Institute of research into the fundamental laws of the Universe

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.

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.

Neutrinoless double beta decay (0n2b) is a theoretical nuclear transition, whose observation would become a major milestone in particle, and in particular, neutrino physics. This process, if it exists, violates lepton number conservation law and confirms Majorana nature of neutrino. The detection of 0n2b is a challenging task, since it is a very rare decay (T1/2>10^26 years), and the experiments require high detection efficiency, energy resolution, radiopurity, large mass and very low background levels. Several ton-scale experiments are in preparation, but in paralell, new approaches have to be investigated for higher sensitivity levels. The TINY project proposes new detection technology, based on cryogenic detectors (measured at mK temperatures). The thesis subject will be mainly dedicated to the development of new thermal sensors, Zr- and Nd-containing detectors characterization, performance evaluation and evaluation of technology applicability for a ton-scale experiment. The student will develop skills on operation of cryogenic facility, signal processing, data analysis and simulations. Finally, a demonstrator will be prepared with the goal to set new limits on 0n2b for 96-Zr and 150-Nd, and perform precision measurements of 2n2b decay.

The CMS group at CEA-Saclay/IRFU/DPhP proposes a thesis on the search for double Higgs boson production in the decay channel with a pair of bottom quarks and a pair of tau leptons. The study of this production gives a direct access to the Higgs boson self-coupling, parameter still to be measured. The selected student will take part to research activities well established within CMS, within the CEA group, in link with other institutes in France and abroad. He or she will have to develop an analysis using the Run 3 data of LHC, and in particular to optimise the trigger strategy with regard to previous such analyses. Several publications are foreseen: a first one using the data collected in 2022 and 2023, combined with Run 2 if the sensitivity gain is as large as expected, and a second one using the full Run 2 and Run 3 data. A contribution to the combination of HH results in the different sensitive channels is also foreseen. In parallel, the student will be able to also take part in activities related to detector upgrades, in particular in calorimetry, benefiting from the great expertise of the group in this domain.

This thesis focuses on measuring the polarization of Lambda hyperons in exclusive deeply virtual meson production processes. The study is rooted in a surprising discovery from the 1970s: in proton-Beryllium collisions, ? hyperons exhibited transverse polarization, challenging the predictions of perturbative Quantum Chromodynamics. Similar polarizations have since been observed in various collision systems.
The proposed research topic leverages deeply virtual exclusive reactions in electron-proton scattering, providing precise control over final states and initial particle polarizations. Specifically, the reaction e+p->e+Lambda+K+ is explored to shed light on the Lambda hyperon's polarization. This process is also sensitive to the poorly known transversity Generalized Parton Distributions (GPDs) of the nucleon, offering valuable insights into nucleon properties.
The thesis aims to analyze data collected with the CLAS12 experiment at the Jefferson Laboratory (JLab) in US, with a focus on e-p collisions with a longitudinally polarized NH3 target. Machine learning algorithms and simulations will be employed to enhance data reconstruction and event candidate selection. The candidate will also contribute to simulation studies for future detectors and their reconstruction algorithms for the EIC.
The research will be conducted within the Laboratory of Nucleon Structure at CEA/Irfu. A background in particle physics, computer science (C++, Python), and knowledge of particle detectors is beneficial for active participation in data analysis.
The student will have the opportunity to collaborate with local and international researchers, to participate in the CLAS collaboration, to join the EIC user group with frequent trips to the USA for data collection and workshops, and present research findings at international conferences.

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 [1] new results leading to the best constraint to date on the parameter dCP, which translates in the theory 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% (3s) and the value most compatible with the data is very close to -90° corresponding to a maximum asymmetry between matter and antimatter. T2K has the best world sensitivity for this fundamental parameter 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 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 SuperKamiokande 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 SuperKamiokande: 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 focus on the analysis of the data for the measurement of the neutrino oscillations with new upgraded near detector installed in 2023. The objective of this new detector is to improve the performance of the ND280 near detector, to measure the neutrino interaction rate and to constrain the neutrino interaction cross sections so that the uncertainty on the number of events predicted at SuperKamiokande is reduced to about 4% (from about 8% today). The upgrade of the near detector will require to put in place a new analysis strategy to enable precise measurement of the neutrino oscillations. 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 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 well bring before ten years a key to understand the mystery of the disappearance of antimatter in our Universe.

