Nuclear Physics Division

One of the strongest research projects in recent years has emerged from a critical, unresolved question about the natural origin of nuclei heavier than iron. The closed neutron shell, N = 126, as the final waiting point in the r-process (rapid neutron capture process), plays an essential role in the formation of these nuclei. However, recent efforts to synthesize superheavy elements and explore N = 126 neutron-rich nuclei have faced significant challenges due to extremely low cross sections using traditional fusion-evaporation reactions.
These factors highlight the urgent need for alternative reaction mechanisms. One alternative has been identified in multinucleon transfer (MNT) reactions, which offer a promising route to neutron-rich heavy nuclei. The challenge is to isolate the desired nuclei from the multitude of products generated during the reaction.
We have been working on this reaction mechanism for several years, performing experiments at Argonne National Laboratory and other international laboratories.
The aim of this thesis is to analyse the data collected during the Argonne experiment (end 2023) and to propose a new experiment at the spectrometer Prisma (Legnaro National Lab) coupled with the Agata germanium detector.

One of the key questions in the field of nuclear structure concerns the emergence of collectivity, and its link with the microscopic structure of the nucleus. Atomic nuclei can exhibit so-called collective behaviours, which means that their constituents, protons and neutrons, move in a coherent way. The main modes of collective nuclear motion are vibrations and rotations. If a nucleus is not deformed, it cannot undergo rotations when excited, but vibrations around its spherical equilibrium shape are possible.
Even-even isotopes of cadmium have been considered textbook examples of vibrational behaviour. However, this interpretation has been questioned following recent experimental studies, which have, with a guidance from theoretical calculations, led to the reorganization of the level schemes of 110,112Cd in terms of rotational excitations, suggesting the presence of a variety of shapes in these nuclei. Thanks to a recent PhD work in our group, this new interpretation has been extended to the 106Cd nucleus. However, questions remain regarding the nature of certain low-lying excited states in this nucleus. In particular, we obtained indications that some excited states may result from a coupling between the so-called octupole (i.e. the nucleus deforms into a pear shape) and quadrupole (i.e. the nucleus oscillates between elongated and flattened shapes) vibrations. To test this hypothesis, a high-precision beta-decay experiment has been proposed at TRIUMF (Vancouver, Canada) using the world's most advanced spectrometer for beta-decay measurements GRIFFIN, to search for weak decay paths in the 106Cd level scheme, and to unambiguously determine the spins of the excited states through the analysis of gamma-gamma angular correlations. Thanks to this measurement it will be possible to solve multiple puzzles concerning the structure of this nucleus, in particular regarding the possible triaxiality of its ground state and the suspected coexistence of multiple shapes.
The student will be in charge of the analysis of this experiment, which will take place in 2025. Then, based on the results of this analysis, they will proceed to a re-evaluation of the population cross sections of excited levels in 106Cd, which were measured with the new generation gamma-ray spectrometer AGATA at GANIL using the Coulomb excitation technique. From this combination of measurements, we hope to obtain, for the first time in the nuclear chart, the complete set of transition probabilities between the states resulting from the coupling between octupole and quadrupole vibrations. We will then proceed to the interpretation of the results in close collaboration with experts in nuclear-structure theory.
This thesis work will make it possible for the student to follow a research project in its entirety, from the preparation of the experiment to its theoretical interpretation, and to become familiar with several experimental gamma-ray spectroscopy techniques, using the most advanced gamma-ray spectrometers in the world.

Protons and neutrons are made of partons (quarks and gluons) that interact via the strong force, governed by Quantum Chromodynamics (QCD). While QCD can be computed at high energies, its complexity reveals itself at low energies, requiring experimental inputs to understand nucleon properties like their mass and spin. The experimental extraction of the Generalized Parton Distributions (GPDs), which describe the correlation of the partons longitudinal momenta and transverse positions within nucleons, provide critical insights into these fundamental properties.
This thesis focuses on analyzing data from the CLAS12 detector, an experiment part of Jefferson Lab's research infrastructure, one the 17 National Laboratory in the USA. CLAS12, a 15-meter-long fixed-target detector with large acceptance, is dedicated to hadronic physics, particularly GPDs extraction. The selected student will study the exclusive photoproduction of the phi meson (gamma p->phi p’), which is sensitive to gluon GPDs, still largely unexplored. The student will develop a framework to study this reaction in the leptonic decay channel (phi -> e+e-) and develop a novel Graph Neural Network-based algorithm to enhance the scattered proton detection efficiency.
The thesis will aim at extracting the cross section of the photoproduction of the phi, and interpret it in term of the proton's internal mass distribution. Hosted at the Laboratory of Nucleon Structure (LSN) at CEA/Irfu in Saclay, this project involves international collaboration within the CLAS collaboration, travel to Jefferson Lab for data collection, and presentations at conferences. Proficiency in particle physics, programming (C++/Python), and English is required. Basic knowledge of particle detectors and Mahine Learning is advantageous but not mandatory.

