Constraining Effective Field Theories without Lorentz

Jul 7, 2025, 9:00 AM → Jul 11, 2025, 4:20 PM Europe/Rome

Room D (SISSA Miramare)

Effective Field Theories (EFTs) are the cornerstone of modern theoretical physics. In the last fifteen years it has been widely appreciated that “not anything goes” in EFTs: fundamental principles such as unitarity and causality place strong constraints on the space of possible theories. The question of how to extend this observation to contexts where Lorentz invariance is spontaneously broken is attracting increased attention. Several approaches have been put forward, each with its own advantages and drawbacks. Time is ripe to bring together experts to share viewpoints and debate recent developments. This workshop will host a diverse group of international researchers, with complementary perspectives and expertise, to discuss open questions and define theoretical targets across different fields, with the ultimate goal of learning what is allowed—and what is not—in our Universe.

 

Organizers: 

Paolo Creminelli (ICTP)

Alessandro Podo (IHES, Bures-sur-Yvette)
Borna Salehian (ETH, Zurich)


 

This workshop is an IFPU Focus Week, organized at the IFPU institute.
The institute is located next to ICTP, in the Miramare campus at Via Beirut 2, Trieste, Italy. You can find information on how to reach it at this webpage: https://www.ifpu.it/how-to-reach-us/

 

Ristorante La Terrazza has been reserved for the workshop dinner on Wednesday, 9th July, at 8:00 PM.

 

Mon, July 7

9:50 AM → 10:00 AM Welcome

10:00 AM → 11:00 AM Positivity bounds: electromagnetic properties of media, and more / Simple properties of Green's function without Lorentz invariance

Part 1: I will start talking about the constraints imposed on the electromagnetic response of media by microcausality (commutators of local fields vanish outside the light cone) and positivity of the imaginary parts (the medium can only absorb energy from the external field). Modeling the medium as composed of small polarizable particles, one finds that similar positivity bounds apply to their electromagnetic polarizability coefficients. This naturally extends to gravitational systems, offering analogous constraints on the tidal response coefficients of weakly gravitating compact objects.

Part 2: We study general properties of retarded Green’s function without Lorentz symmetry. We present constraints on the dissipative part (imaginary part) solely arising from analyticity in forward light cone. Using multi-variable dispersive representation, we show that retarded Green’s functions are Herglotz functions.

Speakers: Ahmadullah Zahed, Borna Salehian

EM positivity_IFPU.pdf

11:00 AM → 11:30 AM Coffee break

11:30 AM → 12:30 PM Free discussion

12:30 PM → 2:30 PM Lunch break

2:30 PM → 3:30 PM The hydrohedron: rigorous bounds on transport from causality

As an effective theory constructed in a gradient expansion, relativistic hydrodynamics is fixed by symmetries up to a set of transport coefficients. I will utilise bootstrap techniques to rule out theories that are inconsistent with microscopic causality. What remains is a universal convex geometry in the space of transport coefficients, "the hydrohedron”. I will also discuss the necessary role played by non-hydrodynamic degrees of freedom in achieving causality. Based on 2212.07434 and 2305.07703.

Speaker: Benjamin Withers

withers.pdf

3:30 PM → 4:00 PM Coffee break

4:00 PM → 5:00 PM Analyticity and causality in Lorentz-breaking backgrounds: scattering vs correlators

We will describe the analytic structure and positivity properties of the perturbative S-matrix of excitations in a relativistic phi^4 superfluid. Motivated by this, we will then discuss how microcausality implies non-perturbative analyticity properties of correlators in a mixed (t,k) representation. This result applies to generic relativistic QFTs in homogeneous and isotropic backgrounds, including finite temperature or density, and cosmological spacetimes.

Speakers: Alessandro Podo, Shengjia Zhou

IFPU-1.pdf

IFPU-workshop.pdf

Tue, July 8

10:00 AM → 11:00 AM Positivity from thermodynamics

I will discuss the extent to which thermodynamic considerations can be leveraged to derive positivity bounds on low energy effective theories.

Speaker: Riccardo Penco

11:00 AM → 11:30 AM Coffee break

11:30 AM → 12:30 PM Free discussion

Convener: Patrick Mcblain

12:30 PM → 2:30 PM Lunch break

2:30 PM → 3:30 PM Dressing and Screening in de Sitter

For a gauge theory in dS, I will discuss how to define and compute properly dressed late-time correlation functions of charged fields, and an analogue of Debye screening that affects the late-time decay of the electric field. I will briefly review the use of conformal symmetry to reveal analyticity and positivity properties of late-time correlation functions, and discuss these properties in the gauge theory example.

Speaker: Lorenzo Di Pietro

Seminar IFPU.pdf

3:30 PM → 4:00 PM Coffee break

4:00 PM → 5:00 PM Free discussion

Wed, July 9

10:00 AM → 11:00 AM An Open System Approach to Gravity and Cosmology

Cosmological models and predictions rely extensively on the well-established field theory framework of particle physics. However, a qualitatively new challenge arises: cosmological systems inherently contain substances with poorly constrained macroscopic properties and entirely unknown microphysics, such as the inflaton sector, dark matter, and dark energy. This results in a rich array of novel phenomena, including dissipation, stochastic fluctuations, out-of-equilibrium dynamics, and non-unitary macroscopic evolution. Moreover, since gravitational observables are of primary interest, and gravity universally couples to all forms of matter, a precise description of all cosmic constituents is required—something feasible only in the simplest toy models. To address these challenges, I propose an open system approach to cosmology.

