Mesure de la réponse intra-pixel de détecteur infrarouge à base de HgCdTe avec des rayons X pour l’astrophysique

In the field of infrared astrophysics, the most commonly used photon sensors are detector arrays based on the HgCdTe absorbing material. The manufacturing of such detectors is a globally recognized expertise of CEA/Leti in Grenoble. As for the Astrophysics Department (DAp) of CEA/IRFU, it holds renowned expertise in the characterization of this type of detector. A key characteristic is the pixel spatial response (PSR), which describes the response of an individual pixel in the array to the point-like generation of carriers within the absorbing material at various locations inside the pixel. Today, this detector characteristic has become a critical parameter for instrument performance. It is particularly crucial in applications such as measuring galaxy distortion or conducting high-precision astrometry. Various methods exist to measure this quantity, including the projection of point light sources and interferometric techniques. These methods, however, are complex to implement, especially at the cryogenic operating temperatures of the detectors.
At the DAp, we propose a new method based on the use of X-ray photons to measure the PSR of infrared detectors. By interacting with the HgCdTe material, the X-ray photon generates carriers locally. These carriers then diffuse before being collected. The goal is to derive the PSR by analyzing the resulting images. We suggest a two-pronged approach that integrates both experimental methods and simulations. Data analysis methods will also be developed. Thus, the ultimate objective of this thesis is to develop a new, robust, elegant, and fast method for measuring the intra-pixel response of infrared detectors for space instrumentation. The student will be based at the DAp. This work also involves collaboration with CEA/Leti, combining the instrumental expertise of the DAp with the technological knowledge of CEA/Leti.

First observations of the TeV gamma-ray sky with the NectarCAM camera for the CTA observatory

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 2026. 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 shipped on site in 2025. 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).

ARTIFICIAL INTELLIGENCE TO SIMULATE BIG DATA AND SEARCH FOR THE HIGGS BOSON DECAY TO A PAIR OF MUONS WITH THE ATLAS EXPERIMENT AT THE LARGE HADRON COLLIDER

There is growing interest in new artificial intelligence techniques to manage the massive volume of data collected by particle physics experiments, particularly at the LHC collider. This thesis proposes to study these new techniques for simulating the rare-event background from the two-muon decay of the Higgs boson, as well as to implement a new artificial intelligence method for simulating the response of the muon spectrometer detector resolution, which is crucial for this analysis.

SEARCH FOR DIFFUSE EMISSIONS AND SEARCHES IN VERY-HIGH-ENERGY GAMMA RAYS AND FUNDAMENTAL PHYSICS WITH H.E.S.S. AND CTAO

Observations in very-high-energy (VHE, E>100 GeV) gamma rays are crucial for understanding the most violent non-thermal phenomena at work in the Universe. The central region of the Milky Way is a complex region active in VHE gamma rays. Among the VHE gamma sources are the supermassive black hole Sagittarius A* at the heart of the Galaxy, supernova remnants and even star formation regions. The Galactic Center (GC) houses a cosmic ray accelerator up to energies of PeV, diffuse emissions from GeV to TeV including the “Galactic Center Excess” (GCE) whose origin is still unknown, potential variable sources at TeV, as well as possible populations of sources not yet resolved (millisecond pulsars, intermediate mass black holes). The GC should be the brightest source of annihilations of massive dark matter particles of the WIMPs type. Lighter dark matter candidates, axion-like particles (ALP), could convert into photons, and vice versa, in magnetic fields leaving an oscillation imprint in the gamma-ray spectra of active galactic nuclei (AGN).
The H.E.S.S. observatory located in Namibia is composed of five atmospheric Cherenkov effect imaging telescopes. It is designed to detect gamma rays from a few tens of GeV to several tens of TeV. The Galactic Center region is observed by H.E.S.S. for twenty years. These observations made it possible to detect the first Galactic Pevatron and place the strongest constraints to date on the annihilation cross section of dark matter particles in the TeV mass range. The future CTA observatory will be deployed on two sites, one in La Palma and the other in Chile. The latter composed of more than 50 telescopes will provide an unprecedented scan of the region on the Galactic Center.
The proposed work will focus on the analysis and interpretation of H.E.S.S observations. carried out in the Galactic Center region for the search for diffuse emissions (populations of unresolved sources, massive dark matter) as well as observations carried out towards a selection of active galactic nuclei for the search for ALPs constituting dark matter. These new analysis frameworks will be implemented for the future CTA analyses. Involvement in taking H.E.S.S. data. is expected.

