Cosmological parameter inference using theoretical Wavelet statistics predictions

Launched in 2023, the Euclid satellite is surveying the sky in optical and infrared wavelengths to create an unprecedented map of the Universe's large-scale structure. A cornerstone of its mission is the measurement of weak gravitational lensing—subtle distortions in the shapes of distant galaxies. This phenomenon is a powerful cosmological probe, capable of tracing the evolution of dark matter and helping to distinguish between dark energy and modified gravity theories.
Traditionally, cosmologists have analyzed weak lensing data using second-order statistics (like the power spectrum) paired with a Gaussian likelihood model. This established approach, however, faces significant challenges:
- Loss of Information: Second-order statistics fully capture information only if the underlying matter distribution is Gaussian. In reality, the cosmic web is highly structured, with clusters, filaments, and voids, making this approach inherently lossy.
- Complex Covariance: The method requires estimating a covariance matrix, which is both cosmology-dependent and non-Gaussian. This necessitates running thousands of computationally intensive N-body simulations for each model, a massive and often impractical undertaking.
- Systematic Errors: Incorporating real-world complications—such as survey masks, intrinsic galaxy alignments, and baryonic feedback—into this framework is notoriously difficult.

In response to these limitations, a new paradigm has emerged: likelihood-free inference via forward modelling. This technique bypasses the need for a covariance matrix by directly comparing real data to synthetic observables generated from a forward model. Its advantages are profound: it eliminates the storage and computational burden of massive simulation sets, naturally incorporates high-order statistical information, and can seamlessly integrate systematic effects. However, this new method has its own hurdles: it demands immense GPU resources to process Euclid-sized surveys, and its conclusions are only as reliable as the simulations it uses, potentially leading to circular debates if simulations and observations disagree.

A recent breakthrough (Tinnaneni Sreekanth, 2024) offers a compelling path forward. This work provides the first theoretical framework to directly predict key wavelet statistics of weak lensing convergence maps—exactly the kind Euclid will produce—for any given set of cosmological parameters. It has been shown in Ajani et al (2021) that the wavelet coefficient L1-norm is extremely powerful to constraint the cosmological parameters. This innovation promises to harness the power of advanced, non-Gaussian statistics without the traditional computational overhead, potentially unlocking a new era of precision cosmology. We have demonstrated that this theoretical prediction can be used to build a highly efficient emulator (Tinnaneri Sreekanth et al, 2025), dramatically accelerating the computation of these non-Gaussian statistics. However, it is crucial to note that this emulator, in its current stage, provides only the mean statistic and does not include cosmic variance. As such, it cannot yet be used for full statistical inference on its own. 

This PhD thesis aims to revolutionize the analysis of weak lensing data by constructing a complete, end-to-end framework for likelihood-free cosmological inference. The project begins by addressing the core challenge of stochasticity: we will first calculate the theoretical covariance of wavelet statistics, providing a rigorous mathematical description of their uncertainty. This model will then be embedded into a stochastic map generator, creating realistic mock data that captures the inherent variability of the Universe.
To ensure our results are robust, we will integrate a comprehensive suite of systematic effects—such as noise, masks, intrinsic alignments, and baryonic physics—into the forward model. The complete pipeline will be integrated and validated within a simulation-based inference framework, rigorously testing its power to recover unbiased cosmological parameters. The culmination of this work will be the application of our validated tool to the Euclid weak lensing data, where we will leverage non-Gaussian information to place competitive constraints on dark energy and modified gravity.

References
V. Ajani, J.-L. Starck and V. Pettorino, "Starlet l1-norm for weak lensing cosmology", Astronomy and Astrophysics,  645, L11, 2021.
V. Tinnaneri Sreekanth, S. Codis, A. Barthelemy, and J.-L. Starck, "Theoretical wavelet l1-norm from one-point PDF prediction", Astronomy and Astrophysics,  691, id.A80, 2024.
V. Tinnaneri Sreekanth, J.-L. Starck and S. Codis, "Generative modeling of convergence maps based in LDT theoretical prediction", Astronomy and Astrophysics,  701, id.A170, 2025.

