Beyond the Standard Model Phenomenology and Cosmology
The Standard Model of particle physics is a milestone in our understanding of particle physics, constantly investigated in the Large Hadron Collider at CERN. However, there are still important open questions, including the mechanism beyond electroweak symmetry breaking, the matter-antimatter asymmetry, the strong CP problem, the origin and the nature of the dark matter.
These open problems are addressed in Beyond the Standard Model (BSM) theories, where the Standard Model is extended with new forces, matter, symmetries. Notable examples are supersymmetry, extra dimensions, composite Higgs models, brane-world. These scenarios also naturally connect BSM particle physics ideas with novel features in the cosmological history of our Universe.
Our group performs research in this area by covering a broad spectrum of topics in BSM physics and cosmology, with the objective of providing a bridge between theory and experiment.
A first research line is centered on the study of novel signatures of BSM physics at colliders. Our investigations explore signals of e.g. supersymmetry, axion-like particles, dark matter, focussing on the high-luminosity LHC and future colliders (such as the FCC or a muon collider). In particular, we are involved in the community effort to explore long-lived particle signatures of BSM physics in colliders. In this context we are also embedded in the EOS consortium to study the Brout-Englert-Higgs boson and its connection to BSM physics.
Another important research line is devoted to the study of unconventional dark matter models and their cosmological history. By exploring novel mechanisms of production of dark matter in the early Universe we aim to identify new signatures or strategies through which we can look for dark matter imprints in current experiments.
Finally, we perform investigations aiming to find gravitational wave signals of well-motivated BSM theories. Gravitational waves have provided a new way to explore the Universe, probing physics at high energies beyond the ones accessible at colliders. This can hence shed new light into fundamental questions of high energy BSM physics. Our research interests are focused primarily on phase transitions that could have occurred during the early Universe in BSM scenarios. They could have generated gravitational waves by their strong dynamics or through the generation of cosmic defects and their subsequent evolution. This research activity combines with the “Gravitational Waves” line described below.
Strings, Supergravity, Geometry and Duality
String Theory is by now well established as a leading candidate for the description of quantum gravity. Its simple premise is to replace point particles with strings, whose quantum excitations include a massless spin 2 mode; the putative graviton of quantum gravity. Indeed, in the low energy regime, general relativity, or its supersymmetric extension namely supergravity, provides an effective description of the string dynamics with the different solutions of supergravity corresponding to string theory vacua. However, in this pointparticle supergravity limit essential features of string theory are not captured. Most notably, the extended nature of strings and their ability to wind around a compact direction in spacetime gives rise to exciting and unexpected behavior that is in complete contrast to that of point particles.
A remarkable phenomenon, known as T-duality, is that seemingly different supergravity backgrounds can give equivalent string theories. In its simplest form, T-duality is the equivalence of strings on a circle of large radius R and those on small circle of radius α’/R (α’
is the square of the string length scale). T-duality interchanges string momenta and winding modes whilst modifying the geometry in such a way that the physics is left invariant. T-duality is just the tip of a much wider class of dualities in String Theory known as U-dualities, which paved the way to the idea of M-theory; an all-encompassing theory that unites all
of the different duality related regimes of string theory.
These dualities pose a fundamental question – do they have a geometrical underpinning? More precisely is there a more appropriate geometrical language in which to formulate string theory that takes into account the extended nature of the string and its dualities. The past few years have seen an upsurge in interest and understanding in addressing this, and related issues, with considerable advances on several fronts. The main themes of our research can be captured by:
- Generalized Geometry
- Double field theory/exceptional field theory (DFT/EFT)
- Doubled worldsheet formalisms
- Branes and backgrounds in DFT/EFT
- Generalized T-duality and their Applications
- T-duality and new integrable models
Gravitational Waves
Since the first observation of gravitational waves by LIGO in 2015, gravitational wave physics has grown explosively. The current generation of gravitational wave interferometers, LIGO/Virgo/Kagra (LVK), has clearly demonstrated that gravitational waves truly opened a new window on our Universe enabling us to address problems and observe novel phenomena otherwise inaccessible. The future 3rd generation observational facilities, such as the Einstein Telescope (ET) in Europe and Cosmic Explorer in the USA, expected to come online in the mid-thirties, will realize a breathtaking and ground breaking scientific program.
