at large quantum numbers and all-orders structures
• Tunnelling phenomena and false vacuum decay at zero and finite temperature
• Hamiltonian truncation and related Hamiltonian approaches
• Topological objects and their phenomenology (instantons, sphalerons, strings, domain walls)
• Nonperturbative phenomena in quantum field theories relevant for phenomenology and model building, including those studied on the lattice
• Interfaces with emerging methods, including quantum computing and quantum simulation, as tools for accessing nonperturbative dynamics

The programme will consist of a mix of research talks and guided discussions, with ample time dedicated to informal collaboration building and exploration of new ideas. A central goal of the workshop is to encourage crosstalk between communities that do not typically interact closely, and to catalyse new ideas and collaborations aimed at tackling nonperturbative problems from multiple perspectives.

Confirmed invited speakers (updated June 15, 2026):
Steve Abel (Durham U., IPPP Durham)
Mohamed Anber (Durham U.)
Daniel Brennan (Birmingham U.)
Oliver Gould (Nottingham U.)
Joerg Jaeckel (U. Heidelberg)
Seth Koren (Notre Dame U.)
Daniel Litim (Sussex U.)
Markus Luty (UC Davis)
Alberto Mariotti (Vrije Universiteit Brussel)
Joan Elias Miro (ICTP Trieste)
Pablo Sesma (IFAE Barcelona)
Mia West (KIT)
Roman Zwicky (U. of Edinburgh)

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Monday 24 August

10:00 → 10:45 Welcome and Icebreakers

10:45 → 11:15 Coffee/tea break

11:15 → 12:00 Generalized symmetries and non-invertible naturalness (30'+15')

Speaker: Seth Koren (Notre Dame U.)

14:00 → 15:00 Talk (45'+15')

Speaker: Roman Zwicky (U. Edinburgh)

15:00 → 15:30 Coffee/tea break

Tuesday 25 August

10:00 → 10:45 Talk (30'+15')

Speaker: Pablo Sesma (IFAE Barcelona)

10:45 → 11:15 Coffee/tea break

11:15 → 12:00 Daisy Resummation Beyond Leading Order (30'+15')

Speaker: Martin Münzenberg (JGU Mainz)

14:00 → 15:00 Metastable defects and gravitational waves (45'+15')

Speaker: Alberto Mariotti (Vrije Universiteit Brussel)

15:00 → 15:30 Coffee/tea break

Wednesday 26 August

10:00 → 10:45 Renormalisable four-fermion interactions in four dimensions (30'+15')

Four-fermion interactions are important in many areas of particle physics, from effective descriptions of weak interactions and QCD to parametrisations of new physics in frameworks such as SMEFT. In this talk I demonstrate that four-fermion interactions may develop strongly coupled UV fixed points in four dimensional QFTs, becoming non-perturbatively renormalisable, and UV completing themselves from the bottom up. The idea will be illustrated in a purely fermionic model of the Gross-Neveu type using the toolbox of the large-N expansion.

Speaker: Charlie Cresswell-Hogg

Four Fermi Theory in Four Dimensions is Renormalisable

10:45 → 11:15 Coffee/tea break

11:15 → 12:00 Non-adiabatic sphaleron biasing and Baryogenesis (30'+15')

Speaker: Miguel Vanvlasselaer (U. Barcelona)

14:00 → 15:00 Talk (45'+15')

Speaker: Litim Daniel

15:00 → 15:30 Coffee/tea break

15:30 → 16:15 Effective methods for bubble nucleation in cosmological phase transitions (30'+15')

I will present an overview of the calculation of the thermal bubble nucleation rate in the context of cosmological phase transitions (PT). Focusing on the case of “SU(2)+Higgs” like theories, I will illustrate the calculation of the nucleation rate in the framework of high-temperature dimensional reduction. I will present next-to-leading order results and discuss the limitations of employing derivative expansion in the perturbation theory.
Finally, I will discuss the most relevant theoretical uncertainties that affect predictions for gravitational-wave (GW) signals from first-order PTs in the early Universe. Furthermore, I will discuss their impact on the possible reconstruction of the underlying BSM physics from measured GW signals.

Speaker: Maciej Kierkla

Finite-temperature bubble nucleation with shifting scale hierarchies

Theoretical uncertainties in reconstructing model parameters with gravitational waves from supercooled phase transitions

Thursday 27 August

10:00 → 10:45 Dynamics of nucleation in first-order phase transitions (30'+15')

I will discuss dynamical effects during bubble nucleation in thermal first-order phase transitions. These effects arise due to lack of local thermal equilibrium and affect the parameters of the transition including the nucleation rate. I will present a general method of calculating the dynamical nucleation rate. I will also present results from real-time classical lattice simulations showing the importance of the non-perturbative contribution to the dynamical prefactor. These results can be important in many areas, from cosmological phase transitions to studies of false vacuum decay in quantum simulators.

