Exoplanets: phase curves observed by JWST
The James Webb Space Telescope (JWST), launched by NASA on December 25, 2021, is revolutionizing our understanding of the cosmos, particularly in the field of exoplanets. With more than 6,000 exoplanets detected, a great variety of worlds have been discovered, some with no equivalent in our Solar System, such as « hot Jupiters » or « super-Earths ». JWST now enables detailed characterization of exoplanetary atmospheres thanks to its spectroscopic instruments covering wavelengths from 0.6 to 27 µm and its large light-collecting area (25 m²). This capability allows determination of molecular composition, the presence of clouds or aerosols, the pressure–temperature profile, and the physical and chemical processes at work in these atmospheres.
The main method used is the so-called transit method, which observes variations in brightness when a planet passes in front of or behind its star (secondary eclipse). Nevertheless, observations over the entire orbital period (phase curve)—which also includes a transit and two eclipses—provide even more information. With phase curves, the energy budget, longitudinal structure, and atmospheric circulation can be directly observed. JWST has already obtained phase-curve data of exceptional quality. Many of these datasets are now publicly available and contain a wealth of information, though they are only partially exploited. The length of these observations, the extremely faint signals (a few tens of ppm), and the presence of subtler instrumental effects make the analysis of these data more complex.
The proposed PhD will first focus on studying and correcting these instrumental effects, then on extracting atmospheric properties using the TauREx software (https://taurex.space/), under the co-supervision of Quentin Changeat (University of Groningen) and Pierre-Olivier Langage (CEA Paris-Saclay). This PhD will contribute to preparing the scientific exploitation of the ESA Ariel mission (launch planned for 2031), entirely dedicated to the study of exoplanetary atmospheres and expected to observe nearly 50 phase-curves.
Magneto-convection of solar-type stars: flux emergence and origin of starspots
The Sun and solar-type stars possess rich and variable magnetism. In our recent work on turbulent convective dynamos in this type of star, we have been able to highlight a magneto-rotational history of their secular evolution. Stars are born active with short magnetic cycles, then slow down due to braking by their magnetized particle wind, their magnetic cycle lengthens to become commensurate with that of the Sun (lasting 11 years) and finally, for stars that live long enough, they end up with a loss of cycle and a so-called anti-solar rotation (slow equator/fast poles). The agreement with observations is excellent, but we are missing an essential element to conclude: What role do sunspots/starspots play in the organization of the magnetism of these stars, and are they necessary for the appearance of a stellar magnetic cycle, e.g. the so-called “paradox of spotty dynamos”? Indeed, our HPC simulations of solar dynamos do not have yet the angular resolution to resolve the spots, and yet we do observe cycles in our simulations of stellar dynamos for Rossby numbers < 1. So, are the spots simply a surface manifestation of an internal self-organization of the cyclic magnetism of these stars, or do they play a decisive role? Furthermore, how do the latitudinal flux emergence and the size and intensity of the spots forming on the surface evolve during the magneto-rotational evolution of these stars? To answer these key questions in stellar and solar magnetism in support of the ESA space missions Solar Orbiter and PLATO, in which we are involved, new HPC simulations of stellar dynamos must be developed, allowing us to get closer to the surface and thus better describe the process of magnetic flux emergence and the possible formation of sun/starspots. Recent tests showing that magnetic concentrations inhibiting local surface convection form in simulations with a higher magnetic Reynolds number and smaller-scale surface convection strongly encourage us to continue this project beyond the ERC Whole Sun project (ending in April 2026). Thanks to the Dyablo-Whole Sun code that we are co-developing with IRFU/Dedip, we wish to study in detail the convective dynamo, the emergence of magnetic flux, and the self-consistent formation of resolved spots, using its adaptive mesh refinement capability while varying global stellar parameters such as rotation rate, convective zone thickness, and surface convection intensity to assess how their number, morphology and latitude of emergence change and if they contribute or not to the closing of the cyclic dynamo loop.
Study of heavy nuclei: from the mass measurement to the spectroscopy of Americium nuclei and commissioning of the double Penning trap PIPERADE
The atomic nucleus is a complex system that continues to be actively studied more than a century after its discovery. Among the open questions, the question of the limits of existence of the nucleus remains central: what are the numbers of protons and neutrons that allow a bound nucleus to form? This question can be addressed using mass measurements that provide access to the binding energy of the nucleus, one of its most fundamental properties. The objective of this thesis is, on the one hand, to perform high-precision mass measurements of the isotopes 234-238Am (Z = 95) isotopes at the University of Jyväskylä, Finland (experiment planned in 2026), and, on the other hand, to participate in the installation and commissioning of the PIPERADE double Penning trap (PIèges de PEnning pour les RAdionucléides à DESIR) at GANIL in Caen.