The PandaX-III collaboration proposes to determine whether the neutrino is a Majorana particle, i.e. its own antiparticle. In this purpose this international collaboration, in which the Research Institute on Fundamental laws of the Universe (IRFU) of CEA Saclay participates, aims to observe neutrinoless double-beta decays of the Xenon-136, where the emission of the two electrons is not compensated by the simultaneous emission of two anti-neutrinos. Such an observation would violate the principle of conservation of leptonic number, in opposition with the predictions of the standard model of particle physics. The search of such rare events requires an enormous quantity of Xenon-136 atoms, a deep underground laboratory protected from cosmic rays and with low radioactive levels, like the Jinping underground laboratory (CJPL, Sichuan province, China), and a very effective particle detector.

The first phase of the experiment aims to construct a first TPC module (Time Projection Chamber) of 145kg of Xenon-136, which will be followed in a second stage by four other 200kg modules. The TPC will be equipped with detectors able to measure the energy of the two beta electrons with an excellent accuracy. The first TPC module will be commissioned end of 2024. The trajectory of the two electrons emitted by the double-beta decay will be reconstructed to measure the initial energy of those electrons, and to recognize the topology of their trajectories to differentiate them from gamma backgrounds which emit only one electron. That module will be equipped with gaseous Micromegas detectors which have a good energy resolution and a very good radio-purity which limits the amount of gamma backgrounds coming from radioactive contamination.

The PandaX-III collaboration is working on the construction of the first TPC module. It will be installed at CJPL during the year 2024. Reconstruction algorithms of detector data using neural networks are being developed, in order to complete the analytical methods already implemented in the REST environment of data reconstruction and analysis, to optimize double-beta events versus gamma backgrounds discrimination, and to improve the quality of the electron energy reconstruction. These algorithms are trained and evaluated on simulated Monte-Carlo events. Data from reduced-size TPC prototype will be also used to test these algorithms in real conditions. As soon as the first module will be installed end of 2024 these algorithms will be used for detector calibrations and for being implemented in real data analysis. They will be then used to extract the first physics results on double-beta events.

The main task of the PhD student will be to contribute on the development of data reconstruction algorithms based on neural networks, in particular by taking into account the defects of the detectors (dead channels, performance inhomogeneity, gas impurities, etc...) and by implementing in REST the data correction methods needed to compensate these defects. That work will include studies of data from prototype TPC chambers, as well as Monte-Carlo simulations. Moreover, as soon as the data from the first TPC module will be available the student will participate to the data analysis and the extraction of the physics results. These studies will be presented in conferences and published in scientific journals. The student will also participate to an R&D to optimize Micromegas detectors in order to improve their energy resolution as well as their general performance in high pressure gaseous Xenon.

A Master internship of 4 to 6 months would be also possible in the IRFU/DPhN PandaX-III group before the start of the PhD thesis.

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.

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.

This thesis project concerns the precise measurement of the electroweak mixing angle with the ATLAS experiment, at the LHC. The evolution with energy of this fundamental parameter will also be tested. The measurement will be based on the di-muon data set from runs 2 and 3 of the LHC, and will use the muon spectrometer as the main instrument.

The determination of the mixing angle is based on the measurement of the forward-backward asymmetry of the Z boson decay muons. For precise control of systematic uncertainties, the internal alignment of the spectrometer must be optimized. This alignment constitutes an important part of the project. The performance of the New Small Wheel, the new muon detector installed for run 3, will also need to be understood in detail. The actual measurement will be carried out at the end of these preliminary studies, and an interpretation of the result will complete the thesis.

