Division of Electronics, Detectors and Computing for Physics (DEDIP)

In 1975, the CEA identified isotopic anomalies in uranium ore at the Oklo site in Gabon. These anomalies were quickly attributed to the presence of natural nuclear reactors that had been in operation for about 2 billion years.

Independently, and more recently, a team of researchers led by A. El Albani discovered in the vicinity of the Oklo site fossils of living organisms also dated to about 2 billion years ago. The size, structure, and demonstration of motility of the associated organisms seem to point to a eukaryotic type of cellular organization.

This doctoral work proposes to establish a link between the two events, by postulating that the uraniferous environment pre-existing the reactors, or the reactors themselves, could have contributed to the appearance of these organisms or their ascending filiation by a mechanism of stimulation of the genetic mutations induced by the ionizing radiations of this environment (dosimetric environment).

Recent developments in quantum computing open opportunities to explore alternative methods for tackling complex problems that could not be solved using classical computers. An accurate solution of nuclear physics many-body problems typically requires sophisticated numerical codes and considerable computational resources. In this context, the proposed PhD thesis aims to explore the use of Rydberg atoms quantum processors for the description of many-fermion systems, with a focus on nuclear physics.



On the one hand, quantum computing algorithms tailored to treat fermions interacting with each other are being developed [1]. While implementing such algorithms on quantum emulators is becoming standard, testing their effectiveness on real quantum machines remains an open issue.



On the other hand, quantum processing units (QPUs) based on manipulating neutral atoms have been put forward as one of the most promising technologies for quantum simulations [2]. Such machines are now becoming available for fundamental research applications, thus offering the opportunity to perform real calculations and gauge the perspectives of so-called Noisy Intermediate Scale Quantum (NISQ) technology.



This project aims to combine these state-of-the-art developments with the following goals:



(a) Implement existing or new quantum algorithms on the neutral atoms QPUs, exploring the different possibilities offered by the machine and respecting/exploiting the specificities of nuclear systems (e.g., the use of symmetry breaking and restoration, understanding entanglement and phase-transition in the description of many-body states [3]).



(b) Demonstrate the usefulness of the Rydberg atoms quantum processor for solving real-world many-body physics problems. This will involve selecting a set of pilot applications that represent typical problems in the field and applying the developed algorithms to solve them. The student will need to compare the results obtained using the quantum computer with those obtained using traditional approaches to validate the effectiveness of the proposed methods.



[1] T. Ayral et al., arXiv :2303.04850 (2023).

[2] L. Henriet et al., Quantum 4, 327 (2020).

[3] D. Lacroix et al, Eur. Phys. J. A 59:3 (2023).

# Overview



Heavy-ion collisions are the golden system to study the quark-gluon plasma (QGP), an exotic state of matter that presumably existed few microseconds after the Big Bang. Among the probes to study the QGP, the production of hadrons containing charm quark (e.g D0 mesons) is one of the historical smoking guns. Indeed, being produced in the very first stages of the collisions, these particles keep track of their subsequent interactions with the QGP. In particular, the study of simultaneous double charm production, never carried out in ion-ion collisions at the LHC, could shed a light on the transport properties of the QGP.



At the end of 2023, the LHCb collaboration will record high energy Pb-Pb collisions at the LHC. These data will benefit from the latest LHCb upgrades which offer enhanced detector capabilities. Notably, the new tracking system of this heavy-quark dedicated detector will allow us to cope with the high detector occupancy in ion-ion collisions. This combined with an increase of the data sample by a factor of two will provide the best Pb-Pb data recorded by the collaboration so far.



Finally, a new upgrade phase of the detector is scheduled for 2030 for which new tracker projects are developed. Among the new detectors, the Upstream Tracker (UT) is the key to reduce the rate of fake tracks reconstructed in head-on ion-ion collisions and ensure good data quality for LHCb heavy-ion program. The development of the UT has started, but the final design is still to be defined. In addition, sophisticated tracking algorithms based on machine learning could be developed to fully exploit the detector future capabilities. Not only this work could improve further the LHCb tracking efficiency in ion data, but also in regular proton-proton collisions, which is at the heart of the collaboration physics program.



# Research project



The PhD research project is divided in two main parts:

- Study of the double charm cross-section in Pb-Pb data with LHCb: this objective implies the participation to the 2023 ion data-taking, the extraction of the signal in these data, and the study of uncertainties using simulations. The study could follow what was done previously by the collaboration with other data sets, but more specific studies to ion-ion collisions will be done in close relationship with the theory community.

- Participation to the UT project: this objective focuses on the development of a new tracking algorithm. In particular, the usage of the Graph Neural Network to the LHCb tracking strategy will be explored to improve both efficiency and computing performance.

The study of so-called 'deformed' atomic nuclei with a non-spherical charge distribution is essential for testing nuclear interactions and structure models. Almost all nuclei have an intrinsic prolate (elongated) shape and very few are oblate (flattened). A very small number of nuclei exhibit coexistence of shapes (e.g. prolate-oblate), a phenomenon allowed by the quantum nature of the atomic nucleus. One of the research themes of the nucleus structure group of the DPhN (Departement de Physique Nucléaire) is to search for these nuclei within the Segrè map in order to study them and characterise their shape.



Experiments will be performed with the European germanium detector array AGATA. The unprecedented efficiency and resolution of this new detector will permit spectroscopic studies further away from the valley of stability than previously possible. The Nuclear Structure Group is strongly involved in the exploitation of AGATA with a particular focus on the study of shape coexistence.