The americium nuclei that will be studied in this thesis are at the boundary between two regions of particular interest: the octupole deformation region (pear-shaped nuclei) and the fission isomer region (meta-stable states of nuclei decaying by fission), and measuring their mass will provide a better understanding of the properties of these exotic nuclei.
PIPERADE is a device that can be used to perform high-precision mass measurements. Currently in the characterisation phase in Bordeaux, its installation at GANIL will enable the study of a wide range of exotic nuclei by measuring their mass. Currently undergoing characterisation in Bordeaux, its installation at GANIL (planned for 2027) will enable the study of a wide range of exotic nuclei by measuring their mass, but also by using separation techniques to purify the radioactive beams before sending them to other experimental devices.
STUDY OF THE NUCLEAR COLLECTIVE PROPERTIES OF 232TH WITH THE AGATA SPECTROMETER
The study of so-called ‘deformed’ atomic nuclei with a non-spherical charge distribution is essential for testing nuclear interactions and structural models. These deformed nuclei exhibit a very particular pattern of excited states, known as ‘rotational bands’. These bands can be constructed on states with different deformations or different intrinsic structures (shape coexistence). The subject of the thesis is the experimental study of the macroscopic and microscopic properties of the nucleus 232Th. This nuclide exhibits a wide variety of rotational bands that are thought to be due to vibrations of the nuclear surface known as quadrupole and octupole vibrations. In particular the latter have attracted a great deal of interest recently, as octupolar deformed nuclei can be used to determine nuclear electric dipole moments, a fundamental question in physics in general. In our particular case, the aim is to characterise for the first time the quadruplet of octupole bands expected in a strongly deformed nucleus. Furthermore, this nucleus is the only example with a rotational band built on a double quadrupole vibration.
We will study these various shapes using the powerful technique of Coulomb excitation, which is the most direct method for determining the shape of nuclei in their excited states. The experiment will be carried out using AGATA, a new-generation gamma spectrometer consisting of a large number of finely segmented germanium crystals, which can identify each point of interaction of a gamma ray inside the detector and then, using the innovative concept of ‘gamma-ray tracking’, reconstruct the energies of all the gamma rays emitted and their emission angles with unprecedented precision. A complementary experiment will be carried out at HIL Warsaw, which will enable better interpretation of the highly complex data provided by AGATA.
Surface technologies for enhanced superconducting Qubits lifetimes
Materials imperfections in superconducting quantum circuits—in particular, two-level-system (TLS) defects—are a major source of decoherence, ultimately limiting the performance of qubits. Thus, identifying the microscopic origin of possible TLS defects in these devices and developing strategies to eliminate them is key to superconducting qubit performance improvement. This project proposes an original approach that combines the passivation of the superconductor’s surface with films deposited by Atomic Layer Deposition (ALD), which inherently have lower densities of TLS defects, and thermal treatments designed to dissolve the initially present native oxides. These passivating layers will be tested on 3D Nb resonators than implemented in 2D resonators and Qubits and tested to measure their coherence time. The project will also perform systematic material studies with complementary characterization techniques in order to correlate improvements in qubit performances with the chemical and crystalline alteration of the surface.
Euclid Weak Lensing Cluster Cosmology inference
Galaxy clusters, which form at the intersection of matter filaments, are excellent tracers of the large-scale matter distribution in the Universe and are a valuable source of information for cosmology.
The sensitivity of the Euclid space mission (launch in 2023) allow blind detection of galaxy clusters through gravitational lensing (i.e. directly linked to the projected total mass). Combined with its wide survey area (14,000 deg²), Euclid should allow the construction of a galaxy cluster catalogue that is unique in both its size and selection properties.
In contrast to existing cluster catalogues, which are typically based on baryonic content (e.g., X-ray emission from intra-cluster gas, the Sunyaev-Zel’dovich effect in the millimeter regime, or optical emission from galaxies), a catalogue derived from gravitational lensing is directly sensitive to the total mass of the clusters. This makes it truly representative of the underlying cluster population, a significant advantage for both galaxy cluster studies and cosmology.
In this context, we have developed a multi-scale detection method specifically designed to identify galaxy clusters based only on their gravitational lensing signal, which has been pre-selected to produce the Euclid cluster catalogue.
The goal of this PhD project is to build and characterize the galaxy cluster catalogue identified via weak lensing in the data collected during the first year of Euclid observations (DR1), based on this detection method. The candidate will derive cosmological constraints from the modelling of the cluster abundance, using the classical Bayesian framework, and will also investigate the potential of Simulation-Based Inference (SBI) methods for cosmological inference.