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 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.

This PhD topic is about the NUCLEUS experiment, which aims to accurately measure the process of coherent elastic neutrino scattering on nuclei (CEvNS) at the Chooz nuclear power plant in the French Ardennes. Although at ~MeV energies, CEvNS is the dominant interaction process of neutrinos with matter, it has remained unobserved for a very long time because of the difficulty of measuring the weak nuclear recoils it induces. It was only 40 years after its first prediction that this process was observed for the first time with neutrinos of a few tens of MeV at the Oak Ridge laboratory accelerator facility. The first detection of CEvNS at a nuclear reactor remains to be achieved, especially because the corresponding nuclear recoils lie in an energy regime (~100 eV) which is difficult to measure with conventional detection technologies, and also because of the unfavorable background conditions nuclear power plant environments generally offer. The NUCLEUS collaboration is therefore working on the design of an innovative detection system using two cryogenic calorimeter arrays (CaWO4 & Al2O3) capable of reaching ~10 eV energy thresholds, and surrounded by a twofold system of instrumented cryogenic vetoes. This set of cryogenic detectors will be protected by an external passive shielding and by a muon veto to improve the identification and discrimination of backgrounds. With this system, NUCLEUS aims at a precise measurement of CEvNS in order to push the study of the fundamental properties of the neutrino as well as the search for beyond standard model physics towards the low energy frontier. Interestingly, CEvNS also exhibits a cross-section 10 to 1000 times larger than the usual ~MeV neutrino detection channels (inverse beta decay reaction, neutrino-electron scattering process), making it possible to miniaturize future long-range neutrino detection setups.

The experiment is currently in its initial commissioning and testing phase at the Technical University of Munich (TUM). This step will be followed in 2024 by several data acquisition runs, aiming at (i) qualifying and validating the performances of the various detectors, (ii) validating the overall background reduction strategy, and (iii) studying and mitigating the "excess", an exponential increase in the count rate of low-energy events observed in the cryogenic calorimeters, which are of unknown origin and could degrade the experiment's sensitivity to a CEvNS signal. The relocation of the experiment to the Chooz nuclear power plant will be led by our team and will take place after the summer 2024. It is in this context that the student will begin his/her PhD work, contributing to all of the integration and commissioning operations. This crucial step will require a serie of various tests and data acquisitions to set up, fine-tune and synchronize the experiment's various detection systems. She/he will focus in particular on the external cryogenic veto and on the muon veto systems, both designed and built by our team. The analysis and the exploitation of data from this on-site commissioning phase at Chooz will enable the student to get acquainted with all existing low- and high-level analysis tools for diagnosing and characterizing these detectors. One of the student's tasks will be to improve these tools, and to set up an automation chain for diagnosing and processing the large volume of daily data (~10 TB) that will be taken during the experiment's first physics run.
Extracting the CEvNS signal from data requires several preliminary studies. The first one is to characterize the energy and time response of the detectors over the data acquisition periods. The student will take charge of one of these tasks, building on the work already accomplished during the commissioning phase. This work will lead to a detailed understanding of the operation of the detectors and the identification of all the factors likely to influence their behavior. It should be noted that our team has proposed and is responsible for an innovative method for the calibration of very low energy nuclear recoils in cryogenic calorimeters, with the installation of a dedicated facility on a low-power research reactor located at the Technical University of Vienna (Austria). The student may eventually get involved in this effort, with a view to interpreting the data collected at Chooz. Building on these results, the student will then focus on the extraction and the study of a specific background component in the collected data. This work will enable the consolidation and the fine-tuning of a predictive model of the experiment's background, using a Monte Carlo simulation framework based on the Geant 4 library. Finally, the student will set up simple statistical tests to characterize the level of confidence with which a CEvNS signal can be extracted from the data after subtraction of the measured backgrounds.
Finally, the student will use the first data from the physics run at Chooz to conduct a search for new physics beyond the standard model (measurement of the weak mixing angle at low energies, search for new neutrino couplings, constraints on the electromagnetic properties of the neutrino, etc.). This work will require the implementation of advanced statistical methods for interpreting the data, in order on the one hand to understand the impact of the various sources of uncertainty on the obtained constraints, and on the other hand to guarantee the reliability of the results.

