Nuclear Physics Division

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.

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.

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.

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.

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.