STUDY OF THE MULTI-SCALE VARIABILITY OF THE VERY HIGH ENERGY GAMMA-RAY SKY

Very high energy gamma ray astronomy observes the sky above a few tens of GeV. This emerging field of astronomy has been in constant expansion since the early 1990s, in particular since the commissioning of the H.E.S.S. array in 2004 in Namibia. IRFU/CEA-Paris Saclay is a particularly active member of this collaboration from the start. It is also involved in the preparation of the future CTAO observatory (Cherenkov Telescope Array Observatory), which is now being installed. The detection of gamma rays above a few tens of GeV makes it possible to study the processes of charged particles acceleration within objects as diverse as supernova remnants or active galactic nuclei. Through this, H.E.S.S. aims in particular at answering the century-old question of the origin of cosmic rays.
H.E.S.S. allows measuring the direction, the energy and the arrival time of each detected photon. The time measurement makes it possible to highlight sources which present significant temporal or periodic flux variations. The study of these variable
Direction de la Recherche Fondamentale
Institut de recherche
sur les lois fondamentales de l’univers

emissions (transient or periodic), either towards the Galactic Center or active nuclei of galaxies (AGN) at cosmological distance allows for a better understanding of the emission processes at work in these sources. It also helps characterizing the medium in which the photons propagate and testing the validity of some fundamental physical laws such as Lorentz invariance. It is possible to probe a wide range of time scales variations in the flux of astrophysical sources. These time scales range from a few seconds (gamma ray bursts, primordial black holes) to a few years (binary systems of high mass, active galaxy nuclei).
One of the major successes of H.E.S.S.'s two decades of data-taking. was to conduct surveys of the galactic and extragalactic skies in the very-high energy range. These surveys combine observations dedicated to certain sources, such as the Galactic Center or certain remains of supernovae, as well as blind observations for the discovery of new sources. The thesis subject proposed here concerns an aspect of the study of sources which remains to be explored: the research and study of the variability of very-high energy sources. For variable sources, it is also interesting to correlate the variability in other wavelength ranges. Finally, the source model can help predict its behavior, for example its “high states” or its bursts.

Disequilibrium chemistry of exoplanets’ high-metallicity atmospheres in JWST times

In little more than two years of scientific operations, JWST has revolutionized our understanding of exoplanets and their atmospheres. The ARIEL space mission, to be launched in 2029, will soon contribute to this revolution. A main finding that has been enabled by the exquisite quality of the JWST data is that exoplanet atmospheres are in chemical disequilibrium. A full treatment of disequilibrium is complex, especially when the atmospheres are metal-rich, i.e. when they contain in significant abundances elements other than hydrogen and helium. In a first step, our project will numerically investigate the extent of chemical disequilibrium in the atmospheres of JWST targets suspected to have metal-rich atmospheres. We will use towards that end an in-house photochemical model. In a second step, our project will explore the effect of super-thermal chemistry as a driver of chemical disequilibrium. This will offer previously-unexplored insight into the chemistry of metal-rich atmospheres, with the potential to shed new light into the chemical and evolutionary paths of low-mass exoplanets.

Nuclear reactions induced by light anti-ions - contribution of the INCL model

The interaction of an antiparticle with an atomic nucleus is a type of reaction that needs to be simulated in order to answer fundamental questions. Examples include the PANDA (FAIR) collaboration with antiproton beams of the order of GeV, which plans to study nucleon-hyperon interactions, as well as the neutron skin by producing hyperons and antihyperons. This same neutron skin is also studied with antiprotons at rest in the PUMA experiment (AD - Cern). At the same site, we are collaborating with the ASACUSA experiment to study the production of charged particles. To respond to those studies, our INCL nuclear reaction code has been extended to antiprotons (thesis by D. Zharenov, defended at the end of 2023). Beyond the antiproton there are antideuterons and antiHe-3. These antiparticles are of more recent interest, notably with the GAPS (General AntiParticle Spectrometer) experiment, which aims to measure the fluxes of these particles in cosmic rays. The idea is to highlight dark matter, of which these particles are thought to be decay products, and whose measured quantity should emerge more easily from the astrophysical background noise than in the case of antiprotons. The proposed subject is therefore the implementation of light anti-nuclei in INCL with comparisons to experimental data.

