Institute of research into the fundamental laws of the Universe

After the discovery of the Higgs boson at the LHC, particle physics community is exploring and proposing next accelerators, to address the remaining open questions on the underlying mechanisms and on the constituents of the present universe. One of the studied possibilities is FCC (Future Circular Collider), a 100-km-long collider at CERN. The hadron version of FCC (FCC-hh) seems to be the only approach to reach energy levels far beyond the range of the LHC, in the coming decades, providing direct access to new particles with masses up to tens of TeV. The electron version of FCC brings a tremendous increase of production rates for phenomena in the sub-TeV mass range, making precision physics studies possible. A first study has shown no major showstopper in the colliders’ feasibility but has identified several specific challenges for the beam dynamics: large circumference (civil engineering constraints), beam stability with high current, the small geometric emittance, unprecedented collision energy and luminosity, the huge amount of energy stored in the beam, large synchrotron radiation power, plus the injection scenarios. This thesis will focus on the optimization of the hadron option of the future circular collider against linear and non-linear imperfections (i.e. magnets alignments and their field quality). A key point of this thesis is the comparison of current advanced correction schemes to techniques based on machine learning. The application of these techniques to accelerators is one of current hot topics in the field and pursued worldwide.

One of the coolest thesis you may do today: Higgs boson physics through one of its rarest and not yet observed decays, standard model processes at their best to fully understand it, and a detector part completely related to it and to the CMS detector upgrades for the High-Luminosity phase of the LHC.



In the quest for understanding our universe, the standard model of particle physics is believed to be a low-energy approximation of a more comprehensive theory. The discovery of the Higgs boson has inserted a new tile in the puzzle, but many questions remains open (naturalness, numbers of lepton generations, asymmetry matter-antimatter, etc.). A precise characterization of the Higgs boson through all of its decays can improve our comprehension of the puzzle.



This thesis proposes a search for the Higgs boson decay into a Z boson and a photon (Zgamma). Almost as rare as the golden decay into two photons, it is harder to observe because of the additional branching fractions of the Z bosons into final state particles: electrons and muons (but also neutrinos and other final states can possibly be exploited to some extent). The decay has not been observed yet -limits on its likelihood have been set so far- but some evidence of it might be found if exploiting the full dataset provided by the Run2 of the LHC (2015-2018) and by the just started Run3, expected to almost double the Run2 dataset by 2025.

The Zgamma decay is related to and can be constrained by other Higgs boson decays: the direct one into two muons plus an additional photon radiated in the final-state, the one into two Z bosons, Dalitz decays in electrons and muons. The standard model production of Z bosons and pairs of vector bosons such as ZZ, ZW, WW can dissimulate a final state as the Zgamma decay and have to be considered in the analysis: searching for the Zgamma decays implies playing with and understanding numerous fundamental SM processes and Higgs boson decays.



The thesis also consists of an experimental part to optimize the time resolution of the electromagnetic calorimeter of CMS (ECAL). While designed for high precision energy measurements, the ECAL also provides an excellent resolution on the arrival time of the photons (about 150 ps in collision events, but 70 ps have been achieved in test beams).

In an environment populated by photons from overlapping events (pileup), the arrival time of a final-state photon can help constraining its provenance to be the same vertex of the Higgs boson decay, i.e. the one of the Z decay products. This feature will be a key for the High-Luminosity LHC phase (2029-) when the ECAL electronics will be upgraded to offer even better time resolution (30 ps for high energy electrons and photons) and the luminosity of the LHC -and the number of overlapping events and photons in the final state- will be a factor of 5 higher than today.



The thesis also proposes the participation at CERN to CMS/ECAL shifts and to laboratory tests foreseen for the newly developed ECAL electronics.

The SPIRAL2 linear accelerator is optimized for light ions (protons, deuterons), but it will also deliver heavier ions (O, Ne, Ar,…Ni) for the S3 spectrometer. The first objective of the PhD, is to propose and study the methods allowing to tune a heavy ion beam in 26 independent accelerating RF cavities in a fast and reproducible way up.

The S3 electromagnetic separator will use the beams from the linac to create and purify radioactive ions with a high efficiency. The complexity of its superconducting magnets requires an optimization of many parameters. Thanks to numerous hexapolar and octupolar corrections, we will be able to reduce the beam optical aberrations. The commissioning of the separator will require numerous measurements with beams and the development of an algorithm to optimize the optics for the 2 different operating modes. The second objective is to provide the simulations tools to nuclear physicists allowing them to prepare their experiments on S3 and to adjust the parameters of the spectrometers during the experiments.

The thesis work will be based on beam dynamics simulations and experimental measurements with beams.

Within the CEA-IRFU (Institut de Recherche sur les Lois Fondamentales de l'Univers), the DACM (Département des Accélérateurs, de Cryogénie et de Magnétisme) is a major player at national and international level in the field of particle accelerators. It has actively participated in most of the accelerator projects of the world's leading research centres over the last decades. An important part of these activities concerns the design of accelerators, linear or circular, for high energy physics or any other scientific application. The field of particle accelerator physics requires in-depth knowledge of the beam dynamics in order to control beams perfectly. In this discipline, the DACM has also turned to new laser-plasma acceleration techniques, with a view to designing laser-plasma wakefield accelerators (LWFA) that will make it possible to significantly reduce the size and cost of future accelerators. Collaborations with international (EuPRAXIA, CERN-AWAKE) or national (LPGP-CNRS, IJCLab-CNRS) partners have been initiated for the design of LWFAs in various configurations and applications. The DACM is currently involved in the design of a reliable and compact LWFA to serve as an electron source for the AWAKE collaboration. Such an accelerator would be a world first. In order to prove its viability, the LWFA must generate reproducible high-quality beams. Detailed physical and numerical optimisations from injection to the end user will have to be implemented. The candidate will also be involved in the other LWFA projects of the DACM.



The thesis will focus on the physical and numerical study of plasma acceleration stages and transport lines between plasma stages or to the end user. The core of the studies will be the control of the quality of the particle beam (size characteristics, divergence, energy spread, ...) that results from the laser-plasma interaction and the applied electromagnetic elements. The optimal integration of the acceleration and transport sections will then be determined. At each stage, the fundamental principles for obtaining the best beam parameters will be sought, and then applied to other ALP design projects in which DACM is involved. Optimizations using machine learning algorithms are also envisaged.



The success of these studies is strongly conditioned by a solid understanding of the physical phenomena in question (6D phase space of the beam, wakefields in plasmas subjected to ultra-intense lasers, multipolar field of electromagnets) and by a good use of the corresponding simulation codes.

Over the last two years the Imaging Air Cherenkov Telescopes (IACTs) H.E.S.S. and MAGIC were able to detect very-high-energy gamma-ray emission from gamma-ray bursts (GRBs). These breakthrough results have triggered renewed discussions of the particle acceleration and emission mechanisms that can be found in these violent explosions [1].



Complementing the detections of GRBs via X-ray satellites, the detection of gravitational waves allows to provide new and complementary insights into the pre-explosion phase, the initial conditions, the geometry of the system, and much more. The proposed thesis project will exploit the exciting possibilities of combining the detection of GWs and the detection of the resulting GRB by VHE gamma-ray observatories in truly multi-messenger observations and analyses.



The core of the proposed project will be H.E.S.S., currently the world’s most sensitive gamma-ray instrument, and CTA, the next generation, global high-energy gamma-ray observatory. We’ll also collaborate closely with partners from around the world including obviously the gravitational wave instrument Advanced VIRGO, the SVOM satellite to detect GRBs, various radio telescopes in Australia and South Africa, optical observatories, and many more. The group at IRFU, CEA Paris-Saclay is leading observations of transient phenomena by both H.E.S.S. and CTA and has long-standing experience with these challenging observations. The group is also driving changes and modernizations of the communication in the astroparticle community (e.g. via the Astro-COLIBRI web/smartphone application, [2]).



The PhD student will first have the opportunity to participate in the development and improvement of the framework that allows to optimize the schedule of follow-up observations of astrophysical transients. Some of the most interesting event are being detected only with large localization uncertainties (i.e. especially GWs, but also GRBs, neutrinos and others). We therefore need specialized tools and algorithms that allow to point the follow-up instruments like H.E.S.S. into the right direction to rapidly catch the associated emission [3]. A year-long observation period by the GW interferometers (called O4) is scheduled to start in spring 2023. This timing is perfectly matching the PhD project presented here, as the selected student will have the opportunity to lead the H.E.S.S. and CTA/LST-1 follow-up observations searching for GRBs and other VHE gamma-ray counterparts to the GWs detected by LIGO/VIRGO/KAGRA during that period. A sizeable amount of observation time with both the H.E.S.S. and CTA/SLT-1 IACTs has been reserved for these exciting searches. We’ll thus have ample opportunities to optimize our follow-up procedures, lots of data to analyze, results to present at international conferences, and papers to publish.



The core of the proposed thesis project will be the real-time search for transient high-energy gamma-ray emission linked to the detection of a gravitational wave (and other multi-messenger astrophysical transients like high-energy neutrinos, gamma-ray bursts, fast radio bursts, stellar/nova explosions, etc.). The combined observations will unequivocally prove the existence of a high-energy cosmic ray accelerator related to these violent multi-messenger phenomena and will allow to derive novel insights into the most violent explosion in the universe.