Chemical recycling of oxygenated and nitrogenated plastic waste by reductive catalytic routes

Since the 1950s, the use of petroleum-based plastics has encouraged the emergence of a consumption model focused on the use of disposable products. Global plastic production has almost doubled over the last 20 years, currently reaching 468 million tons per year. These non-biodegradable plastic are the source of numerous forms of environmental pollution. Since the 1950s, only 9% of the wastes have been recycled. The majority have been incinerated or sent to landfill. In the current context of this linear economy, health, climate and societal issues make it essential to transition to a circular approach to materials. This evolution requires the development of recycling methods that are both effective and robust. While the most common recycling methods currently in use are mainly mechanical processes that apply to specific types of waste, such as PET plastic bottles, the development of chemical recycling methods appears promising for treating waste for which no recycling channels exist. These innovative chemical processes make it possible to recover the carbonaceous material from plastics to produce new ones.
Within this objective of material circularity, this doctoral project aims to develop new chemical recycling routes for mixed oxygen/nitrogen plastic waste such as polyurethanes (insulation foam, mattresses, etc.) and polyamides (textile fibres, circuit breaker boxes, etc.), for which recycling routes are virtually non-existent. This project is based on a strategy of depolymerizing these plastics by the selective cleavage of the carbon-oxygen and/or carbon-nitrogen bonds to form the corresponding monomers or their derivatives. To do that, catalytic systems involving metal catalysts coupled with abundant and inexpensive reducing agents will be developed. In order to optimize these catalytic systems, we will seek to understand how they proceed and the mechanisms involved.

Characterization and calibration of cryogenic detectors at the 100 eV scale for the detection of coherent neutrino scattering (CEvNS)

DESCRIPTIONS:

The NUCLEUS experiment [1] aims to detect reactor neutrinos via coherent elastic neutrino–nucleus scattering (CEvNS). Predicted in 1974 and first observed in 2017, this process provides a unique opportunity to test the Standard Model at low energies. Because the scattering is coherent over the entire nucleus, the cross section is enhanced by several orders of magnitude, making CEvNS also promising for reactor monitoring using neutrinos.

The NUCLEUS experimental setup is currently being installed near the EDF nuclear reactors in Chooz (Ardennes, France), which constitute an intense neutrino source. The only physical signal of a CEvNS event is the tiny recoil of the target nucleus, with an energy below 1 keV. To detect this, NUCLEUS uses CaWO4 crystals of about 1 g, placed in a cryostat cooled to 15 mK. The nuclear recoil produces vibrations in the crystal lattice, equivalent to a temperature rise of about 100 µK, measured with a Transition Edge Sensor (TES) deposited on the crystal. These detectors achieve excellent energy resolutions of only a few eV and detection thresholds on the order of ~10 eV [2]. The NUCLEUS setup was successfully tested and validated in 2024 at TU Munich [3], and data taking at Chooz is scheduled to start in summer 2026, simultaneously with the beginning of the PhD. An initial contribution will involve data acquisition and analysis at the reactor site. More specifically, the PhD student will be responsible for the characterization of the deployed cryogenic CaWO4 detectors — stability, energy resolution, calibration, and intrinsic background of the crystal.

Calibration at the sub-keV scale is a crucial challenge for CEvNS (and dark matter) experiments. Until recently, it was extremely difficult to generate nuclear recoils of known energy to characterize detector responses. The CRAB method [4, 5] addresses this issue by using thermal neutron capture (25 meV) on nuclei that constitute the cryogenic detector. The resulting compound nucleus has a well-known excitation energy — the neutron separation energy — between 5 and 8 MeV, depending on the isotope. When it de-excites by emitting a single gamma photon, the nucleus recoils with a precisely determined energy given by two-body kinematics. A calibration peak in the desired energy range of a few hundred eV then appears in the detector’s energy spectrum. A first measurement in 2022, using a NUCLEUS CaWO4 detector and a commercial ²5²Cf neutron source, validated this method [6].

