QFT constraints on non-equilibrium phenomena

21 Sept 2026, 08:30 → 25 Sept 2026, 15:30 Europe/Zurich

4/3-006 - TH Conference Room (CERN)

Alba Grassi (CERN and Université de Genève), Alexander Zhiboedov (CERN), Alexandre Serantes Rubianes (Ghent University), Benjamin Withers (University of Southampton), Michal Heller

This workshop aims to bring together researchers working on nonequilibrium quantum field theory from a variety of perspectives, including black-hole perturbation theory, holography, transport theory, and effective field theory.

Recent years have seen significant progress in understanding universal constraints on nonequilibrium phenomena, both at weak and strong coupling, as well as in applications ranging from heavy-ion physics to quantum gravity. The goal of the workshop is to foster interactions between these communities and stimulate discussions on common principles underlying real-time dynamics in quantum field theory.

The workshop will cover the following topics:

• Developments in black-hole perturbation theory and holographic real-time correlators, together with their mutual interplay.
• Universal bounds on transport imposed by microcausality and related fundamental principles.
• Effective field theory approaches to nonequilibrium dynamics in quantum field theory and their connections to underlying first principles.

 

Invited Participants

• Ines Aniceto (University of Southampton)

• Paolo Arnaudo (University of Southampton)

• Deniz Bozkurt (DESY)

• Richard Davison (Heriot-Watt University)

• Luca Delacretaz (University of Chicago)

• Lorenzo Gavassino (University of Cambridge)

• Cristoforo Iossa (University of Geneva)

• Robin Karlsson (University of Oxford)

• Pavel Kovtun (University of Southampton)

• Enrico Marchetto (DESY Hamburg)

• Mark Mezei (University of Oxford)

• Mukund Rangamani (UC Davis)

• Michał Spaliński (University of Białystok & NCBJ)

• Yi Yin (Chinese University of Hong Kong)

 . . .

Monday 21 September

10:00 → 10:45 Expiscating the black hole singularity in holography

Speaker: Mukund Rangamani

10:45 → 11:15 Coffee break

11:15 → 12:00 The Lorentzian Geometry of Relaxation

I will present a new geometric approach to the study of linearized excitation spectra in relativistic kinetic theory, transient hydrodynamics, and rheology. The starting point is the observation that, if we rotate the Fourier plane to imaginary directions, we obtain a new plane (which we call the “relaxation plane”) naturally endowed with a Lorentzian structure. In this plane, relaxation, instability, and evanescence correspond to different causal directions. As a result, questions in linear response can be addressed graphically, allowing us to derive universal constraints on dispersion relations, deviations from time dilation, the observer dependence of spectral hierarchies, the regime of validity of hydrodynamics in boosted frames, the maximal allowed diffusivity and viscosity of relativistic media, and the presence of non-hydrodynamic branch cuts in kinetic theory.

Speaker: Lorenzo Gavassino

GavassinoCERN.pdf

12:00 → 13:30 Lunch

13:30 → 14:30 Coffee break

14:30 → 16:30 Free time

16:30 → 17:15 Conformal collider physics at large charge in N=4 SYM

We consider energy correlations in large charge states created by a class of heavy half BPS operators in N=4 SYM known as canonical operators. Correlation functions involving these operators admit a semiclassical description and are exactly computable in a double-scaling limit in which the coupling scales as the inverse of the large charge. Building on these results we study the energy correlations in these states at weak, strong and finite effective coupling, providing a solvable toy model for a conformal collider experiment at high multiplicity.