References:

[1] H.E.S.S. Collaboration: “Revealing x-ray and gamma ray temporal and spectral similarities in the GRB 190829A afterglow, Science, Vol. 372 (2021);

[3] P. Reichherzer, F. Schüssler, et al. : “Astro-COLIBRI-The COincidence LIBrary for Real-time Inquiry for Multimessenger Astrophysics”, ApJS 256 (2021);

[2] H. Ashkar, F. Schüssler, et al. : “The H.E.S.S. gravitational wave rapid follow-up program”, JCAP 03 (2021);

Very high energy gamma-ray astronomy is a relatively young part of astronomy (30 years), looking at the sky above 50 GeV. After the success of the H.E.S.S. array in the 2000s, an international observatory, the Cherenkov Telescope Array (CTA), should start operating by 2024. This observatory will include a total of 50 telescopes, distributed on two sites. IRFU is involved in the construction of the NectarCAM, a camera intended to equip the "medium" telescopes (MST) of CTA. The first NectarCAM (of the nine planned) is being integrated at IRFU and will be installed on the North site of CTA in 2024. Once the camera is installed, the first astronomical observations will take place, allowing to fully validate the functioning of the camera. The thesis aims at finalizing the darkroom tests at IRFU, preparing the installation and validating the operation of the camera on the CTA site with the first astronomical observations. It is also planned for the student to participate in H.E.S.S. data analysis on astroparticle topics (search for primordial black holes, constraints on Lorentz Invariance using distant AGN).

Very-high-energy (VHE, E>100 GeV) gamma-ray observations are crucial to probe the most-violent non-thermal phenomena in the universe. The central region of the Milky Way is very complex and active in VHE gamma rays. Among the VHE gamma-ray sources are the supermassive black hole Sagittarius A* lying at the centre of the Milky Way, supernova remnants and star-forming regions. The diffuse emission detected by the H.E.S.S. telescopes allowed to discover the first Galactic Pevatron - a cosmic accelerator up to PeV energies. The Galactic Center region habours the base of the Fermi bubbles- bipolar structures extending over several ten degrees, possibly linked to an enhanced past activity of Sagittarius A*. The Galactic Center region should also be the brightest source of annihilating dark matter particles VHE gamma rays.

The H.E.S.S. observatory located in Namibia is composed of five atmospheric Cherenkov telescopes. It is designed to detect gamma rays from a few ten GeV up to several ten TeV. The Galactic Centre region is observed by H.E.S.S. for 20 years. These observations allowed to detect the first Galactic Pevatron and to derive the strongest constraints to date on the annihilation cross section of dark matter particles in the TeV mass range.

The proposed PhD work will be focused on the data analysis and interpretation of all the observations carried by H.E.S.S. in the Galactic Center region over the last 20 years.The first part of the work will be dedicated to the analysis of the low-level data and the study of the systematic uncertainties in this massive dataset. In a second part, the student will combine all the available data obtained from the phase 1 and phase 2 of H.E.S.S. to search for diffuse emissions and dark matter signals using multi-component template fitting techniques. The third part will consist in the development and implementation of a new analysis method using Bayesian neural networks to search for new astrophysical emissions in H.E.S.S. data and study the detection potential of CTA. The student will participate to the data taking with the H.E.S.S. telescopes.

The goal of this PhD project is to develop all necessary tools for an efficient and reliable analysis of Euclid weak-lensing and lensing - galaxy cross-correlation data. Starting from the measured Euclid weak-lensing galaxy shapes and spectroscopic galaxy data from Euclid and other surveys such as BOSS, eBOSS, and DESI, the student will construct various estimators of lensing and cross-correlation observables.

Combinations of these observables as function of scale, redshift, and galaxy properties will be optimised to maximally extract cosmological information from the data. In addition, detailed modelling of systematic effects will be carried out to control and minimize their influence on the results.



The student will make use of, and further develop modern statistical inference tools for efficient parameter inference. Theoretical predictions of observables from models of the expansion history and large-scale structure of the Universe will be created in an automatic differentiation framework, e.g. using the library jaxcosmo, exploiting massive parallel computations on GPUs and the ability to compute

gradients of the likelihood to accelerate inference. This will open up efficient (Bayesian) inference methods that make use of the gradients of the models with respect to parameters. Compared to traditional sampling techniques, these methods offer a significant computation time speed-up, and the ability to efficiently explore a large number of parameters. This is important for exploring non-standard models of gravity with additional parameters, and flexible models with many nuisance parameters. It also allows us to include detailed, time-consuming modelling of systematic and higher-order effects.

The goal of this PhD project is to develop and apply methodologies for large-scale Bayesian inference over a differentiable physical simulator, with application to the cosmological analysis of the DESI (Dark Energy Spectroscopic Instrument) galaxy survey.



DESI is a multi-object spectrograph mounted on the Mayall telescope at Kitt Peak, Arizona, which will enable redshift measurements of 35 millions of galaxies and quasars between 0.05 < z < 3.0, yielding a tenfold increase in statistics compared to previous spectroscopic surveys (e.g. BOSS, eBOSS). The first year of DESI data taking, corresponding to one fifth of the total statistics, has been completed, thereby constituting the largest spectroscopic dataset ever assembled.



We propose to develop a theoretically lossless approach to extracting cosmological information from modern galaxy surveys, in particular DESI, which consists in reproducing the observed galaxy density with simulations of the Universe large scale structure. This new approach requires methodological developments to make high-dimensional (~ 10^10) Bayesian inference tractable, and to accelerate numerical simulations with hybrid physical and machine learning modelling. The PhD candidate will apply the developed methodology to the analysis of the DESI first year data to produce state-of-the-art cosmological constraints. This project will lead to three first-author publications.

Exoplanets are now observed in large numbers and around stars with varied properties. Cradles of life development, it is essential to understand the physical processes leading to their formation but also the initial conditions that prevail during the very first steps of their assembly in order to constrain the models of planetary genesis.

Recent observations of protopanetary disks suggest that some planets may already be formed around very young stars, only one million years after the beginning of the star formation process: it seems therefore crucial to explore the earliest phases of solar-type star formation in search of signatures of dust assembly into small planetesimals.

Recently, our team obtained first observational evidence that dust particles around the youngest protostars in our Galaxy might have already evolved significantly with grains perhaps already a few microns large. If these results are confirmed, then the scenario of planetesimal formation will have to be revisited by taking into account the impact of these initial conditions very different from those prevailing in the disks around T-Tauri stars, during the first phase of dust growth.



The subject of this thesis is part of a concerted approach using observations and models to characterize the size of dust grains around protostars only a few hundred thousand years old.

It will analyze data sets obtained with the ALMA radio telescope to measure the spectral indices of the continuum emission of dust grains surrounding Class 0 and I protostars.

Complementary constraints will be obtained at millimeter wavelengths thanks to polarimetric data from a large program of observations with the NOEMA telescope, very sensitive to the size of the largest dust grains.

Our collaboration is currently implementing a new dust model better adapted to the dense gas which, coupled to a radiative transfer code, will allow us to directly compare these observational data to synthetic predictions from MHD models of protostellar evolution.

The intergalactic magnetic field pervading the cosmic voids is suspected to be a relic field originating from the very first epoch of the cosmic history. The goal of this PhD is to look for signatures of this field in the high-energy data of gamma-ray bursts, and to predict the ability of the future CTA observatory to constrain its properties. This work combines both theoretical modelling and analysis of simulated CTA data.

Weak gravitational lensing, the distortion of images of high-redshift galaxies due to foreground matter structures on large scales, is one of the most promising tools of cosmology to probe the dark sector of the Universe.



The student will work on various wide-field instruments which provide imaging surveys of galaxies:

- UNIONS, the Ultraviolet Near-Infrared Optical Northern Sky survey, an ongoing large imaging survey of the Northern sky in 5 bands.

- WFST, the Wide-Field Survey Telescope. This 2.5m optical telescope under construction in China will observe the Northern sky in 5 bands.

- CSST, the 2m Chinese Space Station Telescope, to be launched in 2024 into a low Earth orbit, will observe an area of 17,500 deg2 in 7 optical and near-UV bands.



Starting with ShapePipe, a weak-lensing pipeline created by our group, the student will develop new methods to analyse such large

datasets. The unprecedented statistical precision will require novel approaches for quantifying observational and instrumental systematic errors. Astrophysical and cosmological observables will be modeled carefully for robust and reliable inference of dark-matter and dark-energy properties from weak-lensing data.

Cosmology in the 21st century aims to better our understanding of the Universe by seeking to answer open questions concerning the nature of dark matter and dark energy, and the precise expansion rate of the Universe. In order to tackle these questions, it is essential to take advantage of all the data made available in the current era of multi-messenger astronomy, capitalising on the latest advances in signal processing, machine learning and the handling of big data.

Current and upcoming optical surveys, such as KiDS-450 [1], the Dark Energy Survey Year 1 [2], the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) [3] and Euclid [4], are probing wider and deeper patches of the late-time Universe to improve constraints on cosmological parameters by measuring the shapes and distributions of galaxies. These parameters can be compared to the latest analyses of the cosmic microwave background radiation (i.e. the early Universe) by surveys like Planck [5]. Recent studies [6] highlight some discrepancy between early and late-time analyses indicating some systematic uncertainty or an incomplete model of cosmology.

