Towards a high spatial resolution pixel detector for particle identification: new detectors contribution to physics

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.

Caliste-3D CZT: development of a miniature, monolithic and hybrid gamma-ray imaging spectrometer with improved efficiency in the 100 keV to 1 MeV range and optimised for detection of the Compton effect and sub-pixel localisation

Multi-wavelength observation of astrophysical sources is the key to a global understanding of the physical processes involved. Due to instrumental constraints, the spectral band from 0.1 to 1 MeV is the one that suffers most from insufficient detection sensitivity in existing observatories. This band allows us to observe the deepest and most distant active galactic nuclei, to better understand the formation and evolution of galaxies on cosmological scales. It reveals the processes of nucleosynthesis of the heavy elements in our Universe and the origin of the cosmic rays that are omnipresent in the Universe. The intrinsic difficulty of detection in this spectral range lies in the absorption of these very energetic photons after multiple interactions in the material. This requires good detection efficiency, but also good localisation of all the interactions in order to deduce the direction and energy of the incident photon. These detection challenges are the same for other applications with a strong societal and environmental impact, such as the dismantling of nuclear facilities, air quality monitoring and radiotherapy dosimetry.

The aim of this instrumentation thesis is to develop a versatile '3D' detector that can be used in the fields of astrophysics and nuclear physics, with improved detection efficiency in the 100 keV to 1 MeV range and Compton events, as well as the possibility of locating interactions in the detector at better than pixel size.

Several groups around the world, including our own, have developed hard X-ray imaging spectrometers based on high-density pixelated semiconductors for astrophysics (CZT for NuSTAR, CdTe for Solar Orbiter and Hitomi), for synchrotron (Hexitec UK, RAL) or for industrial applications (Timepix, ADVACAM). However, their energy range remains limited to around 200 keV (except for Timepix) due to the thinness of the crystals and their intrinsic operating limitations. To extend the energy range beyond MeV, thicker crystals with good charge carrier transport properties are needed. This is currently possible with CZT, but several challenges need to be overcome.

The first challenge was the ability of manufacturers to produce thick homogeneous CZT crystals. Advances in this field over the last 20 years mean that we can now foresee detectors up to at least 10 mm thick (Redlen, Kromek).

The main remaining technical challenge is the precise estimation of the charge generated by the interaction of a photon in the semiconductor. In a pixelated detector where only the X and Y coordinates of the interaction are recorded, increasing the thickness of the crystal degrades spectral performance. Obtaining Z interaction depth information in a monolithic crystal theoretically makes it possible overcome the associated challenge. This requires the deployment of experimental methods, physical simulations, the design of readout microelectronics circuits and original data analysis methods. In addition, the ability to localise interactions in the detector to better than the size of a pixel will help to solve this challenge.

Kinetic description of laser-plasma interaction relevant to inertial confinement fusion

Many applications, such as inertial confinement fusion, require an understanding of the physical mechanisms involved when high-energy laser beams propagate in a plasma. In particular, in the case of fusion, the aim is to quantify the deposition of laser energy on a cryogenic deuterium-tritium target, and the efficiency with which this target can be compressed to trigger fusion reactions. However, during their propagation, laser beams create a plasma wave that grows at the expense of the incident laser energy. However, the growth of this wave is not infinite and stops when the wave breaks up. This is accompanied by the production of hot electrons, which can preheat the target and hinder its compression. The breaking of a plasma wave is a physical phenomenon of the kinetic type, which can only be correctly described by calculating the velocity distribution of the electrons in the plasma. The aim of this thesis is to study wave breaking both theoretically and numerically, using Vlasov-type kinetic codes. One of the main difficulties lies in the discontinuity of the distribution functions to be described. In addition, it is necessary to describe the surge from its linear phase to the non-linear regime, enabling the creation of hot electrons to be quantified. The ultimate goal of the thesis is to produce models that are simple enough to run on the CEA's dimensioning codes.