Investigating the nature of Gamma-Ray Bursts with SVOM

Gamma-Ray Bursts are short lived (0.1-100 s) gamma-ray transient sources that appear randomly on the entire sky. Even if they have been discovered at the end of the 1960s, their nature remained mysterious until the end of the 1990s. It is only thanks to the observations of the BeppoSAX satellite at the end of the last century and especially thanks to the observations of the Swift satellite starting from 2004, that the mysterious nature of GRBs started to be elucidated.
These emissions are related to the final stages of very massive stars (30-50 times the mass of the Sun) for the long GRBs (<2 s) or to the merger of two compact objects (typically two neutron stars) for the short GRBs (< 2s). In either case there is the creation of a powerful relativistic jet, which is at the origin of the electromagnetic emission that is measure in gamma-rays and in other energy bands. If this jet points towards the Earth, GRBs can be detected up to very long distances (z~9.1) corresponding to a young age of the Universe (~500 Myr).
Svom is a sino-french space mission dedicated to GRBs, which has been successfully launched on June 22nd 2024, and in which CEA/Irfu/DAp is deeply involved. The PHD subject is aimed at exploiting the multi-wavelength data of SVOM and its partner telescopes in order to investigate the nature of GRBs, and in particular to make use of X-ray data from the MXT telescope in order to try to constrain the nature of the compact object which is at the origin of the relativistic jets.

Search for di-Higgs production in the multilepton channel with the ATLAS detector using 13.6 TeV data

The Higgs boson, discovered in 2012 at the LHC, is at the origin of the electroweak symmetry breaking within the Standard Model (SM). Despite extensive studies on the Higgs properties, the Higgs self-coupling remains unexplored. This parameter is a key factor in determining the Higgs potential and the stability of the universe’s vacuum. Studying Higgs pair production is the only direct method for measuring this self-coupling, which will give crucial insights into the universe’s fundamental structure and the nature of the electroweak phase transition after the Big Bang. Di-Higgs production is predicted to have a very small cross-section within the SM. Among possible detection channels, the multilepton final state is promising due to its unique kinematic signature, though challenging due the need for precise lepton identification and advanced signal separation techniques using machine learning. This PhD project focuses on searching for di-Higgs production in the multilepton channel with 13.6 TeV ATLAS data, taking advantages from the increased data and energy in Run 3 and aiming to approach SM sensitivity.

Development of Reconstruction Algorithms for the New High-Angle Time Projection Chambers in the T2K Experiment and Measurement of CP Violation in Neutrino Oscillations

Neutrinos are promising messengers for detecting physics beyond the Standard Model. Their elusive nature and unexplained mass suggest they could open new pathways for physics. Neutrino oscillation research has entered a precision era with experiments like T2K, which in 2020 observed hints of CP violation in the leptonic sector that could shed light on the question of matter-antimatter asymmetry in the Universe.

The T2K experiment, located in Japan, studies neutrino oscillations by generating an intense beam of muon neutrinos (and anti-neutrinos). This beam is measured at two locations: a near detector, designed to reduce systematic uncertainties related to the neutrino flux and interaction models, and a far detector (Super-Kamiokande), tasked with measuring the disappearance of muon neutrinos and the appearance of electron neutrinos after oscillation.
In 2023, T2K entered its second phase with increased beam power and upgrade of the near detector, including a highly granular new target and High-Angle Time Projection Chambers (HA-TPC). These improvements enable more precise reconstruction of particles produced by neutrino interactions.

IRFU teams contributed by developing HA-TPCs equipped with resistive Micromegas technology. This work improves spatial resolution and the precision of particle momentum. The thesis explores optimizing the particle track reconstruction algorithms in the HA-TPCs using advanced techniques, as well as analyzing T2K data with the upgraded ND280 to achieve a 3 sigma level of significance for CP violation. T2K is thus paving the way for future experiments like DUNE and Hyper-Kamiokande, opening new perspectives for the next two decades.

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