The second part of the PhD will take place within the “high-precision” phase of the project, which consists in performing measurements with a pure thermal neutron beam from the TRIGA-Mark-II reactor in Vienna (TU Wien, Austria). The calibration setup was successfully installed and characterized in 2025 [7]. It consists of a cryostat housing the cryogenic detectors to be characterized, surrounded by large BaF2 crystals for coincidence detection of the nuclear recoil and the gamma ray that induced it. The whole setup is placed directly on the neutron beam axis, which provides a flux of about 450 n/cm²/s. This coincidence technique will significantly reduce background and extend the CRAB method to a wider energy range and to materials used in most cryogenic detectors. These measurements are expected to provide a unique characterization of the response of cryogenic detectors in the energy region of interest for light dark matter searches and coherent neutrino scattering. In parallel with the measurement of nuclear recoils, the installation of a low-energy X-ray source in the cryostat will generate electronic recoils, enabling a direct comparison between the detector responses to sub-keV energy deposits produced by nuclear and electronic recoils.

The arrival of the PhD student will coincide with the completion of the measurement program on CaWO4 and Al2O3 detectors of NUCLEUS and with the start of the measurement programs on Ge (TESSERACT project) and Si (BULLKID project) detectors.
The high-precision measurements will also open a new sensitivity window to subtle effects coupling nuclear physics(nuclear de-excitation times) and solid-state physics (nuclear recoil times in matter, and the creation of crystal defects induced by nuclear recoils) [8].

The PhD student will be deeply involved in all aspects of the experiment: simulation, data analysis, and interpretation of the obtained results.

WORK PLAN:

The PhD student will actively participate in data taking and in the analysis of the first results from the NUCLEUS cryogenic CaWO4 detectors at Chooz. This work will be carried out in collaboration with the Nuclear Physics Department (DPhN), the Particle Physics Department (DPhP) of CEA-Saclay, and the TU Munich team. It will begin with familiarization with the CAIT analysis framework used for cryogenic detectors. The student will focus in particular on detector calibration, studying the detector response to electronic recoils induced by optical photon pulses injected through fibers and by X-ray fluorescence generated by cosmic rays. Once this calibration is established, two types of backgrounds will be investigated: Nuclear recoils in the keV range induced by cosmogenic fast neutrons, and a low-energy background, known as the Low Energy Excess (LEE), intrinsic to the detector.
The comparison between the experimental and simulated fast neutron background spectra will be analyzed in light of the differences between nuclear and electronic recoil responses measured in the CRAB project. The long data-taking periods at the Chooz site will also be used to study the time evolution of the LEE background. This work will be conducted in collaboration with solid-state physics experts from the Institute for Applied Sciences and Simulation (CEA/ISAS) to better understand the origin of the LEE, which remains a major open question in the cryogenic detector community.
The analysis skills acquired on NUCLEUS will then be applied to the high-precision CRAB measurement campaigns planned for 2027 at the TRIGA reactor (TU Wien) with Ge and Si detectors. The student will be deeply involved in the setup, data acquisition, and analysis of results. The planned measurements on germanium, using both phonon and ionization channels, have the potential to resolve the current ambiguity in the ionization yield of low-energy nuclear recoils, a key factor for the sensitivity of future experiments.
The high calibration precision will also be exploited to study fine effects in nuclear and solid-state physics, such as timing effects and crystal defect formation induced by nuclear recoils in the detector. This study will be conducted in synergy with teams from CEA/IRESNE and CEA/ISAS, who provide detailed simulations of nuclear de-excitation gamma cascades and molecular dynamics simulations of nuclear recoil propagation in matter.