Speaker: Cristoforo Iossa

Tuesday 22 September

10:00 → 10:45 Hydrodynamic maps for nonthermal fixed points

Speaker: David García Fariña

10:45 → 11:15 Coffee break

11:15 → 12:00 Causal bounds on hydrodynamic dominance

Speaker: Robbe Brants

12:00 → 13:30 Lunch

13:30 → 14:30 Coffee break

14:30 → 16:30 Free time

16:30 → 17:15 Near-extremal holographic correlators

Near-extremal black holes with an AdS2 throat are of great interest in string theory and in holography due to their ubiquity as classical gravitational saddles. The emergent SL(2,R) symmetry associated to the throat plays an important role in their low-temperature physics. In this talk, I will describe recent progress on analytically computing holographic correlators in black hole spacetimes with a near-extremal AdS2xR2 near-horizon geometry, focusing on shear correlators of the planar Reissner-Nordström AdS black hole. By improving on previous matching calculations, I will show how it is possible to obtain analytical approximations to the correlators that interpolate between the hydrodynamic (frequencies small compared to the temperature) and the non-hydrodynamic low-temperature regimes. The expressions we obtain capture the hydrodynamic poles, the gapped poles controlled by the SL(2,R) symmetry of the AdS2 throat and the successive collisions with them. I will also comment on the appearance of zero temperature gapless poles in the spectrum and their relation to the SL(2,R) spectrum. If time permits, I will describe how quantum corrections originating from the AdS2 region lift these modes.

Speaker: Blaise Goutéraux

CERN-2026-GOUTERAUX.pdf

17:15 → 18:00 Bootstrapping thermal one-point functions: numerical and analytic approaches

Conformal field theories are powerful laboratories for studying QFT: conformal invariance and radial quantization encode the theory-dependent information in a set of CFT data that can be extracted using numerical and analytic bootstrap methods. At finite temperature, additional state-dependent data arise as new thermal one-point functions. These quantities encode the conformal invariance breaking and determine thermal correlation functions. Recent progress in the thermal bootstrap program has shown that thermal one-point functions can be accessed using both numerical and analytic techniques. In this talk, I will explain how the KMS condition can be used to set up a thermal bootstrap problem as a set of sum rules. Their numerical treatment requires control over the contributions of heavy operators, and I will show how Tauberian methods and dispersion relations grant such control. I will illustrate this framework in critical O(N) vector models at finite temperature and conclude with ongoing work on the Maldacena-Polyakov loop in N=4 SYM.

Speaker: Enrico Marchetto

Wednesday 23 September

10:00 → 10:45 Analytic thermal bootstrap in momentum space: From thermal OPE to QNMs

In this talk, I will present how the analytic bootstrap framework connects ultraviolet OPE data to infrared real-time physics in finite-temperature conformal field theories. Focusing on scalar correlators, I will show how dispersion relations and KMS symmetry lead to a decomposition of momentum-space correlators into thermal Polyakov blocks, yielding an asymptotic expansion of retarded thermal correlators valid at arbitrary spatial momentum. This representation gives rise to inversion formulae that directly relate thermal OPE coefficients to quasinormal-mode data. When the retarded correlator is meromorphic, this UV–IR relation translates into a set of nontrivial sum rules that constrain the quasinormal spectrum and its large-momentum asymptotics. I will illustrate the framework through examples ranging from free theories and two-dimensional CFTs to the O(N) model and R-current of \mathcal N=4 SYM.

Speaker: Deniz Bozkurt

cern_bozkurt.pdf

10:45 → 11:15 Coffee break

11:15 → 12:00 Thermalisation of (1+1)d QFTs at high temperature

Interacting quantum field theories typically thermalize, leading to the emergence of hydrodynamics. I will explain how (1+1)d QFTs at large N provide a setting in which this phenomenon is analytically tractable. In particular, I will argue that the thermal two-point function of the stress tensor is universal in the near-lightcone, high-temperature limit of these theories. This universal expression captures the emergence of a fluid-like state at late times and quantitatively predicts how its properties (e.g. viscosity) are related to the microscopic details of the QFT. I will show that explicit computations in holographic theories are consistent with these predictions.

Speaker: Richard Davison

1+1d-Thermalization-Slides.pdf

12:00 → 14:00 Lunch

14:00 → 15:00 Hydrodynamics from microscopics

Like the plasma created in heavy ion collisions, strongly interacting systems at large energy density tend to behave like a liquid. Their long-lived excitations are associated with long wavelength energy excesses, and these are governed by hydrodynamics as an effective field theory (EFT). Can we derive this EFT from the microscopic Lagrangian? In this talk, we show how the EFT of energy diffusion can be derived in the solvable chaotic many-body model, the SYK lattice. We compute the hydrodynamic effective action to high orders in the derivative expansion, determine all the corresponding transport coefficients, and discuss its symmetries. We outline how this framework extends to the computation of probes of quantum information scrambling and entanglement dynamics using a fourfold complex time contour.

