BEL Center - Joint pheno+GW meetings

5 Dec 2025, 10:00 → 1 Jul 2027, 16:00 Europe/Zurich

Tournay-Solvay Castle

Alberto MARIOTTI (Vrije Universiteit Brussel), Benoît Tuybens (Nikhef), Daniel Mayerson (KU Leuven), Gauthier Durieux (CP3 - UCLouvain), Michel Tytgat

Venue location: https://maps.app.goo.gl/32Nt1sWMMsh8w5Hf9 (the parc opening hours are here).

There will be shared office space and meeting rooms to discuss and work outside presentation times.

Friday 12 December

10:00 → 16:00 Joint meeting

10:30 Coffee

11:00 Synergic cosmology across the spectrum

Speaker: Prof. Stefano Camera (Torino U.)

12:30 Lunch

14:00 Quantum Scattering Amplitudes for Gravitational Waves

Speaker: Rafael Aoude

Friday 30 January

10:00 → 13:30 Joint meeting: Pheno morning (Note the special venue!)

10:30 Coffee

11:00 Effective Field Theory for Thermal Phase transitions

I will present recent progress in applying effective field theory (EFT) to the precise computation of thermal phase transition parameters and the associated gravitational wave spectra. I will also briefly comment on byproducts of this work, including new methods for two-loop renormalization in EFT that avoid gauge-breaking counterterms and infrared rearrangement.

Speaker: Mikael Chala (Universidad de Granada)

12:00 Lunch

13:30 → 17:00 Joint meeting: Gravitational wave afternoon (Note the special venue!)

13:30 Introduction to LISA, its data analysis challenges, and modelling of extreme mass ratio inspirals

Speakers: Bert Depoorter (KU Leuven), Robrecht Keijzer (KU Leuven)

14:45 The LISA mission and its distributed data processing center

Speaker: Antoine Petiteau (APC)

16:00 Listening to the millihertz universe: LISA data analysis and the challenge of extreme mass ratio inspirals

The Laser Interferometer Space Antenna (LISA) will open a new window onto the gravitational wave Universe by observing millihertz signals from massive black hole mergers, compact galactic binaries, and extreme mass ratio inspirals (EMRIs). Extracting these signals from a data stream shaped by instrumental noise and astrophysical confusion requires advanced data analysis strategies. In this talk, I will outline the main challenges of LISA data analysis. I will then focus on EMRIs: long lived, highly structured signals produced when stellar mass compact objects spiral into massive black holes. These sources encode precise information about spacetime geometry and black hole astrophysics, yet their detection and characterization demand accurate waveform models and scalable search algorithms. I will discuss recent progress in EMRI modeling and how emerging data analysis pipelines aim to unlock their unique scientific potential.

Speaker: Lorenzo Speri (ESA)

Friday 27 February

10:00 → 17:00 Joint meeting

10:00 Baryogenesis from electroweak textures

The matter-antimatter asymmetry of the Universe represents one of the main open questions in particle physics and cosmology. In this talk, we will present a novel realization of cold baryogenesis (a mechanism involving the formation and decay of topological defects associated with the gauge group of the Standard Model known as SU(2) textures) that relies on the out-of-equilibrium dynamics during a strong first order electroweak phase transition. By performing extensive lattice simulations of the Higgs doublet and gauge field dynamics, we evaluate the related Chern-Simons number production as well as the rate of baryon number violation, as a function of the parameters of the phase transition and the shape of the Higgs potential. We finally provide an estimate for the total baryon asymmetry generated this way.

Speaker: Dr Simone Blasi (DESY)

11:30 Quantum Observables and Their Sensitivity to Fundamental Interactions at Polarized Lepton Colliders

High-energy colliders probe fundamental interactions through direct searches and precision measurements. Yet conventional observables, such as total cross sections and asymmetries do not fully exploit the intrinsically quantum nature of multi-particle final states. A density matrix based description of scattering processes provides access to genuinely quantum observables, offering complementary sensitivity to the underlying dynamics.

In this talk, I will outline the general framework for defining and reconstructing quantum observables in collider experiments, focusing on spin density matrices and their measurable components. I will discuss how different classes of interactions; scalar, vector, and tensor mediators populate distinct structures in the density matrix, thereby motivating the need for multiple, carefully chosen observables. Attention will be given to quantum-information-theoretic quantities such as entanglement, Bell inequality violations, purity, and magic, and to how each probes different aspects of the quantum structure of scattering amplitudes.

As a concrete example, I will consider top-antitop production at future lepton colliders and present analytic results for the spin density matrix. I will demonstrate how beam polarization can be exploited to enhance sensitivity to specific operator structures and improve discrimination among possible interaction scenarios. These results illustrate how quantum observables, combined with polarized beams, provide a systematic framework for probing potential new physics beyond conventional collider analyses.