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 early 2024.

Plutonium is produced in reactors using uranium fuel. Plutonium-239 is produced by neutron capture on uranium-238 then double ß decay (U-238(n,γ)U-239 --> Np-239 --> Pu-239). Plutonium-240 and plutonium-241 are produced by successive captures on plutonium-239. At the end of the cycle, when the fuel is spent, the fissile isotopes of plutonium (Pu-239 and Pu-241) contribute significantly to energy production. In the case of an innovative reactor using plutonium fuel, the contribution of Pu-241 is important from the beginning of the cycle. Pu-241 is not well known because of the difficulties inherent to its study, on the one hand because of its short half-life (~14 years) and on the other hand because of its decay into Am-241 whose very high capture cross-section disturbs the measurement. The Nuclear Energy Agency therefore recommends to improve the accuracy of the capture and fission cross sections of Pu-241.

The capture cross section of Pu-241 is about 4 times lower than the fission cross section in the energy range of interest. In order to realize an accurate measurement of its capture it is thus necessary to develop a device for the detection of gammas and identification of those coming from the fission. In the proposed experiment at the CERN neutron source n_TOF, gammas from the (n,γ) and (n,f) reactions are detected by a 4pi calorimeter (TAC - Total Absorption Calorimeter) while fission events are identified by a fission chamber (CaF) containing Pu-241 samples placed in the center of the TAC. This measurement will significantly improve the accuracy of the Pu-241 capture cross section while providing additional information on the fission reaction (prompt gammas and cross section).

The PhD student will contribute to the preparation and the realization of the experiment, as well as to the analysis of the results: simulations of the device (TAC + CaF + Pu samples), specification of the samples characteristics, development and tests of the fission chamber, implementation and tuning of the device at n_TOF, data taking, data reduction and analysis of the fission data (prompt gammas and effective cross section).

Hadronic matter is composed of fundamental particles called partons (quarks and gluons) and their interactions are described by Quantum Chromodynamics (QCD). Understanding and describing hadronic internal structure is one of the key challenges of nuclear physics. Despite a good description of the dynamics of quarks and gluons at high energy, several elementary hadronic properties (such as mass and spin) cannot be explained through their components with QCD calculations. Phenomenological approaches are therefore required as a theoretical framework to interpret experimental observations. These are the Generalized Parton Distributions (GPDs) and Transverse Momentum distributions (TMDs). These functions offer a 3D description of the nucleon as they give access to the spatial and momentum distributions of quarks and gluons and to parton contribution to the spin of the nucleon.

These distributions are experimentally accessible through electron scattering off proton and neutron (at the CEBAF accelerator in Jefferson Laboratory and the COMPASS experiment at CERN). One can also study it in polarized proton collisions with the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory.

The new detector sPHENIX is being assembled at RHIC and first collisions are scheduled for Spring 2023. About 350 researchers work around this 1000-ton apparatus. The physics program aims at understanding matter and the strong interaction. It covers both heavy-ion physics and matter deconfinement and the study of nucleon internal structure. Data taking will take place between 2023 and 2025 with pp, p-Au and Au-Au collisions at vsnn=200GeV.

The student will be involved in data taking and data analysis of the sPHENIX experiment. The main goal is the study of transverse momentum distributions of partons inside the proton. These results will contribute to deepen our understanding of nucleon structure and parton confinement.



In 2030 a new collider will be operational at RHIC: the electron-ion collider (EIC). The facility should give answers some of the most fundamental questions in nuclear physics. It will give access to a largely unexplored area: the limit of saturation for gluon density and will provide remarkable conditions to study the structure of the nucleon and the effect of a nuclear environment on the dynamics of quarks and gluons.

The CEA is involved in physics simulations and the development of innovative gaseous detectors. Part of the thesis will be dedicated to the study of several prototypes.



The thesis will be hosted by the Laboratory of the Nucleon Structure (Laboratoire de Structure du Nucleon, LSN) composed of several physicists, both some theoreticians and some experimentalists.



The student is expected to be fluent in English to work in the context of a large international scientific collaboration. He/she will have to show interest in detector hardware and software programming (C++).

Several trips should be anticipated, in particular to the United States.

CEA/Irfu staff members are among the principal investigators of ongoing experiments at the Jefferson Lab (JLab) in USA, where a high current electron beam up to 11 GeV in energy collides with fixed targets of several types. The high luminosity available at the JLab allows the study of the properties of the nucleons with high statistical accuracy also via rare processes.

Contrary to the naive expectations, it has been shown that not the valence quarks, but rather the gluons carry the major contribution to the mass and the spin of the nucleons. Therefore, it is crucial to precisely characterize gluons distributions in order to fully understand the strong interactions from which results the protons. In particular, the current knowledge of the GPDs of gluons is rather limited. GPDs are accessible through the study of exclusive processes where all the final state particles are detected, and specifically, gluon GPDs can be accessed via the study of the exclusive electroproduction of the ?-meson. This year, data are being collected with a longitudinally polarized target of protons, providing a unique opportunity to understand the correlation between the spin of the proton and the gluons. The goal of this thesis will be to analyze the data taken with the CLAS12 experiment at the Jefferson Lab to extract target-spin, beam-spin and double-spin asymmetries. The future PhD student will have the opportunity to add a side activity to the data analysis, the choice spanning from detector development to detailed phenomenological studies.