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.

In collaboration with the Thomas Jefferson Laboratory (JLab) in the USA, the researchers in the laboratory of nucleon structure at Irfu want to understand how quarks and gluons interact to form hadrons such protons, neutrons and pions. At JLab, a 11-GeV electron beam is impinged on a proton target. The protos are constituted of three quarks surrounded by a cloud of quark/antiquark pairs whose quantum numbers are similar to pions. The electrons of the beam will interact with these pairs with a structure analogous to a pion. More specifically, we are interested in the deeply virtual Compton scattering (DVCS) giving correlations between longitudinal momentum and transverse position of quarks in a pion. In other words, we are going to perform the very first 3D study of the pion structure. The PhD student will analyze data already available to isolate the DVCS events. A digital twins of the Monte-Carlo simulation/reconstruction chain will be produced with a conditional Generative Adversarial Network in order to caracterize faster and more accurately the background and, in the end, subtract it. The PhD student will travel two to three times a year to JLab, participating to the data taking as well as attending the collaboration meeting. The results will be presented in international conferences and published in peer-reviewed journals.

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.

A few micro-seconds after the Big Bang, the Universe was in a quark gluon plasma (QGP) state. Such state is predicted by Quantum Chromodynamics, which is the theory of strong interactions, and should be reached at very high temperature or energy density. Such conditions are reproduced in ultra-relativistic heavy ion collisions at the LHC at CERN.
Among the various QGP observables, the study of hadrons with heavy-flavour quarks (charm c or beauty b) and quarkonia (c-cbar or b-bbar bound states) is particularly important to understand the properties of the QGP. Indeed, heavy quarks are produced by hard scatterings between partons of the incoming nuclei in the early stage of the collision, and thus experience the full dynamics of the collision.
Thanks to the measurements of J/psi (c-cbar) production in Pb-Pb collisions of Runs 1 and 2 of the LHC, the ALICE collaboration showed the existence of the regeneration mechanism: when the initial number of produced quark/anti-quark pairs is large and if heavy quarks do thermalise in the QGP, then new quarkonia could be produced in the QGP by recombination of heavy quarks. Other mechanisms, such as colour screening, could affect the production of quarkonia. Bc mesons are composed of a b quark and an antiquark c. Their production is therefore strongly disfavored in proton-proton collisions. In Pb-Pb collisions, instead, their production could be largely increased due to the regeneration mechanism.
We propose to study the production of Bc mesons in Pb-Pb collisions at a center-of-mass energy per nucleon pair (sqrt(sNN)) of 5.36 TeV at the LHC with the data of Run 3 (2022-2025). The ALICE apparatus was upgraded in view of LHC Runs 3 and 4 with, in particular, the addition of a silicon pixel tracker (MFT) that complements the ALICE forward spectrometer as well as new readout electronics for the latter. These upgrades will allow us to: Profit from the planned increase in luminosity of the LHC, thus tripling in one year the data collected in the full LHC Run 2 (2015-2018); Measure with high precision secondary vertices of b-hadron decays. The Bc mesons will be measured at forward rapidity by reconstructing three secondary muons with the muon spectrometer and the MFT of ALICE.
The student will first contribute to the optimization and characterization of the muon spectrometer and MFT matching algorithm and the secondary vertex reconstruction. Secondly, the student will study the production of Bc mesons in Pb-Pb collisions. Finally, the results will be compared with other experimental results as well as various theory calculations.
During this work the student will become familiar with the grid computing tools and the simulation, reconstruction and data analysis software of the ALICE Collaboration.

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.