Speaker: Priyanka Lamba

12:45 Lunch

14:00 Geometric resonances for high-frequency gravitational-wave detection

Gravitational waves at kilohertz and higher frequencies provide a unique window onto the early Universe at energy scales far beyond those accessible through the cosmic microwave background, potentially probing physics well above the electroweak scale. Yet existing detector concepts fall many orders of magnitude short of the big-bang nucleosynthesis bound on a stochastic gravitational-wave background in this regime. In this talk, I will present a new interferometric concept that exploits geometric resonances arising from directional changes of light inside high-Q optical cavities. For suitable folded or looped geometries, gravitational-wave–induced phase shifts can accumulate coherently over many traversals, leading to narrowband resonances with a characteristic comb-like structure. This distinctive spectral signature provides a powerful handle to discriminate a stochastic background from instrumental noise, without relying on cross-correlation between detectors. I will explain the underlying physical mechanism, discuss possible realizations compatible with planned tunnel layouts such as the Einstein Telescope, and comment on projected sensitivities, open challenges, and connections to established interferometer designs.

Speaker: Jan Heisig

15:20 Bridging the microhertz gap in the gravitational-wave landscape

Over the past decade, LIGO/Virgo/KAGRA have given us abundant evidence for gravitational waves emitted by binary systems. But binaries can also absorb gravitational waves, leaving potentially detectable imprints on their orbits. In this talk, I will describe how this effect can be used to search for gravitational waves at frequencies that are inaccessible to all other current and future experiments — in particular, how laser-ranging measurements of the Moon and other satellites around the Earth will allow us to explore the “microhertz gap” between the frequencies probed by pulsar timing arrays and future space-based interferometers such as LISA. As examples of the discovery potential of this approach, I will show how it is able to place unique constraints on gravitational waves from cosmological phase transitions, and will discuss how it will shed light on the possible gravitational-wave background recently detected by pulsar timing arrays.

Speaker: Alex Jenkins

Friday 27 March

10:00 → 17:15 Joint meeting

10:00 Neutrino mass bounds from cosmology: How strong? How robust?

Neutrinos of cosmological origin are the second most numerous particles in the universe since billions of years. They are very difficult to measure directly, but we have several indirect (although very clear) indications of their presence. The strongest upper bounds on neutrino masses come from cosmological observations. I will discuss their model-dependence and robustness, given the existence of tensions among different types of recent cosmological data.

Speaker: Julien Lesgourgues (RWTH Aachen university)

11:30 Shockwaves and Time Delays in Einstein-Maxwell Effective Field Theory

We derive the shockwave metric in four-dimensional Einstein-Maxwell effective field theory (EFT) by performing an ultra-relativistic boost of the charged black hole solution accompanied by a rescaling of its mass and charge, including leading order EFT corrections. In contrast to the neutral (Schwarzschild) case, where higher derivative operators leave the shockwave geometry unchanged, we show that electrically charged shockwaves receive nontrivial EFT corrections. We then compute the time delay experienced by a probe photon traversing the resulting charged shockwave. We find that two EFT contributions, the correction to the shockwave geometry and the backreaction induced by the probe photon, are both essential for obtaining a physical time delay that is invariant under field redefinitions of the metric.

Speaker: Christophe Grojean (DESY (Hamburg) and Humboldt University (Berlin))

12:45 Lunch

13:45 PhD introduction for GW seminars

Speakers: Mauro De Haes, Serdar Yildiz

14:45 New physics in the strong field regime

Our recent numerical work has shown the importance of including non-linear effects that modify the gravitational sector in the strong field regime in beyond GR theories and dark matter environments. Advances in mathematically well posed formulations, numerics and data analysis mean that we can now extend our studies and stress-test current diagnostic and modelling tools with fully consistent waveforms. This allows us to identify degeneracies with GR parameters, and so to understand what the waveforms do and do not actually tell us about new physics.

Speaker: Katy Clough (Queen Mary University London)

16:00 Numerical-relativity, fundamental bosonic fields, and compact objects

Bosonic stars are hypothetical compact objects composed of ultralight bosonic fields and have been proposed as viable dark matter candidates. In this talk, I will present recent advances in our understanding of their stability properties and nonlinear dynamics based on numerical-relativity simulations. In particular, I will discuss gravitational synchronization effects that can arise when boson stars interact with other compact objects, such as neutron stars, leading to distinctive dynamical behavior. I will then turn to binary mergers of boson stars, highlighting their merger dynamics and the characteristic gravitational-wave signatures they produce. Finally, I will discuss how current and future gravitational-wave observations may constrain or potentially reveal the existence of such exotic compact objects in our Universe.