Optimisation of the Gbar experiment for the production of antihydrogen ions

The aim of the Gbar experiment (Gravitational Behavior of Antihydrogen at Rest) at CERN is to produce a large number of antihydrogen atoms to measure their acceleration in Earth's gravitational field. The principle relies on the production of antihydrogen ions through two successive charge exchange reactions that occur when a beam of antiprotons passes through a positronium cloud. In 2022, Gbar demonstrated its operational scheme by producing antihydrogen atoms through the first charge exchange reaction. The current focus is on optimizing and improving various elements of the experiment to achieve the production of anti-H+, particularly the positron line leading to the creation of the positronium cloud. The challenge is to increase the number of positrons trapped in the second electromagnetic trap of the line, and then to transport them efficiently to the reaction chamber where they are converted into positronium.
The thesis work will involve operating, diagnosing, and optimizing the two electromagnetic traps of the line, as well as the positron acceleration and focusing devices to yield a sufficient number of positroniums and then the production of antihydrogen ions. The student will also participate in the measurement campaign for studying the mater counterpart of the second charge exchange reaction, relying upon a beam of H- ion instead of the beam of antiprotons.

Development of a neutron/gamma coincidence measurement system for the characterization of radionuclide neutron sources

This PhD work is part of sources calibration activities at the LNHB-MA and R&D activities within the SIMRI aimed at developing neutron measurement systems for the CEA and the nuclear industry. The objective of the PhD work is to develop a new measurement system using neutron/gamma coincidences to enable the characterization of the (alpha,n)-type neutron sources. These sources consists of a homogeneous mixture of an alpha particle emitter and the target substance, the nuclei of which emit neutrons via a nuclear reaction. As for example, we can cite for example: AmBe, PuBe, CmBe, or even exotic source of high emissivity and mixing several alpha radionuclides (ex. AmPuBe). For this familly of sources, the emission of neutron by reaction (alpha,n) is in simultaneous cascade with a characteristic gamma at 4.4 MeV. The detection of the neutron and the gamma in coincidence is likely to provide information of interest in the source characterization in terms of emission rate and spectral fluence. The objective is to measure precisely gamma and neutron signatures as well as gamma/neutron intensity ratios resulting from the nuclear reaction. The new measurement device must also be able to measure neutrons emitted by the spontaneous fission reaction or by (n,2n) reaction in beryllium. Others photon emission can be also provide information of interest, ex. the emission of a gamma at 2.2 MeV resulting from the capture on hydrogen. The neutron/gamma coincidence measurements can be also used to improve the evaluation of nuclear data such as cross sections of certain elements, ex. (n,gamma) reaction on oxygen or hydrogen.

Full isotopic fission fragment distribution measurement of 241Pu using inverse kinematics at GANIL with VAMOS and PISTA

The inverse kinematics technique is used at GANIL to produce the so-called in-flight fission. The accelerated fissioning system is excited by a nuclear reaction, and in particular by a nucleon transfer reaction between the beam and the target. Fission fragments are therefore emitted at forward angles in the laboratory frame due to the kinematic boost of the reaction. The VAMOS wide-acceptance magnetic spectrometer is used to identify the mass and nuclear charge of the various fragments, while silicon telescopes are used to characterize the fissioning system by detecting the ejecta emitted by the transfer reaction.
The fission@VAMOS project involves upgrading the silicon detection system used to identify the fissioning system produced by the transfer reaction. The current device is a highly segmented silicon telescope assembly called PISTA. This improves the sensitivity and precision of the fissioning system formation conditions (mass, atomic number, excitation energy).
The subject of this thesis is therefore a detailed multi-parametric study of fission, with a focus on measuring the fission yields of the fissioning system 242Pu (n+241Pu). Finally, a large part of the work will consist of data analysis and interpretation, followed by publication.

Studying inflation with quasars and galaxies in DESI

Measurements of the statistical properties of the large-scale structure (LSS) of the universe provide information on the physics that generated the primordial density fluctuations. In particular, they enable us to distinguish between different models of cosmic inflation by measuring primordial non-Gaussianity (PNG), the deviation from the initial conditions of the Gaussian random field.

Our strategy for studying LLS is to use a spectroscopic survey, DESI, whose instrument was commissioned at the end of 2019. DESI will observe 40 million galaxies and quasars. Observations take place at the 4-m Mayall telescope in Arizona. In the spring of 2021, the project began a five-year period of uninterrupted observations, covering a quarter of the sky.

For this thesis project, LSS are measured with two tracers of matter: very luminous red galaxies (LRG) and quasars, very distant and very luminous objects. These two tracers enable us to cover a wide redshift range from 0.4 to 4.0.