Additionally, In recent years gravitational wave interferometers, such as LIGO and Virgo, have pushed astronomical observations beyond the electromagnetic spectrum. This has made it possible to detect the interaction of distant neutron stars and black holes.

Radio wavelengths provide a complementary and independent probe of the late-time Universe. Radio astronomy provides the advantage of probing higher redshifts, having a deterministic point spread function (PSF) and being less sensitive to PSF anisotropies [7]. Cross-correlations between radio and optical surveys can additionally alleviate systematics effects such as intrinsic alignments improving cosmological constraints [8-9]. Upcoming radio surveys, such as the Square Kilometre Array (SKA), are designed to reach an order of magnitude greater sensitivity and survey speed than existing instruments. SKA has the potential to add significant additional constraints on cosmological parameters given the vast sky area it will cover (~75%). This, however, comes at the cost of having to manage extremely large scales of data and complicated image reconstruction. SKA is expected to produce ~1 TB of data every second. With typical observations taking ~6h and a total lifespan of 15 years, SKA will produce data in the Exabyte (1018 bytes) scale [10], making it one of the biggest data management problems in modern science.

The CosmoStat team has been pioneering the use of signal processing and machine learning techniques for solving inverse problems in astronomical image reconstruction. Applying these methods to radio-interferometric data, however, brings a host of new challenges, in part due to the additional complexity of the inverse problems to solve, but also due to the extremely large-scale dimensions of the problem. Conventional deep learning approaches will not be able to scale to the typical size of an SKA field, and developing efficient model-parallelism approaches will be necessary.

The James Webb Space Telescope (JWST) is revolutionizing our knowledge of exoplanets. Thanks to its large collecting area (25 m2) and its capacity for spectroscopic observations in a wide range of wavelengths (0.6 - 28 microns), the telescope can probe the atmosphere of a wide range of exoplanets, ranging from warm Jupiters to temperate rocky planets. Thanks to our strong involvement in the development of the JWST Mid-InfraRed Instrument (MIRI), we have guaranteed observing time and coordinate the programme dedicated to the observation of exoplanets. The data provided by the JWST are of remarkable quality.



The thesis work will consist of reducing, analyzing and interpreting the data provided by the JWST during cycle 1 and cycle 2, preparing time requests for cycles 3 and 4 from simulated observations. The laboratory is also very involved in the Ariel mission. One aspect of the thesis will be the study of synergies between JWST and Ariel.

Galaxy clusters, formed at the intersection of the filamentary large scale structures are the most visible tracers of the distribution of matter in the Universe. Composed of dark matter (85% of the total mass) and baryons (hot X-ray emitting gas and galaxies), the evolution of the cluster population over time gives insight on the structure formation and reflects the underlying cosmology.

The sensitivity of Euclid, the upcoming major cosmological mission led by the European Space Agency and planned to be launched in 2023, should allow blind detection of clusters through their weak lensing signal i.e. the coherent distortion of background galaxy images by the intervening mass of the cluster, an effect directly linked to their total (dark and baryonic) projected mass. Combined with the sky coverage, this will allow the construction of a significant galaxy cluster catalogue that is for the first time truly representative of the true cluster population. Indeed, up to now all galaxy cluster catalogues rely on detection through their baryonic signal that represents only 15% of the total mass (e.g. through the intra-cluster gas content in X-rays and the Sunyaev-Zeldovich effect (SZE) at millimetre wavelengths, or through the optical light in the galaxies). The catalogue of galaxy clusters detected through weak lensing is directly linked to the total mass of the cluster. This will provide new constraints on galaxy cluster abundances in the Universe, which has important implications for cosmology.

The thesis project aims at development of innovative methods to detect and characterise clusters through their weak lensing signal. With the imminent launch of the Euclid satellite, the thesis project will take place in a very stimulating context. The ultimate goal of the project being to apply the methods to Euclid data and to take part in the scientific exploitation.

We now know that the first massive groups and clusters were already forming in the first 2-3 billion years of the Universe’s history. Unlike their modern-day descendants, these fascinating systems contained often highly star-forming gas-rich galaxies that later on quenched (stopped forming stars) and transformed morphologically into ellipticals systems through processes that are not yet fully known. This include the recent discovery of peculiar systems with quenched disks and star-forming bulges, which could be seen as ‘anti-galaxies’ respect to the later-time typical objects that are quenched in the centres (bulges) and forming stars in the disks (like our Milky-Way Galaxy). Other exotic components that have been recently discovered are giant reservoirs of cold diffuse hydrogen, possibly connected to cold streams postulated by theory and never fully confirmed, and intracluster light, probably related to early phases of galaxy interactions and mergers that stand in contradiction with current model predictions. The Euclid satellite, with a strong participation from France and CEA/AIM, will be launched in 2023, and will provide ideal ultra-deep multi-wavelength imaging in the optical and near-IR, together with ancillary data at longer wavelengths, to identify the first generation of galaxy groups and clusters that started forming in the distant Universe and study the different exciting physical phenomena that are occurring in dense environments at early times. The PhD student will join Euclid teams and will lead research into these problematics in collaboration with the group in CEA Saclay.

While the Universe is expanding with increasing velocity, the question of what is causing cosmic acceleration remains unsolved. Acceleration seems to act against gravitational attraction, as if a new source of energy, dubbed dark energy, were responsible for it. In addition, neutrino masses are yet to be measured and are degenerate with dark energy evolution.



This PhD proposal is meant to contribute to the Euclid mission, by combining information from galaxy clustering and weak lensing, and integrate it in the Euclid Consortium validated likelihood.

The PhD student will be able to work at the interface between data and theory and concretely collaborate with a large collaboration like the Euclid satellite. Objectives include 1) extending the likelihood software to include early dark energy models 2) contribute to the collaboration effort on comparing theoretical predictions with data and the cross-correlation between galaxy clustering and weak lensing 3) investigate different survey samples and statistics to break the degeneracy between neutrino masses and dark energy evolution.

With ~5,000 exoplanets known (http://exoplanet.eu), it is clear that the observed diversity of exoplanets is linked to their histories. In this setting, exoplanet science is faced with two key questions. The first one is how evolution may have modified the planets’ bulk compositions. The question is timely as various facilities (JWST, launch date: December 2021; ARIEL, launch in 2028; E-ELT: first light in 2027) will study in unprecedented detail the atmospheres of exoplanets with spectroscopy. The second question is how evolution may have shaped the demographics of exoplanets. Notably, our grasp of demographics has changed radically in the last decade, and will continue evolving thanks to new discoveries from photometric space missions (e.g. TESS, PLATO) and radial velocity surveys on the main ground-based telescopes.

Digesting that empirical knowledge requires physically-motivated models that connect the properties of the planets and their host stars throughout their shared histories. To that end, our project will build a sophisticated treatment of radiative transfer (RT) for close-in exoplanets with both hydrogen- and non-hydrogen-dominated atmospheres. The RT modules will be implemented into the team’s photochemical-hydrodynamic models to better understand the temporal evolution of the exoplanets. The predictions will help interpret the constraints that JWST will set on the composition of a few small exoplanets for which observing time has been granted as part of GTO+GO programs.

The interaction of convective motions in the outer convective layer of Sun-like stars with the rotation and the magnetic field is at the origin of the appearance of a dynamo which is responsible for the cycles of magnetic activity. These cycles produce periods of high activity alternated by other less active. This magnetic activity is very important to understand the development and establishment of life as we know it on Earth and to improve the detectability of planets around active stars. Indeed, one of the most important sources of noise to be able to detect and characterise the atmospheres of low-mass planets like the Earth around stars like the Sun is related to the magnetic variability. A better understanding and characterization of this variability is therefore extremely important for the scientific exploitation of data collected by space missions such as Kepler, TESS or JWST of NASA as well as the future satellites PLATO and ARIEL of ESA on which the DAp/CEA is strongly involved in its development and exploitation. Thanks to more than 25 years of data obtained by the ESA/NASA SoHO satellite (still flying around the Sun), the student will start by characterizing seismically the variations of the outermost layers of the Sun during the last two activity cycles (23 and 24) covering already more than 26 years.

The second part of the thesis will consist in searching for activity cycles in the data of ~160,000 stars observed during 4 years by the Kepler satellite and thus better prepare the necessary tools (including "Machine Learning" techniques) to obtain these cycles in the data of the NASA TESS mission and for the future ESA PLATO mission. In parallel to these studies, the student will participate in the characterization of the noise of planetary stars candidates for planetary atmosphere studies with JWST and ARIEL, in collaboration with other members of LDE3, to better characterise the modulations associated with the rotation and magnetism of these stars and thus to improve the characterization of planetary atmospheres. These studies could be done with TESS or ground-based follow-up data.

The Large Scale Structures (LSS) of the Universe come from the growth, under the effect of gravitation, of small primordial fluctuations of density created by inflation. The measurement of the statistical properties of LSS allow us to study the inflation at very large scales (~Gpc), the Dark Energy at smaller scales (~100 Mpc) with Baryonic Acoustic Oscillations (BAO) and the gravity at even smaller scales (~tens of Mpc) with Redshift Space Distortions (RSD).