Speaker: Nico Sanchis-Gual (University of Valencia)

Friday 24 April

10:00 → 16:35 Joint meeting

10:00 New Physics from Geometry

A huge fraction of the signals that could in principle be hiding in LHC data is still unexplored. The difficulty in extracting them is related to a long-standing statistical problem: how to perform a goodness-of-fit test in high-dimensional spaces. I will discuss two new ideas based on a geometric interpretation of collider data. They are completely orthogonal (in a colloquial sense) to existing approaches and scale exponentially better in p-value when the number of input dimensions (collider observables) is increased.

Speaker: Raffaele D'Agnolo (CEA IPhT Saclay)

11:30 Dark Matter circa 2026: the WIMPs and beyond the WIMPs

Almost a century after its discovery, Dark Matter has not been identified yet. Even worse, the candidates that were more promising a few years/decades ago, those that the community was betting to find “just around the corner", have not shown up in the ever-improving searches. Hence theorists have gone back to the drawing board and restarted exploring a wider range of possibilities. Many new ideas have been proposed, and some old ideas have been rediscovered. In this talk I will present a (necessarily biased and personal) view of some Dark Matter candidates, as well as their phenomenology. I will argue that the cherished WIMPs (Weakly Interacting Massive Particles) should not be dismissed yet, but also that other candidates are well motivated, interesting and worth looking for. I will keep track of some of the most promising search directions and highlight the current frontiers for phenomenology and experiments.

Speaker: Marco Cirelli (CNRS LPTHE Jussieu)

12:45 Lunch

14:00 Cosmological inflation: precision physics, exploratory physics, and formal aspects.

The earliest moments of our Universe, over 13 billion years ago, provide a unique laboratory for probing the interplay between gravity and quantum theory through observation. In particular, precise measurements of the Cosmic Microwave Background point towards a rapid inflationary expansion of spacetime, through which vacuum quantum fluctuations seeded the cosmic structures we observe today.
In this talk, I will discuss three complementary avenues of current research about the inflationary paradigm. First, I will explain how inflation acts as a cosmological collider, offering the remarkable prospect of detecting particles up to a billion times more massive than those accessible in terrestrial colliders, using galaxy surveys as our observational tool. Second, I will describe the new perspective on inflation brought by gravitational-wave astronomy. Finally, I will present some recent formal advances in quantum field theory in de Sitter spacetime.

Speaker: Sébastien Renaux-Petel

15:20 Signatures of gauge field amplification and backreaction in axion inflation

Axion inflation with a pseudo-scalar coupling to gauge fields can give rise to a rich phenomenology, including gravitational waves over a broad range of frequencies and primordial black holes. Interesting signals are often reached in a regime in which the gauge fields produced by the rolling axion significantly backreact on the axion dynamics. While this effect was originally computed analytically under the assumption of steady-state evolution of the axion velocity, a more complicated dynamics has emerged from later numerical computations. Exact lattice computations are extremely expensive, justifying the development of approximate computational schemes. We will discuss the results emerging from these approaches and their range of applicability.

Speaker: Marco Peloso

Friday 29 May

09:00 → 18:30 Joint meeting

10:00 Quantum sensors for GW and DM

I will describe some current technologies to detect dark matter and gravitational waves with quantum sensors.

Speaker: Diego Blas (ICREA/IFAE)

11:30 Better and faster particle theory with AI

Machine learning is not only transforming our lives, but also particle theory. I will start by defining something like physics-specific representation learning, with a focus on theory aspects, and give examples how we can use learned representations to improve simulations, include networks in a statistical framework, and properly understand what they learn. Finally, I will briefly show how an agentic interface to MadGraph re-defines the way we do theory predictions for the LHC.

Speaker: Tilman Plehn (Heidelberg University)

12:45 Lunch

14:00 Introduction to gravitational lensing of gravitational waves

Speakers: Disha Hedge (CP3 - UCLouvain), Jef Heynen (CP3 - UCLouvain)

15:15 Gravitational-wave Astronomy through the magnifying glass

Our Universe is still largely a mystery. Elusive dark matter shapes cosmic structures, while the Universe expands at a rate that we struggle to understand. Unraveling these fundamental open questions demands a deeper look into the distant cosmos, where gravitational waves from stellar-mass binary black holes offer a unique window of observation.

Unlike electromagnetic signals, gravitational waves traverse the Universe without alteration, except for gravitational lensing. Their long wavelengths make them susceptible to diffraction by cosmic structures, providing an unprecedented opportunity to map dark matter and test the foundations of gravity. If we can recover these lensed signals, we gain a cosmic magnifying glass to observe the most distant black hole mergers and illuminate their origins. Current gravitational-wave observations have already produced puzzling lensed candidates (e.g., GW231123), and the first definitive detections are expected very soon.