Through this work, the student will receive comprehensive training as an experimental physicist, including strong components in simulation and data analysis, as well as hands-on experience with cryogenic techniques during the commissioning of the NUCLEUS and CRAB detectors. The proposed contributions are expected to lead to several publications during the PhD, with high visibility in the CEvNS and dark matter communities. Within the CEA, the student will also benefit from the exceptionally cross-disciplinary nature of this project, which already
fosters regular interaction among the communities of nuclear physics, particle physics and condensed matter physics.

COLLABORATIONS:

NUCLEUS: Germany (TU-Munich, MPP), Austria (HEPHY, TU-Wien), Italy (INFN), France (CEA-Saclay).
CRAB: Germany (TU-Munich, MPP), Austria (HEPHY, TU-Wien), Italy (INFN), France (CEA-Saclay, CNRS-IJCLab, CNRS-IP2I, CNRS-LPSC).

BIBLIOGRAPHY:

[1] NUCLEUS Collaboration, Exploring CE?NS with NUCLEUS at the Chooz nuclear power plant, The European Physical Journal C 79 (2019) 1018.
15, 48, 160, 174
[2] R. Strauss et al., Gram-scale cryogenic calorimeters for rare-event searches, Phys. Rev. D 96 (2017) 022009. 16, 18, 78, 174
[3] H. Abele et al., Particle background characterization and prediction for the NUCLEUS reactor CE?NS experiment, https://arxiv.org/abs/2509.03559
[4] L. Thulliez, D. Lhuillier et al. Calibration of nuclear recoils at the 100 eV scale using neutron capture, JINST 16 (2021) 07, P07032
(https://arxiv.org/abs/2011.13803)
[5]https://irfu.cea.fr/dphp/Phocea/Vie_des_labos/Ast/ast.php?id_ast=4970
[6] H. Abele et al., Observation of a nuclear recoil peak at the 100 eV scale induced by neutron capture, Phys. Rev. Lett. 130, 211802 (2023) (https://arxiv.org/abs/2211.03631)
[7] H.Abele et al., The CRAB facility at the TUWien TRIGA reactor: status and related physics program, (https://arxiv.org/abs/2505.15227)
[8] G. Soum-Sidikov et al., Study of collision and ?-cascade times following neutron-capture processes in cryogenic detectors Phys. Rev. D
108, 072009 (2023) (https://arxiv.org/abs/2305.10139)

Blended positive electrodes in solid-state batteries: Effect of the electrode fabrication process on electrochemistry

The development of cost-effective, high-energy-density solid-state batteries (SSBs) is essential for the large-scale adoption of next-generation energy storage technologies. Among various cathode candidates, LiFePO4 (LFP) and LiFe1??Mn?PO4 (LFMP) offer safety and cost advantages but suffer from low working voltages and limited kinetics compared to Ni-rich layered oxides such as LiNi0.85Mn0.05Co0.1O2 (NMC85). To balance energy density, rate capability, and stability, this PhD project aims to develop blended cathodes combining LFMP and NMC85 in optimized ratios for solid-state configurations employing sulfide electrolytes (Li6PS5Cl). We will investigate how fabrication methods- including slurry-based electrode processing and binder-solvent optimization- affect the electrochemical and structural performance. In-depth operando and in situ characterizations (XRD, Raman, and NMR) will be conducted to elucidate lithium diffusion, phase transition mechanisms, and redox behavior within the blended systems. Electrochemical impedance spectroscopy (EIS) and titration methods will quantify lithium kinetics across various states of charge. By correlating processing conditions, microstructure, and electrochemical behavior, this research seeks to identify optimal cathode compositions and manufacturing strategies for scalable, high-performance SSBs. Ultimately, the project aims to deliver a comprehensive understanding of structure–property relationships in blended cathodes, paving the way for practical solid-state battery technologies with enhanced safety, stability, and cost efficiency.