Our strategy for studying the LSS is to use a spectroscopic survey, DESI that will observe tens of millions of galaxies and quasars. The observations take place at the 4-meter Mayall telescope in Arizona .Since spring 2021, the project has started an uninterrupted observation period that will last 5 years and that will cover a quarter of the sky.



For this PhD, LSS are measured with a single tracer of the matter: the quasars, very distant and very luminous objects. This tracer allows us to cover a wide redshift range from 0.9 to 3.5 and to the Universe clustering at all scales, from a few tens of Mpc to Gpc.



During the first year, the PhD student will participate in the analysis of the first observation year. The PhD student will be able to devote to a global measurement of the cosmological parameters which will simultaneously cover all the scales. The thesis will end with the study of the first three years of observation of DESI.

The InterStellar Medium (ISM) is the complex intertwining of phases filling the volume of a galaxy between the stars. It is constituted of: (i) gas, principally hydrogen (75%) and helium (23%), but also heavier elements (C, N, O, etc.; 1%) that can be found in molecular forms; and (ii) dust grains (the remaining 1% of the ISM mass), which are small solid particles of sub-micronic sizes. The ISM is a fundamental constituent of the Universe, as stars are born out of the collapse of dense interstellar clouds, and return some of their mass, enriched in freshly synthesized heavy elements, at the end of their lifetime.



Interstellar dust is particularly important component of the ISM, as it absorbs the visible light and reemits it thermally in the infrared. Certain interstellar regions are totally opaque to visible photons and can only be probed by their infrared emission. In addition, dust is an important agent of the gas heating, by photoelectric effect. It is also the catalyst where dihydrogen, the most abundant molecule in the Universe, forms. Yet, interstellar grain properties (composition, abundance, size distribution, etc.) are still poorly known. This uncertainty thus impedes our understanding of the physics of the ISM, and by extension, of galaxy evolution.



This thesis project aims to progress in our understanding of grain properties, focusing on the variation of these properties in the nearby Universe. Dust indeed evolves in the ISM, and this evolution depends on local conditions (gas density, UV field, etc.). This evolution can be studied in an empirical way, modeling multiwavelength observations, spatially-resolved at the scale of a few hundred parsecs, in nearby galaxies.



Our group has a renowned expertise in this area. We are currently leading a large millimeter observation program of these objects, with the instrument NIKA2 (https://irfu.cea.fr/dap/Phocea/Vie_des_labos/Ast/ast_visu.php?id_ast=4644). We have recently been instrumental in modeling spectral energy distributions for the large European collaboration, DustPedia (http://dustpedia.astro.noa.gr/?AspxAutoDetectCookieSupport=1). We have also developed a unique spectral energy distribution fitting code, implementing a hierarchical Bayesian method, HerBIE (Galliano, 2018 ; https://ui.adsabs.harvard.edu/abs/2018MNRAS.476.1445G/abstract). This code allowed us to estimate the characteristic timescales of cosmic dust evolution (https://irfu.cea.fr/dap/Phocea/Vie_des_labos/Ast/ast.php?t=fait_marquant&id_ast=4929).



The thesis work will have two parts. The first task will consist in developing the HerBIE code, implementing the emission from stellar populations. Besides, this modeling will need to be executed self-consistently with the chemical evolution computation. We will also need to model the radiative transfer through the ISM, in a large number of possible topologies. The second part will consist in applying this code to multiwavelength observations of nearby galaxies, in order to demonstrate trends between grain properties and the physical conditions of the ISM, and constrain dust evolution processes.

X-ray binaries (or microquasars) represent excellent laboratories for testing physical phenomena in the most extreme environments. Composed of a compact star (black hole or neutron star) accreting matter from a companion star, they have been observed for several years at various wavelengths, allowing to characterize a set of complex activities. A very varied physics is thus opened to the modelers.



The goal of this thesis will be to study the activities of binary black holes recently discovered in our Galaxy. The main objective is to understand the connections between accretion and ejection processes, but more particularly to constrain the energy of their relativistic jets from calorimetric measurements made during their interactions with the surrounding medium. The modeling of such interactions brings new constraints on the energy balance of black holes, information that is essential for the understanding of these systems.

The aim of this thesis is to explore the Universe at low surface brightness on a large sample of galaxies in the nearest cosmic filaments (z<0.1) by exploiting the unique imaging and spectroscopy capabilities of Euclid, a mission that will be launched in 2023 by the European Space Agency and in which France is heavily involved. Cosmological simulations and theoretical models predict a morphological segregation of galaxies at the edge and within the large filaments that form the cosmic web, as well as notable correlations between their orientation and that of the large cosmic filaments. This segregation results from tidal effects via i) the infall of galaxies towards the gravitational potential wells caused by the dark matter of the filaments and ii) the flow of galaxies within these same filaments. To date, this signature has not been explored in depth in the nearby Universe because of a lack of an adequate large imaging+spectroscopic survey. However, understanding these intrinsic alignments of galaxies has a twofold interest, not only to understand the formation and evolution of galaxies as such (nature vs nurture) but also because these alignments contaminate the cosmic shear signal which is measured from coherent deformations of the shapes of galaxies. Thus, the Euclid space telescope will constrain the evolution models of the Universe while providing evidence on the intrinsic alignments of galaxies which affect at higher redshift the measurements of gravitational shear at the heart of the Euclid’s core science on the question of dark energy. Recent advances in the exploration of the outer regions of galaxies indicate that the morphology inferred from the central, bright, section generally does not reflect the one derived from the outermost regions related to the halo. These outer regions are a tracer of the evolution over the last billion years during which galaxies have evolved within, or near, a large structure, the interactions between galaxies in the filament representing a nuisance factor. The nearby Universe is the ideal laboratory because the cosmological extinction still has little effect on the measurement of the faint brightness of the external regions of the galaxies. Euclid will provide a unique sample of galaxies whose membership in filaments will be guaranteed by spectroscopy while its imaging capabilities at low surface brightness from the optical to the near-infrared (a whole new observation window ideal for capturing the old stellar population from the outer regions) will reveal the morphology at the true edges of the galaxies. This project is a rare case in the Euclid legacy science making a critical use of all three instrumental modes, exploiting the VIS, Y, H, J imaging and the spectroscopy. Before focusing this study on the Euclid data which will arrive in 2024, the first part of the thesis will consist in a pilot study based on data from the CFIS-UNIONS northern sky survey carried out with the MegaCam camera on the Canada-France-Hawaii Telescope. This ground-based survey spawned from the need of securing photometric data to derive photometric redshifts for Euclid but it also carries a diverse galactic and extragalactic science that stands on its own merit. The pilot study will exploit this deep optical dataset optimized for low surface brightness studies in particular, over an area that covers the nearest dense filament, the Pisces-Perseus supercluster at z=0.02 covering 80 degrees (!) in the sky. The full imaging dataset has now been acquired over this region of interest and is ready for analysis in the context of a thesis effort. Spectroscopic redshifts are available for nearly two thousand galaxies from the Sloan surveys and the HI Arecibo Legacy Fast ALFA Survey. The CFIS-UNIONS survey, just as Euclid will, also probes the large voids around the filament, thus completing the comparison of morphology segregation of galaxies directly linked to the filament.

The scientific objective of the Franco-Chinese SVOM satellite, whose launch is scheduled for the end of 2023 in phase with the start of the proposed thesis, is the study of gamma-ray bursts (GRBs), ephemeral astrophysical events in the sky, releasing tremendous energy as bursts of gamma rays from a point in the sky. Previous missions focused on the detection of short GRBs with average durations of 1 s (resp. long GRBS, emitting for around 10 s), associated with the coalescence of compact objects such as neutron stars (resp. supernovae of very massive stars). Very promising targets for SVOM are the class of ultra-long GRBs (with a gamma emission up to 10000 s and whose origin remains an enigma), and GRBs at cosmological distances (with a high redshift, and therefore detectable in the low-energy range). The ECLAIRs telescope on board SVOM is particularly well suited to observing this type of burst, thanks to its detection threshold as low as 4 keV and thanks to the gamma-ray burst "trigger", the scientific software on board ECLAIRs, developed at CEA. This trigger detects and localizes gamma-ray bursts on board the satellite, requests its repointing for follow-up observations with the other instruments on board, and alerts the ground community of the occurrence of the event. The thesis proposes the study of the first gamma-ray bursts observed by the ECLAIRs instrument operational in flight, in particular the long and ultra-long duration bursts and the bursts rich in X-rays. The thesis candidate will take part in the observation campaigns and analyze the data produced by the instrument. In particular, the understanding of the background noise observed at low energy, including the known sources referenced in an onboard catalog, and the scientific analysis of the operation of the trigger on time scales ranging from 20 s to 20 min are crucial to increase the number of triggers on these innovative sources.