During the first year of his/her thesis, the student will contribute to the final analysis of the first year of DESI observations. In particular, he/she will study LSS with quasars and galaxies (LRG). His/her work will also involve assessing all possible sources of bias in the selection of quasars and LRGs that could contaminate a cosmological signal. In a second phase, the student will develop a more sophisticated analysis using three-point statistics such as the bispectrum with an extended sample to the first three years of DESI observations.

Implementation of covariant QRPA to describe deformed atomic nuclei

All other things being equal, what differences can be expected from the choice of a relativistic or non-relativistic interaction in the QRPA description of the excited states of the atomic nucleus? In order to answer it, the student will on one hand use numerical tools to solve non- relativistic interaction QRPA matrix equations and on the other hand use a solver of the finite amplitude method to produce QRPA response functions with relativistic interactions. These numerical tools leverage supercomputers and are widely used for nuclear data and astrophysics issues as well as to conduct academic nuclear structure studies. The relativistic extension of the matrix QRPA solver will make it possible to transfer all the expertise of nuclear data production to the case of interactions from relativistic lagrangians. Thus, an analysis of the respective merits of the two functionals will be conducted and exploited with a view to the development of new generation effective interactions.

Search for Higgs boson production with a single top and study of the CP properties of the top-Higgs coupling in the diphoton channel with the CMS experiment at the LHC.

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.

Study of low-frequency radiation produced by particle acceleration at ultra-high laser intensity in relativistic plasmas

Today, petawatt laser sources deliver optical pulses lasting a few tens of femtoseconds with an intensity larger than 1020 W/cm2. When such a light beam interacts with a gas or a solid target, the electrons accelerated by the laser ponderomotive force become relativistic and acquire high energies, in excess of the GeV. These laser systems also produce various radiations such as hard X photons or electron-positron pairs by quantum conversion of gamma photons. As laser technology is advancing rapidly, these light sources have increasingly compact dimensions and they nowadays complement many international laboratories hosting synchrotrons or conventional particle accelerators.
If this extreme light makes it possible to generate radiation in the highest frequencies regions of the electromagnetic spectrum, it also fosters, through the production mechanisms of plasma waves and particle acceleration, conversion processes towards much lower frequencies belonging to the gigahertz and terahertz (THz) ranges.

Having high-power transmitters operating in this frequency band is attracting more and more interest in Europe, overseas and in Asia. On the one hand, the generation of intense electromagnetic pulses with GHz-THz frequencies is harmful for any electronic device close to the laser-plasma interaction zone and the diagnostics used on large-scale laser facilities like, e.g., the PETAL/LMJ laser in the Aquitaine region. It is therefore necessary to understand their nature to better circumvent them. On the other hand, the waves operating in this field not only make it possible to probe the molecular motions of complex chemical species, but they also offer new perspectives in medical imaging for cancer detection, in astrophysics for the evaluation of ages of the universe, in security as well as environmental monitoring. The processes responsible for this violent electromagnetic field emission, if properly controlled, can lead to the production of enormous magnetic fields in excess of 1000 Tesla, which presents exciting new opportunities for many applications such as particle guiding, atomic physics, magnetohydrodynamics, or modifying certain properties of condensed matter in strong field.

The objective of this thesis is to study the physics of the generation of such giant electromagnetic pulses by ultrashort laser pulses interacting with dense media, to build a model based on the different THz/GHz laser-pulse conversion mechanisms, and validate this model by using dedicated experimental data. The proposed work is mainly oriented towards an activity of analytical modeling and numerical simulation.

The doctoral student will be invited to deal with this problem theoretically and numerically using a particle code, whose Maxwell solver will be adapted to describe radiation coming from different energy groups of electron/ion populations. A module calculating online the field radiated by each particle population in the far field will be implemented. Particular attention will be given to the radiation associated with the acceleration of electrons and ions on femto- and picosecond time scales by dense relativistic plasmas and their respective roles in target charging models available in the literature. This field of physics requires a new theoretical and numerical modeling work, at the crossroads of extreme nonlinear optics and the physics of relativistic plasmas. Theory-experiment confrontations are planned within the framework of experiments carried out on site at CELIA facilities and experiments carried out in collaboration with US laboratories (LLE/Rochester). The thesis will be prepared at CELIA laboratory on the campus of Bordeaux university.