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.

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.

Impact of irradiation parameters on the alpha’ phase formation in oxide dispersion strengthened steels

Ferritic-martensitic oxide dispersion strengthened steels (ODS steels) are materials of great interest in the nuclear industry. Predominantly composed of iron and chromium, these materials can become brittle due to the precipitation of a chromium-rich phase, called a', under irradiation. This phase, known to be sensitive to irradiation conditions, provides an ideal topic for a deeper exploration of the capability to emulate neutron irradiation with ions. Indeed, while ion irradiations are frequently used to understand phenomena observed during neutron irradiations, the question of their representativeness is often raised.

In this thesis, we aim to understand how the irradiation parameters can affect the characteristics of the a' phase in ODS steels. To do so, various ODS steels will be irradiated under different conditions (flux, dose, temperature, and type of particles, such as ions, neutrons, electrons), and subsequently analyzed at the nanoscale. The a' phase (size, chromium content) obtained for each ion irradiation condition will be compared to the one after neutron irradiation.

Testing the Standard Model in the Higgs-top sector in the multilepton final using the ATLAS detector at the LHC

The thesis proposes to measure in a coherent way the different rare processes of production of top quarks in association with bosons in the final state with multiple leptons at the Large Hadron Collider (LHC). The thesis will be based on the analysis of the large dataset collected and being collected by the ATLAS experiment at a record energy. The joint analysis of the ttW, ttZ, ttH and 4top processes, where one signal process becomes background when studying the other ones, will allow to get complete and unbiased measurements of the final state with multiple leptons.
These rare processes, which became accessible only recently at the LHC, are powerful probes to search for new physics beyond the Standard Model of particle physics, for which the top quark is a promising tool, in particular using effective field theory. Discovering signs of new physics that go beyond the limitations of the Standard Model is a fundamental question in particle physics today.

Construction of a Micromegas tracker for the P2 experiment, and measurement of the electroweak mixing angle in electron-proton scattering

This thesis project concerns the precise measurement of the electroweak mixing angle with the P2 experiment, at the MESA accelerator, in Mainz. The measurement will make it possible to test, for the first time, the prediction of the Standard Model for the evolution of this fundamental parameter as a function of the energy scale, and the effects of possible new particles or interactions.

The determination of the mixing angle is based on a precise measurement of the variation of the scattering cross section of an electron beam on a liquid hydrogen target, as a function of the polarization of the beam. This asymmetry, measured in scattering at forward angles, is affected by significant systematic uncertainties linked to the structure of the proton. A measurement of the scattering asymmetry in the backward direction, using a dedicated detector, makes it possible to reduce these uncertainties, and constitutes the subject of this thesis.

The thesis project arrives at a crucial moment in the development of the experiment, and will allow the student to participate directly in the construction of a very high performance detector, its installation in the P2 experiment, and its scientific exploitation.

ADVANCED ARTIFICIAL INTELLIGENCE TECHNIQUES FOR PARTICLE RECONSTRUCTION IN THE CMS DETECTOR USING PRECISION TIMING AND ATTENTION MECHANISM

Particle reconstruction in collider detectors is a multidimensional problem where machine learning algorithms offer the potential for significant improvements over traditional techniques. In the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC), photons and electrons produced by the collisions at the interaction point are recorded by the CMS Electromagnetic Calorimeter (ECAL). The large number of collisions, coupled with the detector's complex geometry, make the reconstruction of clusters in the calorimeter a formidable challenge. Traditional algorithms struggle to distinguish between overlapping clusters created by proximate particles. In contrast, It has been shown that graph neural networks offer significant advantages, providing better differentiation between overlapping clusters without being negatively affected by the sparse topology of the events. However, it is crucial to understand which extracted features contribute to this superior performance and what kind of physics information they contain. This understanding is particularly important for testing the robustness of the algorithms under different operating conditions and for preventing any biases the network may introduce due to the difference between data and simulated samples (used to train the network).
In this project, we propose to use Gradient-weighted Class Activation Mapping (Grad-CAM) and its attention mechanism aware derivatives to interpret the algorithm's decisions. By evaluating the extracted features, we aim to derive analytical relationships that can be used to modify existing lightweight traditional algorithms.
Furthermore, with the upcoming High Luminosity upgrade of the LHC, events involving overlapping clusters are expected to become even more frequent, thereby increasing the need for advanced deep learning techniques. Additionally, precision timing information of the order of 30 ps will be made available to aid in particle reconstruction. In this PhD project, we also aim to explore deep learning techniques that utilize Graph and Attention mechanisms (Graph Attention Networks) to resolve spatially proximate clusters using timing information. We will integrate position and energy deposition data from the ECAL with precision timing measurements from both the ECAL and the new MIP Timing Detector (MTD). Ultimately, the developed techniques will be tested in the analysis of a Higgs boson decaying into two beyond-the-standard-model scalar particles.

We are seeking an enthusiastic PhD candidate who holds an MSc degree in particle physics and is eager to explore cutting-edge artificial intelligence techniques. The selected candidate will also work on the upgrade of the CMS detector for the high-luminosity LHC.

INVESTIGATION OF THE NUCLEAR TWO-PHOTON DECAY IN SWIFT FULLY STRIPPED HEAVY IONS

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 2024.