Blind and semi-blind unmixing problems are classical inverse problems and ubiquitous in a very wide range of scientific domains from sound processing, medical signal processing to remote sensing or astrophysics. In these domains, the fast development of high resolution/high sensitivity multispectral sensors mandates the development of dedicated analysis tools. For such type of data, the observations can be modelled as the linear or non-linear combination of various elementary physical components, which are to be retrieved by the astrophysicist. However state-of-art methods suffer two major bottlenecks when facing real-world applications: i) the ability to retrieve physically interpretable solutions, ii) their high computational cost, which largely limit their applicability. To that end, the objective of this work is to investigate new approaches, based on machine learning, to tackle blind and semi-blind (when one has access to no or only partial knowledge about the components to be restored) unmixing problem. More precisely, we introduced a novel algorithm based on unrolling techniques to tackle supervised unmixing. We showed that unrolling techniques allow to account for physics-driven information to unmix the data, leading to more physically relevant solutions at a very low computational cost. The goal is then to generalize this prior work to the more challenging blind/semi-blind cases, which will require both revisiting the model architecture as well as optimisation. The results will be tested and validated with astrophysical X-ray data (e.g. Chandra) as well as gravitational wave simulations in preparation for LISA.

The LISA space observatory, scheduled for launch in 2035, will consist of three satellites 2.5 million kilometers apart and will allow the direct detection of gravitational waves undetectable by terrestrial interferometers, opening a new window of observations in astrophysics. In order to maximize the scientific potential of such a mission, the analysis of the data will involve several steps, one of which is the rapid analysis pipeline, whose role is the detection of new events, as well as the characterization of events. Beyond the interest for LISA, this low latency analysis pipeline plays a key role for the fast follow-up of events detected by electromagnetic observations (ground or space observatories, from radio waves to Gamma rays). If fast analysis methods have been developed for ground-based interferometers, the case of space-based interferometers such as LISA remains a field to explore. Thus, an adapted data processing will have to take into account the mode of transmission of the data by packet, thus requiring the detection of events from incomplete data, marred by artifacts. These methods will have to allow the detection and characterization of events as diverse as black hole mergers, EMRIs (extreme mass ratio inspirals), bursts and binaries of compact objects. All this must be done in real time. To this end, this thesis will aim at generalizing classical methods, based on matched filtering, to the analysis of LISA data and at developing a new approach based on machine learning for the detection and early characterization of black hole mergers. These methods will be done in the framework of the LISA consortium and will contribute to the development of a fast analysis pipeline in France.

In order to increase performances of future particle accelerators, high field superconducting electromagnets (higher than 16 T), based on REBCO materials, are being studied. The LEAS at CEA Paris-Saclay already built two dipoles based on these materials through the two major projects EuCARD and EuCARD2. From these magnets we understood that we have to improve the design and protection scheme for such magnets in order to avoid a local damage during resistive transition (from the superconducting state to the resistive state). Indeed during such transition the heat dissipated locally is very high and led to irreversible damages. A possibility is to reuse the technology developed for UHF (Ultra High Field) solenoid like our recent NOUGAT magnet which reached a world record of 32.5 T in 2019 (14.5 T coming from the HTS part and 18 T from a resistive magnet). This MI (Metal-as-Insulation) winding technique allows the current to automatically bypass any local defect and the quench at 32.5 T confirmed that it is efficient to avoid local burning, even at very high Field/current. The PhD candidate will adapt the existing numerical models to the dipole magnets in order to find the best set of windings parameters which allows a good protection and a good magnetic field generation for the purpose of the accelerator magnets. The PhD student will also participate to the design, the fabrication and the tests of one or more prototypes but also needed technological developments.

Positron emission tomography (PET) is a medical nuclear imaging technique widely used in oncology and neurobiology. The decay of the radioactive tracer emits positrons, which annihilate into two back-to-back photons of 511 keV. These pairs of photons are detected in coincidence and used to reconstruct the distribution of the tracer activity in the patient's body.

In this thesis, we propose to contribute to the development of the cutting-edge patented technology ClearMind. The first prototype is currently being tested in the laboratory. The proposed detector uses a monolithic lead tungsten crystal in which Cherenkov and scintillation photons are produced. Those photons are converted to electrons by the photo-electric layer and multiplied in a microchannel plate. The induced electrical signals are amplified by gigahertz amplifiers and digitized by the fast acquisition modules SAMPIC. The opposite surface of the crystal will be equipped with a matrix of the silicon photo-multiplier. Machine-learning techniques will be applied for processing the complex acquired signals in order to reconstruct the time and coordinates of the gamma-conversion in the crystal.

The candidate will work on the development of high-efficient machine learning algorithm for the reconstruction of the gamma-conversion vertex in the monolithic crystal. In particular, this work consists in the evolution and improvement of the existing Geant4 detector simulation for its adjustment to the prototype performances as measured in the laboratory. This simulator will be used to feed a training database for the development and optimization of deep neural network algorithms for the efficient reconstruction of the gamma-interaction using the full signal shape and/or the pre-processed data (aka features engineering). The reconstruction performances of these algorithm will be assessed on real test data acquired with the available ClearMind prototype. Special attention will be made on the development of compact, efficient and fast networks together with a robust uncertainty estimation of the reconstructed parameters in the context of trustworthy AI. The possibility of embedding these algorithms in FPGAs for fast on-line reconstruction will be studied.

In 1975, the CEA identified isotopic anomalies in uranium ore at the Oklo site in Gabon. These anomalies were quickly attributed to the presence of natural nuclear reactors that had been in operation for about 2 billion years.

Independently, and more recently, a team of researchers led by A. El Albani discovered in the vicinity of the Oklo site fossils of living organisms also dated to about 2 billion years ago. The size, structure, and demonstration of motility of the associated organisms seem to point to a eukaryotic type of cellular organization.

This doctoral work proposes to establish a link between the two events, by postulating that the uraniferous environment pre-existing the reactors, or the reactors themselves, could have contributed to the appearance of these organisms or their ascending filiation by a mechanism of stimulation of the genetic mutations induced by the ionizing radiations of this environment (dosimetric environment).

Recent developments in quantum computing open opportunities to explore alternative methods for tackling complex problems that could not be solved using classical computers. An accurate solution of nuclear physics many-body problems typically requires sophisticated numerical codes and considerable computational resources. In this context, the proposed PhD thesis aims to explore the use of Rydberg atoms quantum processors for the description of many-fermion systems, with a focus on nuclear physics.



On the one hand, quantum computing algorithms tailored to treat fermions interacting with each other are being developed [1]. While implementing such algorithms on quantum emulators is becoming standard, testing their effectiveness on real quantum machines remains an open issue.



On the other hand, quantum processing units (QPUs) based on manipulating neutral atoms have been put forward as one of the most promising technologies for quantum simulations [2]. Such machines are now becoming available for fundamental research applications, thus offering the opportunity to perform real calculations and gauge the perspectives of so-called Noisy Intermediate Scale Quantum (NISQ) technology.



This project aims to combine these state-of-the-art developments with the following goals:



(a) Implement existing or new quantum algorithms on the neutral atoms QPUs, exploring the different possibilities offered by the machine and respecting/exploiting the specificities of nuclear systems (e.g., the use of symmetry breaking and restoration, understanding entanglement and phase-transition in the description of many-body states [3]).



(b) Demonstrate the usefulness of the Rydberg atoms quantum processor for solving real-world many-body physics problems. This will involve selecting a set of pilot applications that represent typical problems in the field and applying the developed algorithms to solve them. The student will need to compare the results obtained using the quantum computer with those obtained using traditional approaches to validate the effectiveness of the proposed methods.



[1] T. Ayral et al., arXiv :2303.04850 (2023).

[2] L. Henriet et al., Quantum 4, 327 (2020).

[3] D. Lacroix et al, Eur. Phys. J. A 59:3 (2023).

It is proposed to study the salient effects of coupling between discrete and continuous states near various particle emission thresholds using the shell model in the complex energy plane. This model provides the unitary formulation of a standard shell model within the framework of the open quantum system for the description of well bound, weakly bound and unbound nuclear states.

Recent studies have demonstrated the importance of the residual correlation energy of coupling to the states of the continuum for the understanding of eigenstates, their energy and decay modes, in the vicinity of the reaction channels. This residual energy has not yet been studied in detail. The studies of this thesis will deepen our understanding of the structural effects induced by coupling to the continuum and will provide support for experimental studies at GANIL and elsewhere.

Plutonium is produced in reactors using uranium fuel. Plutonium-239 is produced by neutron capture on uranium-238 then double ß decay (U-238(n,γ)U-239 --> Np-239 --> Pu-239). Plutonium-240 and plutonium-241 are produced by successive captures on plutonium-239. At the end of the cycle, when the fuel is spent, the fissile isotopes of plutonium (Pu-239 and Pu-241) contribute significantly to energy production. In the case of an innovative reactor using plutonium fuel, the contribution of Pu-241 is important from the beginning of the cycle. Pu-241 is not well known because of the difficulties inherent to its study, on the one hand because of its short half-life (~14 years) and on the other hand because of its decay into Am-241 whose very high capture cross-section disturbs the measurement. The Nuclear Energy Agency therefore recommends to improve the accuracy of the capture and fission cross sections of Pu-241.

The capture cross section of Pu-241 is about 4 times lower than the fission cross section in the energy range of interest. In order to realize an accurate measurement of its capture it is thus necessary to develop a device for the detection of gammas and identification of those coming from the fission. In the proposed experiment at the CERN neutron source n_TOF, gammas from the (n,γ) and (n,f) reactions are detected by a 4pi calorimeter (TAC - Total Absorption Calorimeter) while fission events are identified by a fission chamber (CaF) containing Pu-241 samples placed in the center of the TAC. This measurement will significantly improve the accuracy of the Pu-241 capture cross section while providing additional information on the fission reaction (prompt gammas and cross section).