In this talk, I will explain why gravitational-wave lensing is uniquely informative, outline how deep‑learning methods can accelerate its discovery, and show what the first detections could reveal about dark‑matter substructure and astrophysical populations.

Speaker: Jose María Ezquiaga (Instituto de Física Teórica UAM-CSIC)

16:30 Wave-optics lensing: methods and applications

Light travelling through the Universe is deflected by the intervening gravitational fields, a phenomenon known as gravitational lensing. This effect was an early prediction of General Relativity (GR) and it has become an essential tool in astronomy. The existence of gravitational
waves was another prediction of GR, but in this case we had to wait until 2015 for the first detection. Since then, hundreds of events have been detected in LVK, a number that will increase significantly in the next observing run and with future detectors like LISA and the Einstein Telescope.

This large number of detections guarantees the eventual observation of lensed gravitational waves. Similarly to light, gravitational waves are affected by intervening matter, but with some important differences, e.g. they can experience diffraction. In this talk I will discuss the phenomenology of gravitational wave lensing and the numerical tools needed for computations in the wave-optics regime. Finally, I will cover the exciting implications of the recently announced event GW231123, which can be considered to be the first serious candidate for a lensed GW.

Speaker: Hector Villarrubia-Rojo (Complutense University of Madrid)

Friday 25 September

10:00 → 17:00 Joint meeting

10:00 Imagine Dragons: Alternative Realities To Understand our Own

Ancient cartographers marked the edge of the known world, where the coastline stopped and the unmapped sea began, with the phrase Hic Sunt Dracones: here be dragons. Theoretical physics has a similar map. Infinitely many alternative universes can be written down on paper as effective field theories, but many of them cannot arise as the low-energy limit of any standard high-energy theory: no unitary, causal, relativistic UV completion reduces to them in the infrared. They are not merely disfavoured by data. They are impossible, and imaginary experiments suffice to rule them out.
In this talk I will show how quantum mechanics, relativity, unitarity, and causality draw this coastline. The tools are scattering amplitudes, analyticity, crossing, and dispersion relations; the result is a set of positivity bounds that carve away impossible EFTs. In this sense, imagined universes and imaginary experiments become a way of learning about our own.

Speaker: Brando Bellazzini (Université Paris-Saclay (FR))

11:30 Vacuum decay in field theory

The decay of a metastable vacuum in quantum field theory is a phenomenon that has a variety of applications, ranging from condensed matter systems to the possible production of gravitational waves in the early universe. The decay rate has the form A exp(-B). The prefactor A is difficult to compute and is often simply guessed from the dimensionful parameters of the model.
I plan to summarize and discuss pedagogically recent work with my collaborators on the calculation of A.
We find that in the thin-wall approximation both the naive guess and some detailed calculations in earlier literature get A wrong by orders of magnitude.

Speaker: Lorenzo Ubaldi

12:45 Lunch

Friday 6 November

10:00 → 16:15 Joint meeting

10:00 Primordial high energy neutrinos

Speaker: Thomas HAMBYE (ULB)

11:30 Dark Matter Production in the Early Universe: Advancing the Standard Treatment

Dark matter constitutes about 27% of today's energy density of the Universe, yet its particle nature remains unknown. Its observed abundance provides key guidance, making a precise understanding of how dark matter was produced in the early Universe crucial. In this talk, I will discuss recent progress on both freeze-out and freeze-in scenarios. I will show how bound-state formation can impact dark matter production via freeze-out and how thermal effects from the primordial plasma can significantly alter freeze-in predictions. These effects prove essential for an accurate interpretation of experimental results.

Speaker: Julia Harz

12:45 Lunch

14:00 Searching for stochastic gravitational-wave backgrounds with a network of ground-based interferometric gravitational-wave detectors

Speaker: Federico De Lillo (ULB)

15:15 Building the infrastructure for the detection of gravitational wave microlensing

Speaker: Mick Wright (University of Utrecht)

Friday 27 November

10:00 → 16:00 Joint meeting

10:00 Alejandro Ibarra

Speaker: Alejandro Ibarra (Technical University of Munich)

11:30 Adam Falkowski

Speaker: Adam FALKOWSKI

Friday 29 January

09:30 → 17:30 Joint meeting

Friday 26 February

09:30 → 17:30 Joint meeting

10:00 Davide Racco

Speaker: Dr Davide Racco (Universita e INFN Roma Tre (IT))

Friday 19 March

09:30 → 17:30 Joint meeting

Friday 23 April

09:30 → 17:30 Joint meeting

Friday 28 May

09:30 → 17:30 Joint meeting