Speaker: Mark Mezei

15:00 → 17:15 Free time

17:15 → 18:00 Causality and instability in Gauss-Bonnet gravity

Speaker: Pavel Kovtun

Kovtun-CERN-2026-09-23.pdf

18:15 → 20:15 Dinner (meet at CERN at 18:15)

Thursday 24 September

10:00 → 10:45 Non-hydrodynamics simplified: symmetry and reduction

Speaker: Yi Yin

Non-hydro-simplified_09_24_2026_CERN_YiYin.pdf

10:45 → 11:15 Coffee break

11:15 → 12:00 OPE = QNM: Analytic Constraints on Thermal Real-Time Response

Thermal correlators admit two complementary descriptions. At short times they are controlled by local operator data through the thermal OPE, while at late times they are governed by collective excitations, or quasinormal modes. We show that these descriptions overlap in a common domain of complex time and therefore constrain one another through analyticity. This relation can be used constructively in both directions. Starting from OPE data, we recover low-lying quasinormal modes by analytic continuation and determine large-overtone frequencies and residues from the singularity structure of the correlator. In Mellin space, the relation becomes a set of spectral sum rules connecting the OPE spectrum to QNM data and constraining the allowed spectral structure. I will illustrate these ideas for holographic thermal correlators, in particular scalar perturbations of the Schwarzschild-AdS_5 black brane. More broadly, this suggests a bootstrap perspective on real-time thermal response in which microscopic data, causality and collective spectra are treated as mutually constrained.

Speaker: Paolo Arnaudo

12:00 → 13:40 Lunch

13:40 → 15:00 Visit to ALICE cavern

15:00 → 16:30 Free time

16:30 → 17:15 Analytic approaches to perturbations of strongly coupled Yang-Mills plasma

In this talk I will study perturbations of Yang-Mills plasma, represented by scalar quasi-normal modes of AdS black branes, as functions of the wave number q. This will be done using a combination of Seiberg-Witten and exact WKB approaches. The instanton expansion naturally provides an analytic understanding of the validity of the classical spectral methods based on the truncation of the boundary value problem. I will then show that a large wave number asymptotic analysis based on exact WKB provides an alternative description that remains accurate far beyond the strict large-q regime, and that can be analytically continued in the entire range of q from zero to infinity.

Speaker: Inês Aniceto

20260924-CERN-presentation.pdf

17:15 → 18:00 Hydrodynamic evolution and the gradient expansion

Speaker: Michał Spaliński

x.pdf

Friday 25 September

10:00 → 10:45 Bounding the emergence of hydrodynamics

Interacting many-body systems thermalize, leading to the emergence of hydrodynamics. This can take a long time, especially at weak coupling. Strong coupling speeds up local thermalization, but not indefinitely: it has been conjectured that there is a universal strong coupling bound on the thermalization time \tau_eq. I will discuss two different proofs of this conjecture, with slightly different scopes: First, hydrodynamic EFTs predict their own breakdown (strong coupling scale) at \tau_eq > \beta / (s_o)^{1/d}, where s_o = s/T^d is the dimensionless entropy density; this bound is very general, but becomes weak at large N. Second, analyticity in the complex time strip | Im t | < \beta/2 implies that the two-sided correlator G_2 cannot exhibit hydrodynamic behavior before \tau_eq > \beta; this bound is specific to G_2, but applies at large N. I will comment on hopes to translate this into a Planckian bound on QNMs.

Speaker: Luca Delacrétaz

CERN hydro bounds.pdf

10:45 → 11:15 Coffee break

11:15 → 12:00 AdS BH EFT and Love numbers

Love numbers characterise a compact object's response to external perturbations and offer a means to distinguish between different heavy objects. We study a heavy-point particle effective field theory in AdS, parameterised by the Love numbers, and its holographic imprints on thermal two-point functions in the boundary theory. By matching against the exact UV results, we explore differences between heavy states and black holes, including the transition from normal to quasi-normal modes, as well as the connection to black hole Love numbers in flat space.

Speaker: Robin Karlsson

12:00 → 13:30 Lunch

13:30 → 14:45 Free time

14:45 → 15:30 Farewell coffee