The PhD student will contribute to the preparation and the realization of the experiment, as well as to the analysis of the results: simulations of the device (TAC + CaF + Pu samples), specification of the samples characteristics, development and tests of the fission chamber, implementation and tuning of the device at n_TOF, data taking, data reduction and analysis of the fission data (prompt gammas and effective cross section).

The structure of doubly magic nuclei is a milestone of the nuclear structure studies. In this project, we propose to revisit the structure of one of them, 56Ni, by a new probe which is the production of this exotic nucleus by fast neutrons delivered by the GANIL/SPIRAL2/NFS facility. The structure of the nucleus will be studied by high resolution gamma spectroscopy using the EXOGAM detector. This work will be pioneering for nuclear structure studies combining for the first time high energy neutrons, a high efficiency gamma spectrometer and (n,Xn) reactions. Such new spectroscopic information’s are also relevant for nuclear reaction mechanism formalism (like TALYS) and nuclear data evaluation in the framework of the CEA-CNRS network NACRE supported by the multipartner research program NEEDS (https://needs.in2p3.fr). This is an experimental project that will allow the student to acquire advanced skills in gamma spectroscopy and neutrons physics. Finally, the student will be in charge of applying for the first time at GANIL, a FAIR approach (https://www.panosc.eu/data/fair-principles/) of the large volumes of data that will be recorded. Data collection is scheduled for fall 2023.

# Overview



Heavy-ion collisions are the golden system to study the quark-gluon plasma (QGP), an exotic state of matter that presumably existed few microseconds after the Big Bang. Among the probes to study the QGP, the production of hadrons containing charm quark (e.g D0 mesons) is one of the historical smoking guns. Indeed, being produced in the very first stages of the collisions, these particles keep track of their subsequent interactions with the QGP. In particular, the study of simultaneous double charm production, never carried out in ion-ion collisions at the LHC, could shed a light on the transport properties of the QGP.



At the end of 2023, the LHCb collaboration will record high energy Pb-Pb collisions at the LHC. These data will benefit from the latest LHCb upgrades which offer enhanced detector capabilities. Notably, the new tracking system of this heavy-quark dedicated detector will allow us to cope with the high detector occupancy in ion-ion collisions. This combined with an increase of the data sample by a factor of two will provide the best Pb-Pb data recorded by the collaboration so far.



Finally, a new upgrade phase of the detector is scheduled for 2030 for which new tracker projects are developed. Among the new detectors, the Upstream Tracker (UT) is the key to reduce the rate of fake tracks reconstructed in head-on ion-ion collisions and ensure good data quality for LHCb heavy-ion program. The development of the UT has started, but the final design is still to be defined. In addition, sophisticated tracking algorithms based on machine learning could be developed to fully exploit the detector future capabilities. Not only this work could improve further the LHCb tracking efficiency in ion data, but also in regular proton-proton collisions, which is at the heart of the collaboration physics program.



# Research project



The PhD research project is divided in two main parts:

- Study of the double charm cross-section in Pb-Pb data with LHCb: this objective implies the participation to the 2023 ion data-taking, the extraction of the signal in these data, and the study of uncertainties using simulations. The study could follow what was done previously by the collaboration with other data sets, but more specific studies to ion-ion collisions will be done in close relationship with the theory community.

- Participation to the UT project: this objective focuses on the development of a new tracking algorithm. In particular, the usage of the Graph Neural Network to the LHCb tracking strategy will be explored to improve both efficiency and computing performance.

The study of so-called 'deformed' atomic nuclei with a non-spherical charge distribution is essential for testing nuclear interactions and structure models. Almost all nuclei have an intrinsic prolate (elongated) shape and very few are oblate (flattened). A very small number of nuclei exhibit coexistence of shapes (e.g. prolate-oblate), a phenomenon allowed by the quantum nature of the atomic nucleus. One of the research themes of the nucleus structure group of the DPhN (Departement de Physique Nucléaire) is to search for these nuclei within the Segrè map in order to study them and characterise their shape.



Experiments will be performed with the European germanium detector array AGATA. The unprecedented efficiency and resolution of this new detector will permit spectroscopic studies further away from the valley of stability than previously possible. The Nuclear Structure Group is strongly involved in the exploitation of AGATA with a particular focus on the study of shape coexistence.

Hadronic matter is composed of fundamental particles called partons (quarks and gluons) and their interactions are described by Quantum Chromodynamics (QCD). Understanding and describing hadronic internal structure is one of the key challenges of nuclear physics. Despite a good description of the dynamics of quarks and gluons at high energy, several elementary hadronic properties (such as mass and spin) cannot be explained through their components with QCD calculations. Phenomenological approaches are therefore required as a theoretical framework to interpret experimental observations. These are the Generalized Parton Distributions (GPDs) and Transverse Momentum distributions (TMDs). These functions offer a 3D description of the nucleon as they give access to the spatial and momentum distributions of quarks and gluons and to parton contribution to the spin of the nucleon.

These distributions are experimentally accessible through electron scattering off proton and neutron (at the CEBAF accelerator in Jefferson Laboratory and the COMPASS experiment at CERN). One can also study it in polarized proton collisions with the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory.

The new detector sPHENIX is being assembled at RHIC and first collisions are scheduled for Spring 2023. About 350 researchers work around this 1000-ton apparatus. The physics program aims at understanding matter and the strong interaction. It covers both heavy-ion physics and matter deconfinement and the study of nucleon internal structure. Data taking will take place between 2023 and 2025 with pp, p-Au and Au-Au collisions at vsnn=200GeV.

The student will be involved in data taking and data analysis of the sPHENIX experiment. The main goal is the study of transverse momentum distributions of partons inside the proton. These results will contribute to deepen our understanding of nucleon structure and parton confinement.



In 2030 a new collider will be operational at RHIC: the electron-ion collider (EIC). The facility should give answers some of the most fundamental questions in nuclear physics. It will give access to a largely unexplored area: the limit of saturation for gluon density and will provide remarkable conditions to study the structure of the nucleon and the effect of a nuclear environment on the dynamics of quarks and gluons.

The CEA is involved in physics simulations and the development of innovative gaseous detectors. Part of the thesis will be dedicated to the study of several prototypes.



The thesis will be hosted by the Laboratory of the Nucleon Structure (Laboratoire de Structure du Nucleon, LSN) composed of several physicists, both some theoreticians and some experimentalists.



The student is expected to be fluent in English to work in the context of a large international scientific collaboration. He/she will have to show interest in detector hardware and software programming (C++).

Several trips should be anticipated, in particular to the United States.

The nuclear two-photon, or double-gamma decay is a rare decay mode in atomic nuclei whereby a nucleus in an excited state emits two gamma rays simultaneously. Even-even nuclei with a first excited 0+ state are favorable cases to search for a double-gamma decay branch, since the emission of a single gamma ray is strictly forbidden for 0+ ? 0+ transitions by angular momentum conservation. The double-gamma decay still remains a very small decay branch (<1E-4) competing with the dominant (first-order) decay modes of atomic internal-conversion electrons (ICE) or internal positron-electron (e+-e-) pair creation (IPC). Therefore we will make use of a new technique to search for the double-gamma decay in bare (fully-stripped) ions, which are available at the GSI facility in Darmstadt, Germany. The basic idea of our experiment is to produce, select and store exotic nuclei in their excited 0+ state in the GSI storage ring (ESR). For neutral atoms the excited 0+ state is a rather short-lived isomeric state with a lifetime of the order of a few tens to hundreds of nanoseconds. At relativistic energies available at GSI, however, all ions are fully stripped of their atomic electrons and decay by ICE emission is hence not possible. If the state of interest is located below the pair creation threshold the IPC process is not possible either. Consequently, bare nuclei are trapped in a long-lived isomeric state, which can only decay by double-gamma emission to the ground state. The decay of the isomers would be identified by so-called time-resolved Schottky Mass Spectroscopy. This method allows to distinguish the isomer and the ground state state by their (very slightly) different revolution time in the ESR, and to observe the disappearance of the isomer peak in the mass spectrum with a characteristic decay time. After a first successful experiment establishing the double-gamma decay in 72Ge a new experiment has been accepted by the GSI Programme Committee and its realization is planned for early 2024.

The study of exotic nuclear matter is at the forefront of modern subatomic physics. Atomic physics techniques - more specifically, optical measurements of the atomic structure - readily yields fundamental and model-independent data on the structure of ground and isomeric nuclear states. The competition and balance between nuclear shell and collective effects results in a range of shapes and sizes within nuclear systems. Such shapes and structures perturb the atomic energy levels of atoms and ions at the ppm level that it is readily probed and measured by laser spectroscopic methods, i.e. in-source laser spectroscopy at Jyväskylä and in-gas jet laser spectroscopy at S3-LEB. These techniques are suitable for the study of short-lived radionuclides with lifetimes of only few milliseconds, and production rates often only a few hundred isotopes/isomers per second. Both laboratories permit access to regions of the nuclear chart which are currently either inaccessible to the majority of facilities, or are challenging to probe spectroscopically due to the complexity of the atomic structure. One such region lies between the refractory systems of Zr (Z=40) which exhibit changes in nuclear shape, and the more single-particle-dominated region around Sn (Z=50). This transitional region serves up a landscape of shape transitions, shape coexistence and triaxiality.