I begin with a pedagogical introduction to open quantum system techniques, formulated within the Schwinger-Keldysh path integral framework. Then, I present the open effective field theory of inflation as a general class of theories of single-field inflation in the presence of an unknown medium. This local dissipative single-field effective theory yields a new class of predictions for cosmological correlators, generalizing existing models. I then tackle the challenge of formulating general relativity in the presence of an unspecified medium. As a warmup, I present a Schwinger-Keldysh formulation of electromagnetism in a medium, incorporating dissipation and fluctuations while ensuring a consistent treatment of gauge symmetries within an open system framework. Building on these results, I introduce the general and systematic construction of dissipative extensions of general relativity and explore their implications for modeling open dark energy and the late-time evolution of the universe. Finally, I study the implications for the dissipative propagation of gravitational waves through the dark sector medium.

Speaker: Enrico Pajer

Trieste_ICTP_July_2025.pdf

11:00 AM → 11:30 AM Coffee break

11:30 AM → 12:30 PM Free discussion

12:30 PM → 2:30 PM Lunch break

2:30 PM → 3:30 PM Bispectrum Islands

Motivated by the success story of the modern S-matrix bootstrap in mapping out the space of scattering amplitudes consistent with fundamental principles, I explore how a similar framework can be applied to cosmological correlators generated during inflation. In particular, I focus on the bispectrum of curvature perturbations sourced by interactions with a generic hidden sector during inflation, and I show that the result must obey a set of positivity constraints stemming from the interplay between unitarity and de Sitter isometries. While I assume certain broad features of the hidden sector, I remain agnostic about its microscopic details. As a result, the derived positivity bounds are broadly applicable to a wide range of inflationary UV completions—whether the bispectrum arises from the exchange of weakly coupled fields, such as Kaluza-Klein modes or moduli in extra-dimensional models, or from operators with continuous spectra, as in conformal field theories.

Speaker: Sadra Jazayeri

BispectrumIslandsv2.pdf

3:30 PM → 4:00 PM Coffee break

4:00 PM → 5:30 PM Discussion session: Discussion session: Cosmology

Conveners: Diptimoy Ghosh, Lam Hui, Paolo Creminelli

8:00 PM → 9:00 PM Social Dinner

Thu, July 10

10:00 AM → 11:00 AM Graviton loops and negativity

I will explain some challenges introduced by loop corrections to gravitational EFT bounds on Wilson coefficients and how they can be overcome. Time permitting, I will show that bounds without IR divergences from massless graviton exchange can be obtained under certain reasonable assumptions.

Speaker: Julio Parra-Martinez

25-07_IFPU.pdf

11:00 AM → 11:30 AM Coffee break

11:30 AM → 12:30 PM Free discussion

Convener: Giulia Isabella

12:30 PM → 2:30 PM Lunch break

2:30 PM → 3:30 PM All but bounds on Love numbers

I will discuss the EFT describing compact objects, characterized by Wilson coefficients knowns as Love numbers. I will present a method for computing this gravitational EFT to high orders and highlight the challenges involved in placing bounds on these coefficients.

Speaker: Giulia Isabella

Trieste-1.pdf

3:30 PM → 4:00 PM Coffee break

4:00 PM → 5:30 PM Discussion session: Targets and Synergies

Conveners: Joan Elias Miro, Julio Parra-Martinez, Luca Santoni

6:30 PM → 7:30 PM Social Dinner: Osmiza

Fri, July 11

10:00 AM → 11:00 AM Effective field theories of hydrodynamics and magnetohydrodynamics

I will discuss recent advances in constructing and analysing effective field theories (EFTs) that capture the dynamics of collective thermal systems, in particular of dissipative fluids and plasmas described by hydrodynamics and magnetohydrodynamics. I will begin by reviewing how these EFTs, which encode the global symmetries of the underlying microscopic dynamics, are formulated within the Schwinger–Keldysh closed-time-path formalism. Holographic duality has provided valuable insights into the structure of these theories and the behaviour of hydrodynamic observables by offering explicit, calculable toy models. Building on this, I will examine what has been learnt so far about the analytic properties of hydrodynamic correlators when (stochastic) loop corrections are taken into account. Finally, I will present our recent efforts to better understand the derivation of dissipative Schwinger–Keldysh EFTs within the framework of the Wilsonian renormalisation group.

Speaker: Saso Grozdanov

Trst_julij2.pdf

11:00 AM → 11:30 AM Coffee break

11:30 AM → 12:30 PM IFPU Colloquium: Spontaneously broken spacetime symmetries and a quantum bound on transparency

I will show how the low-energy dynamics of different phases of matter---different "materials"---can be conveniently described within the framework of relativistic quantum field theory, in particular in terms of spontaneous symmetry breaking, Goldstone excitations, and effective field theory. As an application of the formalism, I will show how to derive an absolute bound on how transparent a material can be.

Speaker: Alberto Nicolis

IFPU_Trieste.pdf

12:30 PM → 2:30 PM Lunch break

2:30 PM → 4:00 PM Discussion session: Condensed Matter

Conveners: Angelo Esposito, Subham Dutta Chowdhury