Here is a snapshot of some of the topics that I work on. It is not complete, I am interested in a lot of things!
We know that the Standard Model is incomplete, but finding what lies beyond is currently a puzzle: we do not yet have direct signs as to where new physics might be hiding, even if many people were optimistic that the LHC would discover a plethora of new colourful particles. There are several indirect signs that new physics might still be at reachable energies; and there is no convincing reason to believe that there must not be particles lying at higher masses or in hidden valleys. So the challenge at the energy and intensity frontier now is to confront new theories to data with higher precision. This is the motivation behind my research in generic phenomenology, or trying to perform calculations and create tools that work for any new theory, rather than exploring them one at a time. See also my codes section.
One of the major sources of information that is still under-exploited from the BSM perspective is, amazingly, the Higgs boson. On the experimental side, its mass has been determined to within 0.2% (!) and its most important couplings to other particles have been measured. The Higgs mass now qualifies as an electroweak precision observable. But on the theoretical side, given a top-down theory where we know the couplings, we can calculate the Higgs mass and at present the uncertainty can be several percent (and the uncertainty itself is also difficult to determine). Since the Higgs mass depends on the masses of new particles logarithmically, if we were to turn the calculation around and try to infer the scale of new physics in our theory from the Higgs mass then even a few percent uncertainty can lead to an order of magnitude error. For example, in the Standard Model we know that the quartic coupling runs to near (and most likely below) zero at a scale well below the Planck mass that we can predict relatively accurately.
Historically Higgs mass calculations have been very important in supersymmetric models, where a lot of development has taken place in the MSSM. In the last few years the KUTS workshops have brought together experts to push new developments (see e.g. the recent review paper on our progress) and I have been very much involved. My own perspective has been to perform and implement generic calculations so they can be used for any theory.
My switch to working on BSM phenomenology started with exploring the properties of Dirac gaugino models, which are a very interesting and well-motivated class of non-minimal supersymmetric models. They have several special features thanks to the supersoft operator, and phenomenologically this makes them both somewhat protected from collider constraints, and excellent playgrounds for testing out new calculations and searches. I've been interested in the formal aspects (related e.g. to building or constraining models from the UV, especially in the context of gauge mediation) and phenomenological: dark matter, Higgs mass, collider constraints.
Dark matter (whether or not it is actually a particle!) unequivocally requires new physics (modulo some attempts to explain it in terms of a hypothetical metastable baryon). My research has mainly focused on exploring the parameter space of theories that have a WIMP candidate or a WISP (see below), and I'm increasingly interested in complementarity with collider searches and unitarity constraints. I also harbour a secret penchant for theories that can offer an explanation for the predictions of modified gravity.
While there are plenty of experimental constraints that we can apply to new theories, there are far fewer theoretical constraints: things we can use to rule out theories before we start! One very important class of such constraints is the requirement of unitarity. In the last few years I was involved in automating such constraints for the scalar sector of renormalisable theories. There is plenty more that needs to be done in this direction! There are also potentially many connections with formal theory, as well as dark matter and even the anomalous magnetic moment of the muon.
In the last few years I have been increasingly interested in recasting existing LHC searches so that they can be applied to other models. Initially, I just wanted to be able to apply the latest constraints on my favourite Dirac gaugino models. Naively for supersymmetric models you can look up the limits from the analysis papers, but once you read the small print it is apparent that they are usually very dependent on (often very) favourable assumptions. Applying them to other variants of the MSSM is then hard, and applying to other models may be impossible without doing a simulation: namely, recasting.
However, it was quickly apparent that the set of searches that have been performed is much larger than the number for which reusable codes exist. We could check for the most important constraints on the colourful particles, the squarks and gluino that exist in the MSSM too, but for other colourful particles -- the octets -- the most important channel (four tops) was missing. And when we started to examine the electroweak sector of our models -- essential for the dark matter properties -- the most powerful searches had not been recast, and I started to look at it myself. Sometimes the work of reinterpreting an analysis can be an almost straightforward translation of the cuts in the analysis paper, but so far in my experience it has actually been very complicated, even in the cases where the experimental collaborations provide materials to help with the task (which was rare but is thankfully becoming more frequent).
The models I was interested in also often had long-lived particles (LLPs), almost by accident, and when this is the case they should stick out like a sore thumb at the LHC. So I started to be interested in recasting LLP searches, and it turns out that this is the next great hope for discoveries at Run 3 of the LHC, since many new triggers are being implemented and new types of searches will be done that have not been looked for before. So a group of us at the LPTHE (myself, Benjamin Fuks and Filippo Sala) are now involved in an ANR-funded project with two experimental groups in ATLAS based at the LPNHE (Paris) and IPSC (Grenoble) to help in this effort, and maybe even discover something!
In 2024, I became interested in the searches for electroweak-charged particles which have so far been difficult to constrain, especially for compressed spectra. In these parts of the parameter space of models there can still be a large enough number of signal events at the LHC, but they are swamped by backgrounds. And yet there are excesses! We pointed out that there are four excesses that can all be compatible with the same types of models; I've been working on recasting the relevant searches, building models to explain the excesses, and working out how to best find the most interesting regions of their parameter spaces since then.
From my work in string theory (see below) and collaborating with Andreas Ringwald at DESY I became very interested in WISPs: very light and very weakly interacting particles. This is really an umbrella term for hidden photons and axion-like particles. Such new particles are ubiquitous in BSM constructions and string theory models, and I worked on mapping out predictions for their properties from string theory, especially within the LARGE volume scenario, which is one of the leading frameworks for obtaining low-energy physics from strings.
I also worked on WISPs from a bottom-up perspective, e.g. as dark matter candidates in this paper. I have also given lectures about the formal aspects of the strong CP problem and I'm very interested in the chiral effective theory approach to computing axion properties.
I started my career in string theory doing calculations for intersecting brane models, at tree-level and at one loop. I developed technology for computing amplitudes, and applied it to models of non-commutative geometry, stringy instantons, and models with kinetic mixing of hidden photons with the visible one. I also applied some of these techniques (with Joe Conlon and Eran Palti) to computing the anomaly mediation term from string theory: the idea was that there were ambiguities in the field theory calculation, so we used string theory as a UV completion to attempt to resolve it! I'm still interested in research in this area, and discuss with my colleagues in the Paris area about it.