The fission process is a violent reaction in which a heavy nucleus is split in two fission fragments. More than 300 different isotopes are produced from one fissioning system and their relative production is strongly determined by the nuclear structure along with the nuclear dynamics that drives the system from an initial state to the final break-up through different states of deformation. Nowadays, the experimental access to th identification of fission fragments is still very challenging and this prevents a complete understanding of the fission process. The VAMOS++ spectrometer offers the opportunity to identify fission fragments in terms of mass, nuclear charge and velocity vector. The subject of this PhD is the study of the fission process of minor actinides produced in inverse kinematics. This experiment will benefit from the combined setup of the VAMOS++ spectrometer and the new PISTA silicon telescope in order to measure simultaneously both mechanism, the production and decay through fission of actinides around Thorium. The objective of this PhD is the determination of isotopic fission-fragment yields and the scission configuration of exotic actinides.

Ten years ago, the ATLAS and CMS experiments at LHC at CERN discovered a new boson, with a dataset of proton-proton collisions of about 10 fb-1 at the centre of mass energy of 7 to 8 TeV [1,2]. Since then, the properties of this particle have been tested by both experiments and are compatible with the Higgs boson properties predicted by the Standard Model of particle physics (SM) within the uncertainties. In absence of direct probes of New Physics, increasing the accuracy of the measurements of the properties of the Higgs boson (its spin, its parity and its couplings to other particles) remains one of the most promising path to pursue.

The measurement of the ttH production allows the direct access to the top quark Yukawa coupling, fundamental parameter of the SM. ttH production is a rare process, two orders of magnitude smaller than the dominant Higgs boson production by gluon fusion. This production mode has been observed for the first time in 2018 [3, 4] separately by the CMS and ATLAS experiments, by combining several decay channels. More recently, with the full Run 2 dataset (data recorded between 2016 and 2018, with a total of 138 fb-1 at 13 TeV), this production mode was observed also using solely the diphoton decay channel, and a first measurement of its CP properties was provided again by both experiments, with the exclusion of a pure CP odd state at 3s [5, 6]. The associated production with a single top quark is about 5 times smaller than the ttH production and has never been observed. Thanks to the searches in the diphoton and multilepton channel, very loose constraints on this production modes were set for the first time recently (see Ref. [7]). This production mode is very sensitive to the H-tt coupling CP properties, since in case of CP-odd coupling, its production rate is largely increased. We propose in this thesis to study jointly the two production modes (ttH and tH) and the H-tt coupling CP properties with Run 3 data (data being recorded now and until 2026, with potentially about 250 fb-1 at 13.6 TeV) in the diphoton decay channel. If there was some CP violation in the Higgs sector, excluding small pseudo-scalar contributions will require more data. Pursuing these studies with Run 3 and beyond may allow to pinpoint small deviations not yet at reach. We propose to bring several improvements to the Run 2 analysis strategy and to use novel reconstruction and analysis techniques based on deep-learning, developped in the CEA-Saclay group by our current PhD students but not yet used in physics analyses, in order to make the most of the available dataset.

[1] ATLAS Collaboration, “Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC,” Phys. Lett. B 716 (2012) 1.

[2] CMS Collaboration, “Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC,” Phys. Lett. B 716 (2012) 30.

[3] ATLAS Collaboration, “Observation of Higgs boson production in association with a top quark pair at the LHC with the ATLAS detector”, Phys. Lett. B 784 (2018) 173.

[4] CMS Collaboration, “Observation of ttH Production”, Phys. Rev. Lett. 120 (2018) 231801.

[5] CMS Collaboration, “Measurements of ttH Production and the CP Structure of the Yukawa Inter- action between the Higgs Boson and Top Quark in the Diphoton Decay Channel”, Phys. Rev. Lett. 125, 061801.

[6] ATLAS Collaboration, “CP Properties of Higgs Boson Interactions with Top Quarks in the ttH and tH Processes Using H ? ?? with the ATLAS Detector” , Phys. Rev. Lett. 125 (2020) 061802.

[7] CMS Collaboration, “A portrait of the Higgs boson by the CMS experiment ten years after the discovery”, Nature 607 (2022) 60.

Why is the observable Universe today made of matter, without any significant amount of antimatter? Neutrinos shed light on this cosmic mystery.

In 2020, the T2K collaboration in Japan published in the journal Nature new results leading to the best constraint to date on the degree of asymmetry between matter and antimatter. The T2K results exclude for the first time nearly half of the possible values at 99.7% of confidence level and the value most compatible with the data correspond to a maximum asymmetry between matter and antimatter (notably between neutrinos and antineutrinos). T2K has the best world sensitivity for this crucial measurement and is going to collect new data from 2023 with an upgraded detector to search for a possible discovery of symmetry violation.

T2K is a neutrino experiment designed to study the transition of neutrinos and antineutrinos from one flavor to another as they travel (neutrino oscillations). An intense beam of muon neutrinos is generated at the J-PARC site on the East coast of Japan and directed to the Super-Kamiokande neutrino detector in the mountains of western Japan. The beam is measured once before leaving the J-PARC site, using the ND280 near-field detector, and again at Super-Kamiokande: the evolution of the measured intensity and the composition of the beam are used to determine the properties of the neutrinos.

The thesis work will be twofold, including the analysis of the new T2K data for the measurement of the (anti)neutrino oscillations and the commissioning and scientific exploitation of the High-Angle Time Projection Chamber (High-Angle TPC). The objective of this new detector is to improve the performance of the ND280 near detector, to measure the neutrino production and interaction rate so that the uncertainty on the number of events predicted neutrinos at Super-Kamiokande is reduced to about 4%. The student will use cosmic data to align the TPC modules. Then, he will exploit the first data to calibrate the TPC and evaluate its performance.



The student will perform the analysis of the new data which will be collected by T2K to measure the matter-antimatter symmetry violation in neutrino oscillations. The upgrade of the near detector will require to put in place a new analysis strategy. For the first time, the measurement of low momentum protons and neutrons produced by neutrino interactions will be exploited. Another important part of the analysis which must be updated to cope with increased statistics, is the modeling of the flux of neutrinos produced by the accelerator beamline.



A new generation of experiments is expected to multiply by a factor 20 the data production in the next decades. In Japan, the Hyper-K experiment, and in the USA, the DUNE experiment, will be operational around 2027-2028. This thesis work will explore new analysis strategies crucial also for such next-generation experiments. If their new data confirm the preliminary results of T2K, neutrinos could bring before ten years the key to understand the mystery of the disappearance of antimatter in our Universe.

CEA/Irfu staff members are among the principal investigators of ongoing experiments at the Jefferson Lab (JLab) in USA, where a high current electron beam up to 11 GeV in energy collides with fixed targets of several types. The high luminosity available at the JLab allows the study of the properties of the nucleons with high statistical accuracy also via rare processes.

Contrary to the naive expectations, it has been shown that not the valence quarks, but rather the gluons carry the major contribution to the mass and the spin of the nucleons. Therefore, it is crucial to precisely characterize gluons distributions in order to fully understand the strong interactions from which results the protons. In particular, the current knowledge of the GPDs of gluons is rather limited. GPDs are accessible through the study of exclusive processes where all the final state particles are detected, and specifically, gluon GPDs can be accessed via the study of the exclusive electroproduction of the ?-meson. This year, data are being collected with a longitudinally polarized target of protons, providing a unique opportunity to understand the correlation between the spin of the proton and the gluons. The goal of this thesis will be to analyze the data taken with the CLAS12 experiment at the Jefferson Lab to extract target-spin, beam-spin and double-spin asymmetries. The future PhD student will have the opportunity to add a side activity to the data analysis, the choice spanning from detector development to detailed phenomenological studies.

Future experiments on linear colliders (e+e-) with low hadronic background require improvements in the spatial resolution of pixel vertex detectors to the micron range, in order to determine precisely the primary and secondary vertices for particles with a high transverse momentum. This kind of detector is set closest to the interaction point. This will provide the opportunity to make precision lifetime measurements of short-lived charged particles. We need to develop pixels arrays with a pixel dimension below the micron squared. The proposed technologies (DOTPIX: Quantum Dot Pixels) should give a significant advance in particle tracking and vertexing. Although the principle of these new devices has been already been studied in IRFU (see reference), this doctoral work should focus on the study of real devices which should then be fabricated using nanotechnologies in collaboration with other Institutes. This should require the use of simulation codes and the fabrication of test structures. Applications outside basics physics are X ray imaging and optimum resolution sensors for visible light holographic cameras.

The thesis proposes to measure for the first time in a coherent way the different rare processes of production of top quarks in association with bosons, with three charged leptons in the final state at the Large Hadron Collider. The thesis will be based on the analysis of the large set of data collected and being acquired by the ATLAS experiment. The joint analysis of the ttW, ttZ, ttH and 4top processes where one signal is the background of the other will allow for the first time to have complete and unbiased measurements of the final state with three leptons.