Growth and Characterization of AlScN: A New Promising Material for Piezoelectric and Ferroelectric Devices

III-nitride semiconductors — GaN, AlN, and InN — have revolutionized the lighting market and are rapidly entering the power electronics sector. Currently, new nitride compounds are being explored in the search for novel functionalities. In this context, aluminum scandium nitride (AlScN) has emerged as a particularly promising new member of the nitride family. Incorporating scandium into AlN leads to:

* Enhanced Piezoelectric Constants: Making AlScN highly attractive for the fabrication of piezoelectric generators and high-frequency SAW/BAW filters.
* Increased Spontaneous Polarization: The enhanced polarization can be exploited in designing high-electron-mobility transistors (HEMTs) with very high channel charge densities.
* Ferroelectricity: The recently discovered (2019) emergence of ferroelectric properties opens up possibilities for developing new non-volatile memory devices.

Over the past five years, AlScN has become a major focus of research, presenting numerous open questions and exciting opportunities to explore.

This PhD thesis will focus on the study of the growth and properties of AlScN and GaScN synthesized by molecular beam epitaxy (MBE). The student will receive training in the use of an MBE system for the synthesis of III-nitride semiconductors and in the structural characterization of materials using atomic force microscopy (AFM) and X-ray diffraction (XRD). The variation of the polarization properties of the materials will be investigated by analyzing the photoluminescence of quantum well structures. Finally, the student will be trained in the use of simulation software to model the electronic structure of the samples, aiding in the interpretation of the optical results.

Development of photo-printed interferometric biosensors on multi-core optical fibers for molecular diagnostics

Optical fibers are minimally invasive devices commonly used in medicine for in vivo tissue imaging by endoscopy. However, at present, they only provide images and no molecular information about the tissues observed. The proposed thesis is part of a project aimed at giving optical fibers the ability to perform molecular recognition in order to develop innovative biosensors capable of performing real-time, remote, in situ, and multiplexed molecular analysis. Such a tool could lead to significant advances in the medical field, particularly in the study of brain pathologies, where knowledge of the tumor environment, which is difficult to access using conventional biopsies, is essential.
The proposed approach is based on 2-photon polymerization printing of interferometric structures at the end of each core of a multifiber assembly. The detection principle is based on the interference occurring in these structures and their modification by the adsorption of biological molecules. Each fiber in the assembly will act as an individual sensor, and measuring the intensity of the light reflected at the functionalized end will provide information about the biological interactions occurring on that surface. By modeling the interference phenomenon, we determined parameters to optimize the shape and sensitivity of interferometric structures (PTC InSiBio 2024-2025). These results enabled the printing and characterization of the sensitivity of interferometric structures on single-core fibers. The objectives of the thesis are to continue this optical characterization on new samples and to develop original photochemical functionalization methods in order to graft several biological probes onto the surface of the fiber assemblies. This multi-functionalization will enable multiplexed detection, which is essential for future medical applications. Depending on the progress of the thesis, the biosensors will be validated through the detection of biological targets in increasingly complex environments, up to and including a brain tissue model.

Coupled Friction Effects of Dirac sea and Electromagnetic Vacuum on Atomic movements

Quantum fluctuations induce conservative macroscopic forces such as the Casimir effect. They could also cause dissipative forces, termed vacuum (or quantum) friction. Up to now, this friction effect has been calculated with consideration of the electromagnetic fluctuations only, i.e. without taking into account the Dirac Sea. This project is devoted to the extension of our research in this direction: electrons, as main contributors of the matter-field interaction, also interact with electron-positron virtual pairs in the quantum vacuum. How much of quantum friction, at zero or finite vacuum temperature, could be due to this type of interaction? A first step will be adapting the present semi-classical framework to include vacuum polarization and pair creation. In doing so, one will encounter finite frequency cut-offs, traditionally linked to virtual pair creation; thus one will determine a friction component linked with the finite cut-off of Fourier integrals. On this research path, one shall pay attention to maintaining the mathematical coherence of the whole framework. A longer-term goal remains a complete and consistent quantum relativistic treatment of quantum friction at the atomic level.