From Detector to Discovery: Constructing the ATLAS Inner Tracker and Probing Higgs Physics at the HL-LHC
This PhD project combines work on the construction of the new Inner Tracker (ITk) for the ATLAS experiment and an analysis of ATLAS sensitivity at the High-Luminosity LHC (HL-LHC) to key processes related to Higgs boson physics using ITk. The candidate will take part in the development, operation, and optimization of the test benches for ITk pixel modules at CEA. Together with two other partner laboratories in the Paris region, CEA will assemble and test about 20% of the ITk pixel modules. The student will contribute to the commissioning of the detector at CERN. The candidate will also carry out HL-LHC sensitivity studies of the interactions between the Higgs boson and the top quark, including for instance a CP-violation analysis in the ttH channel and an analysis of tH production, a process particularly sensitive to the Higgs–top and Higgs–W couplings. The first two years of the PhD are expected to be based at CEA Saclay, while the last year will be based at CERN.
Joint simulation-based inference of tSZ maps and Euclid's weak lensing
Context:
The Euclid mission will provide weak lensing measurements with unprecedented precision, which have the potential to revolutionise our understanding of the Universe. However, as the statistical uncertainties decrease, controlling systematic effects becomes even more crucial. Among these, baryonic feedback, which redistributes gas within galaxies and clusters, remains one of the key astrophysical systematic effects limiting Euclid’s ability to constrain the equation of state of dark energy. Understanding baryonic feedback is one of the urgent challenges of cosmology today.
The thermal Sunyaev-Zel’dovich (tSZ) effect provides a unique window into the baryonic component of the Universe. This effect arises from the scattering of cosmic microwave background (CMB) photons by hot electrons in galaxy groups and clusters. This is the same hot gas that has been redistributed by baryonic feedback and is particularly relevant for weak lensing cosmology. The cross-correlation between tSZ and weak lensing (WL) probes how baryons trace and modify the cosmic structures, allowing joint constraints on cosmology and baryonic physics.
Most current tSZ-WL analyses rely on fitting angular power spectra under the assumption of a Gaussian likelihood. However, the tSZ signal is highly non-Gaussian, as it traces the massive structures of the Universe, and the power spectra fail to fully capture the information in the data. To unlock the scientific potential of the tSZ-WL analyses, it is essential to move beyond these simplifying assumptions.
PhD thesis:
The goal of this PhD project is to develop a novel simulation-based framework to jointly analyse tSZ and Euclid’s WL data. This framework will combine physically motivated forward models with advanced statistical and machine-learning techniques to provide accurate measurements of baryonic feedback and cosmological parameters. By jointly analysing tSZ and WL measurements, this project will increase the accuracy of Euclid’s cosmological analyses and improve our understanding of the dark matter-baryon connection.
Cosmology with the Lyman-alpha forest from the DESI cosmological survey.
We use the large-scale distribution of matter in the universe to test our cosmological models. This is primarily done using baryon acoustic oscillations (BAO), which are measured in the two-point correlation function of this distribution. However, the entire matter field contains information at various scales, allowing us to better constrain our models than BAO alone. At redshifts greater than 2, the Lyman-alpha forest is the best probe of this matter distribution. The Lyman-alpha forest is a set of absorption lines measured in the spectra of distant sources. The large DESI spectroscopic survey has collected approximately one million of these spectra. Using the partial data set "DR2," we measured the BAO with an accuracy of 0.7%, which strongly constrains the expansion rate of the universe during the first billion years of its evolution.
This thesis aims to exploit the full set of large-scale Lyman-alpha data from DESI to obtain the strongest constraints on cosmological models possible. First, the student will apply a method known as reconstruction to improve the accuracy of BAO measurements by exploiting information from the matter density field. For the remainder of the thesis, the student will implement a new method known as simulation-based inference. Similar efforts have been carried out in our group with DESI galaxies. In this approach, the entire matter field is used directly to estimate cosmological parameters, particularly dark energy. Thus, the student will make an important contribution to DESI's final cosmological measurements with Lyman-alpha.
An internship is preferred before beginning this thesis.
Unbiased Shear Estimation for Euclid with Automatically Differentiable and GPU Accelerated Modeling
This PhD project focuses on achieving unbiased measurements of weak gravitational lensing — the tiny distortions in galaxy shapes caused by the matter along the line of sight. This technique is key to studying dark matter, dark energy, and gravity, and lies at the heart of the Euclid space mission launched in 2023. Traditional shape-measurement methods introduce systematic biases in shear estimation. The goal of this PhD is to develop and extend an innovative forward-modelling approach that directly infers the shear by simulating realistic galaxy images using deep-learning architectures. The student will adapt this framework to real Euclid data, accounting for the complexity of the Science Ground Segment (SGS) and implementing GPU-accelerated and high-performance computing solutions to scale to the full sky coverage. The project is timely, coinciding with Euclid’s first public data release in 2026. The expected outcome is a more accurate and robust shear estimation method, enabling the next generation of precision cosmology analyses.