High-precision measurements of nuclear recoil on the 100 eV scale for cryogenic detectors

The CRAB method aims to provide an absolute calibration of cryogenic detectors used in dark matter and coherent neutrino scattering experiments. These experiments have in common the fact that the signal they are looking for is a very low-energy nuclear recoil (around 100 eV), requiring detectors with a resolution of a few eV and a threshold of O(10eV). Until now, however, it has been very difficult to produce nuclear recoils of known energy to characterize the response of these detectors. The main idea of the CRAB method, detailed here [1, 2], is to induce a nuclear capture reaction with thermal neutrons (25 meV energy) on the nuclei constituent the cryogenic detector. The resulting compound nucleus has a well-known excitation energy, the neutron separation energy, being between 5 and 8 MeV, depending on the isotope. If it de-excites by emitting a single gamma ray, the nucleus will recoil with an energy that is perfectly known, given by the two-body kinematics. A calibration peak, in the desired range of some 100 eV, then appears in the energy spectrum of the cryogenic detector. A first measurement performed in 2022 with a CaWO4 cryogenic detector from the NUCLEUS experiment (a coherent neutrino scattering experiment supported by TU-Munich, in which CEA is heavily involved) has validated the method [3].

This thesis comes within the scope of the second phase of the project, which involves high precision measurements using a thermal neutron beam from the TRIGA-Mark-II reactor in Vienna (TU-Wien, Austria). Two complementary approaches will be used simultaneously to achieve a high precision: 1/ the configuration of the cryogenic detector will be optimized for very good energy resolution, 2/ large crystals of BaF2 and BGO will be placed around the cryostat for a coincident detection of the nuclear recoil in the cryogenic detector and the gamma ray that induced this recoil. This coincidence method will significantly reduce the background noise and will enable the CRAB method to be extended to a wider energy range and to the constituent materials of most cryogenic detectors. We expect these measurements to provide a unique characterization of the response of cryogenic detectors, in an energy range of interest for the search for light dark matter and coherent neutrino scattering. High precision will also open up a window of sensitivity to fine effects coupling nuclear physics (nucleus de-excitation time) and solid-state physics (nucleus recoil time in matter, creation of crystal defects during nucleus recoil) [4].

The PhD student will be heavily involved in all aspects of the experiment: simulation, on-site installation, analysis and interpretation of the results.

Optimization of gamma radiation detectors for medical imaging. Time-of-flight positron emission tomography

Positron emission tomography (PET) is a nuclear medical imaging technique widely used in oncology and neurobiology. The decay of the radioactive tracer emits positrons, which annihilate into two photons of 511 keV. These photons are detected in coincidence and used to reconstruct the distribution of tracer activity in the patient's body.
We are offering you the opportunity to contribute to the development of an ambitious, patented technology: ClearMind.
You will work in an advanced instrumentation laboratory in a particle physics environment.
Your first task will be to optimize the "components" of ClearMind detectors, in order to achieve nominal performance.
We'll be working on scintillating crystals, optical interfaces, photo-electric layers and associated fast photo-detectors, readout electronics.
We will then characterize the performance of the prototype detectors on our measurement benches, which are under continuous development. The data acquired will be interpreted using in-house analysis software written in C++ and/or Python.
Finally, the physics of our detectors will be modeled using Monté-Carlo simulation (Geant4/Gate software), and we will compare our simulations with our results on measurement benches. A special effort will be devoted to the development of ultra-fast scintillating crystals in the context of a European collaboration.

Drell-Yan production measurement in proton-proton collisions and preequilibrium dilepton production in heavy-ion collisions with the LHCb experiment at the LHC

At the Large Hadron Collider (LHC) at Geneva, collisions of lead nuclei are used to create a thermodynamic system described by fluid dynamics under extreme conditions. The temperature of the short-lived system is sufficiently large in order to release the building blocks of matter at a subnucleonic scale, quarks and gluons. This state of matter is commonly called Quark Gluon Plasma (QGP). The space-time evolution of heavy-ion collisions at the LHC is described by close-to-ideal hydrodynamics after a short lapse of time. However, key features of the early stages of these collisions are largely unknown. These characteristics are crucial to understand the applicability limits of hydrodynamics and to understand thermalisation of a strongly interacting system.
In recent publications, it was pointed out that the dilepton production in the intermediate mass scale between 1.5 and 5 GeV/c² is highly sensitive to the ´thermalisation´ time scale towards the equilibrium QGP.

In addition, the LHC provides highly energetic proton and heavy-ion beams. They allow us to access the hadronic structure of the projectiles at very small fractional longitudinal momenta and at the same time still relatively large four momentum transfers. This configuration enables hence for perturbative calculations allowing the extraction of hadron structure information at very small fractional longitudinal momenta.
The theoretically best understood process in hadronic collisions is the production of dilepton pairs, the so-called Drell-Yan process. However, so far, no measurement down to 3 GeV/c² at a hadron collider has been published despite its theoretical motivation to test the lowest fractional momenta. In fact, at masses below around 30 GeV/c², semileptonic decays from heavy-flavour hadron decays start to dominate the dilepton production. This process has obscured any attempt to extract dilepton production in this kinematic domain.

The first goal of the thesis is the first measurement of Drell-Yan dimuons at low invariant masses at the LHC in proton-proton collisions that will be taken in 2024. This measurement will be based on novel background rejection techniques exploiting the forward geometry of LHCb. In a second part, the feasibility of the measurement in heavy-ion collisions will be investigated in the present and the future LHCb set-up. Depending on the outcome of the studies, a measurement in heavy-ion collisions will be conducted.

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