Modeling of a magnonic diode based on spin-wave non-reciprocity in nanowires and nanotubes

This PhD project focuses on the emerging phenomenon of spin wave non-reciprocity in cylindrical magnetic wires, from their fundamental properties, to their exploitation towards realizing magnonic diode based devices. Preliminary experiments conducted in our laboratory SPINTEC on cylindrical wires, with axial magnetization in the core and azimuthal magnetization on the wire surface, revealed a giant non-symmetrical effect (non-symmetrical dispersion curves with different speeds and periods for left- and right-propagating waves), up to an extent of creating a band gap for a given direction of motion, related to the circulation of magnetization (right or left). This particular situation has not been yet described theoretically or modeled, which sets an unexplored and promising ground for this PhD project. To model spin-wave propagation and derive dispersion curves for a given material we plan to use different numerical tools: our in-home 3D finite element micromagnetic software feeLLGood and open source 2D TetraX package dedicated to eigen modes spectra calculations. This work will be conducted in tight collaboration with experimentalists, with a view both to explain experimental results and to guide further experiments and research directions.

Chemical biology approaches to rare earth toxicology in Humans

Recent technological developments have expanded and intensified the use of lanthanides in domains as diverse as renewable energy, computing, and medicine. Increasing usage of these metals raises the question of their impact on the environment and human health. However, the potential toxicity of these metal ions, and its underlying molecular mechanisms, are still little known and rarely investigated in human cell models. The goal of the PhD will be to investigate the human cells response to exposure to Ln ions, and to systematically identify the proteins involved in this response, using a set of chemical and biological tools. In particular, we want to address the following questions: which protein networks are activated or deactivated following Ln exposure? Do Ln ions affect phosphorylation of proteins? Which proteins are directly interacting with Ln ions? will thus decipher what are the key biological interactors of lanthanides, their roles in living systems and the features that enable efficient binding to metals. We expect that our findings will give insights into the toxicology of those elements and inform environmental and occupational safety policies. On the longer term, new bio-inspired strategies for their extraction, recycling, decorporation and remediation will arise from the molecular understanding of metal-life interactions, enabling a well thought-out usage of these elements to support the environmental and numerical transitions.s

Ultra-fast pathogenic bacteria detection in human blood

This project aims to develop a versatile and easy-to-use surface plasmon resonance imaging (SPRi) instrument for the rapid and broad-spectrum detection of low concentrations of pathogenic bacteria in complex samples, particularly blood. SPRi is a label-free technique that allows real-time probing of a sample (regardless of its optical transparency). Due to the high sensitivity of the plasmon phenomenon, the dynamic range of measurable index variation is limited by SPRi detection when reading is performed at a fixed angle, as is the case in commercially available devices. This reduces the use of such optical instruments to the study of environments whose index remains relatively stable during the experiment and whose molecular probes have molecular weights comparable to the targets (monitoring of bimolecular interactions).
This considerably limits the detection of growing bacteria in complex environments. Our laboratory has developed original solutions for the detection of very low levels of contamination in food matrices (creation of a start-up in 2012), but SPRi is unsuitable for the detection of bacteria in blood, partly due to the very high intrinsic variability of this matrix.
These limitations will be overcome by integrating five complementary components:
1. The design of an instrument optimized for real-time recording of SPR images over a defined range of illumination angles;
2. The development of dedicated SPR data analysis and processing to extract the most relevant information for each probe from the images in real time;
3. The functionalization of biochips through a combination of appropriate probes (series of peptides such as antimicrobial peptides (AMPs), antibodies, and even bacteriophages) to optimize the number of possible identifications with a reduced set of probes;
4. The learning of specific “4D-SPRi signatures” of model strains in blood matrices;
5. Validation of the performance of the new “4D-SPRi” instrument as a tool for detecting and characterizing bacteria from hospital strains compared to reference techniques.

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