Speaker: Andrey Shkerin (Perimeter Institute)

Thermal false vacuum decay in (1+1)-dimensions: Evidence for non-equilibrium dynamics

Thermal False Vacuum Decay Is More Than It Seems

10:45 → 11:15 Coffee/tea break

11:15 → 12:00 Thermal nucleation in perturbation theory (30'+15')

Cosmological first-order phase transitions may have generated a gravitational wave background detectable by LISA, offering a unique window into physics beyond the Standard Model. A crucial step in predicting this background is the reliable computation of bubble nucleation rates. In this talk, we overview the current state of perturbative calculations based on effective field theories and Boltzmann equations. We also introduce a recent approach to nucleation based on Wigner functions, which provides a solid quantum-mechanical foundation for these existing frameworks. In addition, it allows for including further quantum effects to the rate, thereby extending the validity to lower temperatures, and enabling higher precision. We conclude by discussing remaining open questions in perturbative approaches, highlighting directions for future developments.

Speaker: Joonas Hirvonen (Nottingham U.)

Effective field theory approach to thermal bubble nucleation

Quantum field nucleating and Wigner functions

Real-Time Nucleation and Off-Equilibrium Effects in High-Temperature Quantum Field Theories

14:00 → 15:00 Some thoughts on field redefinitions and/or low energy potentials (45'+15')

Speaker: Joerg Jaeckel (Heidelberg U.)

15:00 → 15:30 Coffee/tea break

15:30 → 16:15 Another Brick in the Wall: Domain Wall Networks Beyond Z₂ (30'+15')

Domain walls beyond the textbook Z₂ kink have been studied analytically and through effective models, but seldom simulated directly on the lattice. We close that gap, evolving Z₃ and Z₄ wall networks with CosmoLattice. These networks are qualitatively richer than Z₂: walls come in distinct species, meet along junctions, and can transmute into one another rather than simply annihilating. To capture this we develop a wall-counting algorithm for multi-component fields that separates each species, and use it to measure the late-time scaling area of each. In Z₄ the simulations directly exhibit a stability transition, where "opposite" walls split into pairs of "adjacent" walls, confirming our analytic prediction. Finally, we extend the velocity-dependent one-scale model from Z₂ to general Zₙ, encoding the reduced annihilation efficiency and junction friction so that the measured scaling plateaus feed directly into the network dynamics, and ultimately the gravitational-wave signal.

Speaker: Dhruv Pasari (Durham University)

Friday 28 August

10:00 → 10:45 Higher Charge Monopoles and GUT Physics (30'+15')

Speaker: Daniel Brennan (Birmingham U.)

10:45 → 11:15 Coffee/tea break

11:15 → 12:00 Supercooling in confining theories (30'+15')

Speaker: Gaurang Ramakant Kane

Monday 31 August

10:00 → 10:45 Real-time tunneling calculations in the steadyon picture (30'+15')

Quantum tunnelling is conventionally formulated through Euclidean instantons, obscuring the role of physical time and complicating non-vacuum initial states. I will review the steadyon picture, in which infinitesimally regulated real-time path integrals are controlled by complex saddles whose projections recover familiar instantons. I will summarize its applications to finite-temperature decay, resonance states and states carrying conserved Noether charge, highlighting an emerging unified description of tunnelling beyond the false vacuum.

Speaker: Thomas Steingasser (IFT/CSIC, UAM)

10:45 → 11:15 Coffee/tea break

11:15 → 12:00 Dynamical evolution of the pressure on the bubble wall (30'+15')

First-order cosmological phase transitions proceed through the nucleation and growth of bubbles of the new phase, and can be a leading source of gravitational waves, baryogenesis, and dark matter in the early Universe. A central question is what happens to a bubble immediately after it nucleates: does the wall settle into a slow, subsonic regime, or does it accelerate to ultra-relativistic velocities? This outcome is controlled by hydrodynamic obstruction, whereby heating of the surrounding plasma counteracts the vacuum pressure that drives the wall.

In this talk I focus on the early evolution of the bubble right after nucleation, a stage usually left out of wall-velocity calculations. Standard analyses assume that the fluid around the wall has already relaxed to a steady-state profile, but this assumption can break down precisely while the wall is still accelerating. Working in local thermal equilibrium (LTE) and combining analytical approximations with numerical hydrodynamic simulations, we find that the heating wave often takes longer to form than the wall takes to accelerate, so steady-state predictions fail near the speed of sound. From this we derive a revised criterion for the maximal driving pressure, separating deflagration and hybrid solutions from detonations and runaway walls. Validated against simulations, the criterion shows that hydrodynamic obstruction is less
restrictive than steady-state LTE predictions suggest.