These rare processes, recently accessible at the LHC, can probe the models explaining the current anomalies observed in flavor physics. These anomalies could be the first signs of new physics beyond the Standard Model of particle physics. The ttH process also makes possible the direct study of the coupling between the top quark and the Higgs boson, which could provide new sources of matter-antimatter asymmetry. Discovering signs of new physics that go beyond the limitations of the Standard Model and in particular new sources of matter-antimatter asymmetry is a fundamental question in particle physics today.

The Higgs boson discovered at the LHC in 2012 is the cornerstone of the Standard Model (SM). Its properties, such as its mass or spin, are now better and better known. Nevertheless, the total width of the Higgs boson remains a fundamental parameter that is very difficult to measure at the LHC without the support of theoretical assumptions.

In this PhD thesis, we propose to pursue an original approach to measure this parameter, approach only possible in the diphoton decay channel of the Higgs boson. Indeed, in this channel the position of the mass peak depends on the interference between the Higgs boson signal and the background noise. The resulting shift depends on the natural width of the Higgs boson. This is a very small effect in the SM but could be larger when considering Higgs bosons produced at high transverse momentum.

This type of analysis requires a thorough mastery of the various uncertainties related to the experimental apparatus, in particular to the electromagnetic calorimeter (ECAL), and to the reconstruction of the electromagnetic objects. In order to improve the latter, the student will develop a new approach to electromagnetic-object reconstruction based on a technique initiated at CEA-Irfu by the CMS group and using state-of-the-art methods in artificial intelligence (Convolutional NN and Graph NN).

These two aspects will be addressed in parallel during the thesis. The student will be supervised by the CMS group of Irfu whose expertise in the ECAL and in the two-photon Higgs boson decay channel is internationally recognised.

After a very successful operation period crowned with the discovery of the Higgs boson, the Large Hadron Collider (LHC) will undergo a luminosity upgrade where it is planned to increase the collision rate by a factor of ten, resulting with extremely large number of simultaneous collisions. The particle detectors of the LHC will also be upgraded to cope with these challenging environment. Furthermore, with increased granularity and more advanced readout electronics, they are aimed to achieve a better event reconstruction for instance with new high-granularity calorimeters.



In this project we will develop an ultra-low latency; machine learning based electromagnetic or hadronic particle reconstruction algorithm for the collider experiments. This state-of-the-art algorithm will be distributed into large number of high-capacity low-latency components, which will drastically improve the readout efficiency and reconstruction capability of the future collider experiments. This unified algorithm will have significant impact on the ambitious physics program of these colossal detectors. We will demonstrate the impact of this development for Higgs precision measurements, focusing on the analysis of the Higgs self coupling.



Implementing advanced machine learning algorithms in low-level electronics such as Field Programmable Gate Arrays (FPGA) is a newly-emerging exciting field. To accomplish the goals of the project we will be collaborating with other international laboratories and institutes such as CERN, Fermilab, CalTech, with frequent visits to these labs. The successful candidate will be working with the High Level Synthesis tools to optimize the neural networks to their limits. They will need to have basic programming knowledge on C++ and python, and some exposure to readout systems will be a plus.

The neutrinoless double beta decay (0nbb) is a very rare nuclear transition that plays a key role in

(astro)particle physics for the study of the nature of neutrinos and lepton number violation. CUPID is

a proposed next-generation experiment to study the 0nbb. The analysis and control of the radiogenic

background is a major challenge for the experiment. CUPID uses scintillating bolometers operating at

temperatures of a few mK. In this thesis work, prototypes of CUPID will be developed and analysed in

surface and underground laboratories (Gran Sasso Laboratory in Italy and Canfranc in Spain). The

radiogenic background model of CUPID, aiming at evaluating the sensitivity of the experiment to new

physics, will be refined by simulations based on the experimental performance of the prototypes,

using the GEANT-4 package and a boosted-decision-tree analysis. The overall objective of the thesis is

the definition of the final configuration of CUPID, based on both the optimization of the CUPID

modules and the improvement of the radiation background model.

The discovery of the Standard Model Higgs boson in 2012 is undoubtedly a bright success for the Standard Model of particle physics. This discovery however does not bring any answer to many of the questions that are still open in cosmology and particle physics. Among others, there is the nature of drak matter and dark energy, the origin of the Higgs potential, and the fact that the Standard Model does not provide an explanation for the very small masses of the neutrinos. Natural solutions to these problems could come from the existence of new interaction types or new particles.

This is why since the discovery of the Higgs boson efforts are focused on the search for new phenomena, beyond the Standard Model. One of the important aspects of the comparison between experimental measurements and theory is the need to normalize as precisely as possible experimental results to theory. This means in practice being able to measure as precisely as possible the luminosity of the LHC. The goal is to reach a precision better than 1% within the next few years. This is a factor two or three better than the precision that has been reached up to now.



LHC experiments are equipped with dedicated luminosity measurement subsystems, and several observables can be used to measure the luminosity. However, the techniques used have various stability and linearity issues, that complicate their exploitation.



Over LHC run-3, that started during summer 2022, it is planned to increase the luminosity by a factor of about two. To make the best out of this luminosity increase, the calorimeter trigger system has been significantly modified and upgraded. The upgraded trigger system is based on real time analysis of the digitized detector signals. Irfu is one of the key contributors to the design and the production to the necessary hardware elements, as one of the instituts in charge of the design and production of the LTDB (LAr Trigger Digitizing Board, see figure 1), i.e. the board that digitizes the detector analog signals and transmits them to the back-end system.

An essential feature of the upgraded trigger system is its ability to measure the energy deposited in the calorimeter bunch crossing by bunch crossing. Combined with the stability, excellent linearity and response uniformity of the ATLAS Liquid Argon calorimeter, the upgraded trigger system offers the potential to measure the luminosity with excellent linearity and stability performances. Preliminary studies performed on a prototype trigger chain show that the 1% precision level should be reachable.These preliminary studies have been done with standard deconvolution algorithms, based on an a priori knowledge of the signal pulseshape. A very promising improvement would be to use a neural net.



An other feature of the upgraded trigger system is its ability to keep track of all the interactions taken place in the detector over a much longer period of time than the main readout. The main readout system is able to keep in memory only up to four or five consecutive interactions. The trigger system has the capability to keep track of each individual bunch crossing over a period of time corresponding to several tens of consecutive bunch crossings. The spatial granularity of the information is however somewhat coarser than the granularity available to the main readout. This long term memory feature gives the possibility to compensate real time the effect of charge space accumulation, which will be crucial for data taken after 2025, at very high luminosity. More importantly, this also opens up the possibility to detect particles reaching the detector long (several tens or even hundreds of ns, to be compared to the 25 ns between two consecutive bunch crossings) after their production. Such particles are slow and very heavy, and can be detected almost up to the kinematic limit of 7 TeV. This is significantly higher than the limits reachable by more classic techniques. Such particles typically appear in many classes of supersymmetric models.

The GBAR experiment at CERN aims at measuring the gravitational acceleration of antimatter on Earth using ultra-cold antihydrogen atoms. In order to obtain these ultra-cold anti-atoms, the key is to first produce positive antihydrogen ions (two positrons and one antiproton, equivalent to H–), using positronium (bound state of an electron and a positron) for that purpose.

The PhD topic is dedicated to the study of the charge exchange reaction between an antihydrogen atom and a positronium atom, producing a positive antihydrogen ion. The first objective is to measure the cross sections for this reaction, for which only theoretical values exist, using hydrogen instead of antihydrogen and producing H–. The second objective is to observe the production of antihydrogen ions and optimise it. An experimental measurement of the cross sections will provide a test for several low-energy atomic collision models that currently provide disagreeing theoretical values. The first ever detection of an antihydrogen ion will be a major milestone for GBAR and will also open new opportunities for future antimatter experiments. Finally, an application of the measured cross sections to the positron annihilation in the interstellar medium will be explored.

From 2023 to 2025, GBAR will receive beams of antiprotons and H– and the experimental program of this thesis will be carried out during this period at CERN. 2026 will mainly be dedicated to finalising the data analysis and PhD dissertation writing.

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Résultats de traduction

Résultat de traduction

State of the art: Many studies have been devoted to the digital revolution, to the changes it induces in uses, in work, production, consumption, relationships with others, knowledge, information… But very little on the concept of frugality as such, considered both from a technical point of view (how to design and achieve it) and from a social and economic point of view (what effects will it on the "practice" of digital?)



Scientific question to address: How is frugality conceived and practiced by researchers who are interested in it or claim it?



Methodology and study techniques: the first step will be to carry out a survey on the way in which frugality is thought about and integrated into the work of researchers, in particular by looking at the work of other CEA PhD students who are working on this subject.

Then (or in parallel), to question the human and social sciences in order to determine the effects that frugality, presented as a necessity with regard to the energy transition, could induce on behaviors and uses: "rebound effect", link with sobriety, etc. It will be a question in particular of answering this question: does frugality encourage sobriety or on the contrary allow to free oneself from it? What effect will it have on energy consumption?

Finally, it will be a question of proposing scenarios for the future, for example by examining various foresight works and science fiction stories.



Expected results: An original thesis in that it will combine technical analyses, sociological analyzes and forecasting.