As members of the Virgo collaboration we actively participate to the activities of the stochastic gravitational wave background group (SGWB) of the LVK collaboration. We develop new data analysis techniques and perform data analysis targeting the discovery of the SGWB of astrophysical origin. Furthermore we are engaged in theoretical studies aiming at modeling sources for the SGWB of cosmological origin such as strong first order phase transitions in the early universe and domain walls. Indeed, the observation of gravitational waves allows us to explore elementary particle physics at energy scales not accessible by colliders. On the national level we coordinate our scientific efforts with other groups in Belgium through the BelGrav consortium.
In addition to the GW physics program we are also involved in instrumentation. We are one of the founding partners of ETpathfinder, an R&D facility currently under construction in Maastricht (NL) where cryogenic silicon based optics, required for the low frequency instrument at ET, will be developed. This work is performed in close collaboration with Brussels Photonics BPhot. We are also directly involved in the preparations for ET, of which we hope that it will be constructed at the BE-DE-NL three country point. In particular we actively contribute to Division 2 “Cosmology” of the Einstein Telescope Observational Science Board (OSB).
Holography, quantum entanglement and the emergence of spacetime
Arguably the most profound theoretical breakthough in the past 25 years has been the discovery of Holography. This by now strongly supported conjecture states a remarkable equivalence between gravity theories in certain spacetimes and lower-dimensional quantum field theories that can be thought of as living at the boundary of the gravitational spacetimes. Whilst this conjecture was originally formulated in a deductive (or top-down) manner in the context of string theory, it has now also been applied in an inductive (bottom-up) fashion to a wide range of physical systems including condensed matter systems, fluid mechanics, and the quark-gluon plasma created in heavy ion collisions.
An important question is how exactly the higher-dimensional gravitational spacetime is encoded in the lower-dimensional quantum field theory. Research of the last decade suggests that quantum entanglement plays a crucial role in this. In some cases, the relevant entanglement is between degrees of freedom in different spatial regions, but in other cases, most notably in matrix models, one also needs entanglement between “internal” degrees of freedom. Motivated by this, one aim of our research is to learn how to describe and compute entanglement between the degrees of freedom in systems of interacting matrices.
A specific question is how to decode what happens inside a black hole, and relatedly, how the Hawking radiation of an evaporating black hole contains information about the matter from which the black hole was formed. Recent progress suggests that entanglement plays a key role in this, and we are investigating possible ways in which radiation may or may not “know” about black hole interiors.
Black holes, thermalization, quantum chaos and complexity
Holography relates black holes in certain spacetimes to thermal states in dual quantum field theories. The process of thermalization, which in strongly coupled systems is hard to study using conventional tools, is therefore related to black hole formation in gravity, which has opened a new holographic window on thermalization. It is believed that chaos underlies thermalization, and black holes have been argued to be maximally chaotic. This has led to new developments in quantum chaos theory.
Our group has made many contributions to holographic thermalization, and has recently explored quantum chaos in several contexts, holographic and otherwise. One question we have addressed is how information is scrambled by extremal black holes, which often appear in string theory constructions. We are also interested in how quantum chaos observables interpolate between integrable and chaotic models, e.g. in the rich framework of quantum resonant systems which we have developed.
Quantum complexity has been conjectured to be related to what happens inside a black hole horizon, e.g. to the spatial volume inside a black hole. A challenge in making this correspondence precise is that it is not straightforward to define and compute complexity in quantum field theory. We are studying geometric notions of quantum complexity, again making use of the framework of quantum resonant systems.