Speaker: Benoit Laurent (Perimeter Institute)

Dynamical evolution of the pressure on the bubble wall

14:00 → 15:00 Chiral Gauge Theory on a Computer (45'+15')

Speaker: Markus Luty (UC Davis)

15:00 → 15:30 Coffee/tea break

Tuesday 1 September

10:00 → 10:45 Extracting zero-damped modes using exact WKB (30'+15')

Zero-damped quasinormal modes are a characteristic feature of near-extremal black holes and are expected to play an important role in their late-time ringdown. In this talk, I will give a pedagogical introduction to the exact WKB method and show how it can be used to study the zero-damped spectrum of near-extremal Reissner--Nordström black holes, highlighting recent developments that make this analysis possible.

Speaker: Nils Wagner (TU Munich)

10:45 → 11:15 Coffee/tea break

11:15 → 12:00 Towards the exotic infrared phases of chiral gauge theories (30'+15')

The infrared structure of gauge theories with chiral fermions remains largely unexplored. In this talk I will discuss the Bars-Yankielowicz class using the functional renormalisation group, building on recent developments in gauge-fermion systems that provide clear criteria for confinement and dynamical symmetry breaking. I will show how two distinct phases arise: one exhibiting both confinement and symmetry breaking at small numbers of colours, and another characterised by confinement without symmetry breaking in the large-colour limit. The latter realises a novel regime, opening the possibility of exotic spectra and phenomena that can now be studied within a systematic framework.

Speaker: Alvaro Pastor Gutierrez

Confinement without symmetry breaking in chiral gauge theories

14:00 → 15:00 Bounding a dark sector ChiPT Lagrangian with unitarity (45'+15')

Speaker: Mia West (KIT)

15:00 → 15:30 Coffee/tea break

15:30 → 16:15 Theoretical uncertainties in (H)EFT: Equations of Motion vs. Field Redefinitions (30'+15')

Effective field theories rely on truncated expansions whose interpretation is intertwined with redundancies from field redefinitions. This interplay raises important questions about power counting and the consistent assignment of theoretical uncertainties associated with neglected higher-dimensional operators. In this talk, I use field redefinition invariance as a tool to assess truncation effects, contrasting equation of motion substitutions with explicit field redefinitions. As a case study in the Higgs Effective Field Theory, I illustrate how universal Higgs modifications interplay with process-dependent sensitivities in realistic observable

Speaker: Shakeel Ur Rahaman (IPPP Durham)

Wednesday 2 September

10:00 → 10:45 UV/IR Mixing Through the Lens of Tensor Networks (30'+15')

Speaker: Steven Abel

10:45 → 11:15 Coffee/tea break

11:15 → 12:00 Effective field theories for phase transitions (30'+15')

Speaker: Oliver Gould (Nottingham U.)

14:00 → 15:00 Talk (45'+15')

Speaker: Joan Elias Miro (ICTP Trieste)

15:00 → 15:30 Coffee/tea break

Thursday 3 September

10:00 → 10:45 Sphaleron Transitions in Real Time

Speaker: Pedro Bittar (Perimeter Institute)

10:45 → 11:15 Coffee/tea break

11:15 → 12:00 Excited-Vacuum Decay from Large-Order Perturbation Theory (30'+15')

Speaker: Yan Luo (DESY and USTC, Hefei)

14:00 → 15:00 Non-perturbative QFT on the twisted four-torus (45'+15')

I will review the old 't Hooft construction of fractional instantons on the twisted four-torus and its relation to the modern language of 1-form symmetry. Then, I will describe how this setup enables us to perform nonperturbative calculations in QFT. In particular, I will discuss how to compute the gaugino condensates in supersymmetric theories on the four-torus.

Speaker: Mohamed Anber (Durham)

15:00 → 15:30 Coffee/tea break

Friday 4 September

10:00 → 10:45 The fermion sign problem in Gauss law sectors of quantum link models with dynamical matter (30'+15')

Speaker: Pallabi Dey (Saha Institute of Nuclear Physics, Kolkata, India)

10:45 → 11:15 Coffee/tea break

11:15 → 12:00 Nonlocal Effective Field Theory Approach to Thermal Photon Condensation in a Strongly Magnetized Vacuum (30'+15')

Speaker: Selym Villalba-Chavez (HHU Düsseldorf)