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+ to 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).

The thesis project has two distinct experimental parts: First, we store bare (fully-stripped) ions in their excited 0+ state in the heavy-ion storage ring (ESR) at the GSI facility to search for the double-gamma decay in several nuclides. 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 is identified by so-called time-resolved Schottky Mass Spectroscopy. This method allows to distinguish the isomer and the ground 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. Successful experiment establishing the double-gamma decay in several nuclides (72Ge, 98Mo, 98Zr) were already performed and a new experiment has been accepted by the GSI Programme Committee and its realization is planned for 2025.

The second part concerns the direct observation of the emitted photons using gamma-ray spectroscopy. While the storage ring experiments allow to measure the partial lifetime for the double gamma decay, further information on the nuclear properties can be only be achieved by measuring the photon themselves. A test experiment has been performed to study its feasibility and the plans a more detailed study should be developed with the PhD project.

Heavy-ion collisions provide a unique opportunity to study the quark-gluon plasma (QGP), an exotic state of matter where quarks and gluons are no longer confined within hadrons and believed to have existed just a few microseconds after the Big Bang. Charm quarks are among the key probes for investigating the QGP. Indeed, they retain information about their interactions with the QGP, making them essential for understanding the properties of the plasma. The production of charm quarks and their interactions with the QGP is studied through the measurements of hadrons, mesons and baryons, containing at least one charm quark or antiquark, like D0 mesons or Lambda_c baryons. However, the hadronization process—how charm quarks become confined within colorless baryons or mesons—remains poorly understood.

A promising approach to gaining deeper insights into charm hadronization is to measure the elliptic flow of charm hadrons, which refers to long-range angular correlations and is a signature of collective effects due to thermalization. By comparing the elliptic flow of D0 mesons and Lambda_c baryons, researchers can better understand the charm hadronization mechanism, which is sensitive to the properties of the created medium.

To measure elliptic flow, the selected student will develop an innovative method that leverages the full capabilities of the detector. This method, which has never been applied before, provides a more intuitive and theoretically sound interpretation of the results. The candidate will adapt this technique for use with the LHCb detector to measure, compare, and interpret the elliptic flow of Lambda_c charm baryons and D0 mesons with the PbPb samples collected by LHCb in 2024.

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

This proposal focuses on enhancing tracking performance for the LHCb experiments during Run 5 at the Large Hadron Collider (LHC) through the exploration of various machine learning-based algorithms. The Upstream Tracker (UT) sub-detector, a crucial component of the LHCb tracking system, plays a vital role in reducing the fake track rate by filtering out incorrectly reconstructed tracks early in the reconstruction process. As the LHCb detector investigates rare particle decays, studies CP violation in the Standard Model, and study the Quark-Gluon plasma in PbPb collisions, precise tracking becomes increasingly important.

With upcoming upgrades planned for 2035 and the anticipated increase in data rates, traditional tracking methods may struggle to meet the computational demands, especially in nucleus-nucleus collisions where thousands of particles are produced. Our project will investigate a range of machine learning techniques, including those already demonstrated in the LHCb’s Vertex Locator (VELO), to enhance the tracking performance of the UT. By applying diverse methods, we aim to improve early-stage track reconstruction, increase efficiency, and decrease the fake track rate. Among these techniques, Graph Neural Networks (GNNs) are a particularly promising option, as they can exploit spatial and temporal correlations in detector hits to improve tracking accuracy and reduce computational burdens.

This exploration of new methods will involve development work tailored to the specific hardware selected for deployment, whether it be GPUs, CPUs, or FPGAs, all part of the futur LHCb’s data architecture. We will benchmark these algorithms against current tracking methods to quantify improvements in performance, scalability, and computational efficiency. Additionally, we plan to integrate the most effective algorithms into the LHCb software framework to ensure compatibility with existing data pipelines.