From Few-body to High-Energy antinuclei Collision Kinematics

Because rare antinuclei in space could carry information about exotic production mechanisms—including, potentially, dark-matter annihilation or decay—their study has become a high-impact frontier connecting nuclear physics, astroparticle physics, and collider measurements. Interpreting present and future antinuclei searches, however, is limited by a lack of key nuclear input data: low-energy scattering, annihilation, and breakup processes of antinuclei on ordinary matter are difficult to measure directly, precisely because producing and manipulating antinuclei is so challenging. This motivates a complementary, theory-driven strategy. Our project adopts a bottom-up approach: we will establish a controlled, ab initio description of the simplest low-energy antimatter nuclear systems and collisions, identify the underlying many-body mechanisms of annihilation, and then propagate these constraints to transport and event-level modeling at the many-body and higher-energy scales. In doing so, we aim to both deepen our understanding of matter–antimatter interactions at the nuclear level and deliver validated inputs for the simulation tools used in astroparticle and collider applications.
Two-way transfer between the two fields: In this project, we simplify the problem to the simplest case that can be treated by the ab initio method: in INCL the annihilation of the antideuteron is identified as an annihilation with a quasi-deuteron in a large target. Two key questions must be addressed in part using ab initio calculations:
1. Which quasi-deuteron will interact?
2. Which output channel will result?

Elliptic Flow of Charmed Hadrons in Heavy-Ion Collisions at LHCb?

The FLOALESCENCE project explores one of the most fundamental questions in Quantum Chromodynamics (QCD): how quarks and gluons transition from a deconfined Quark–Gluon Plasma (QGP) into ordinary hadrons.?This transition, called hadronization, occurred microseconds after the Big Bang and can be recreated today in ultra-relativistic lead–lead collisions at CERN’s Large Hadron Collider (LHC).
The PhD will focus on charm quarks—excellent probes of the QGP because they are produced early in the collision and interact throughout its evolution. Using the LHCb detector, uniquely sensitive in the forward rapidity region, the project aims to measure the elliptic flow (v2) of charmed baryons (?c+) and mesons (D0) in Pb–Pb collisions.?The goal is to test whether these heavy quarks thermalize and hadronize through a coalescence mechanism, a key feature of QGP dynamics.

Objectives and tasks:
- Extract and analyze ?c+ and D0 signals in newly collected 2024–2025 Pb–Pb datasets at LHCb.
- Implement a novel flow analysis method (based on the reformulated Lee–Yang Zeros approach) for the first time at LHCb.
- Develop an event-by-event multiplicity metric to correlate flow with system energy density.
- Compare results to theoretical models and cross-check with measurements at central rapidity (ALICE).
- Publish results and present findings at international conferences.

The successful candidate will:
- Develop advanced data-analysis expertise with CERN’s LHCb software framework, ROOT, and machine learning–based signal extraction.
- Gain in-depth knowledge of QCD and relativistic heavy-ion physics, especially QGP properties and collective phenomena.
- Learn modern statistical methods for flow analysis and uncertainty estimation.
- Acquire collaborative and communication skills within a major international experiment (LHCb), including presentations in collaboration meetings and conferences.
- Build strong experience in scientific computing, big-data handling, and detector physics, valuable for both academic and industry careers.

Study of uranium-235 fission induced by neutrons from 0.5 to 40 MeV at NFS-SPIRAL2 using the FALSTAFF spectrometer and the FIFRELIN code

The presented project has two main objectives. The first one is the realization (building, calibration, data taking and data analysis) of a first experiment with the FALSTAFF detector in its configuration with two detection arms. In such a configuration, FALSTAFF will be able to detect in coincidence both fragments emitted by fast-neutron triggered fission reactions. These neutrons will be provided by the neutron beam of SPIRAL2-NFS in GANIL. The advantage of using direct kinematics is the ability to determine on an event-by-event basis the excitation energy of the fissioning nucleus by the measurement of the incident-neutron kinetic energy.
For this first experiment, we will have a uranium 235 target. 235U is the main source of fission neutrons in nuclear reactors and therefore at the heart of the system. Hence, the understanding of neutron-induced fission of 235U is essential and the rather exclusive data FALSTAFF will provide, with not only the identification of the fission fragments but also their kinematics will permit to reconstruct also the fissioning system. Such a measurmement in direct kinematics have never been done, to our knowledge, with the accuracy we are aiming at.
To perform this exepriment, we have improved and added detection capabilities to the FALSTAFF spectrometer, in particular with the financial support of the Région Normandie over the last two years. This experiment will be completed by a work to be done on a theoretical model developed by our collaborators of CEA-Cadarache. We will compare our detailled data with predictions of the model and have the model evolve, according to the laws of nuclear physics in order to obtain results from the model close to the data. Such a test of this model on as complete data as those we will obtain with FALSTAFF have never been done so far.

Stochastic Neutron Noise Estimation Using a Rare-Event Simulation Approach. Application to the Monitoring of Nuclear System Reactivity

This PhD project aims to develop an innovative method to characterize the reactivity of fissile systems by analyzing their stochastic fluctuations, known as zero-power neutron noise. In a subcritical fissile medium, neutrons originating from spontaneous fission can initiate short and random chain reactions, generating a fluctuating signal. This noise carries essential information on the distance of the system to criticality, a key parameter both for the safety of nuclear installations (prevention of criticality accidents) and for the detection of undeclared fissile materials (nuclear security and non-proliferation).

Existing theoretical approaches to infer system reactivity from neutron noise are limited to idealized situations and become unsuitable in realistic configurations, particularly when the system is strongly subcritical or when significant uncertainties exist regarding its geometry or composition (as in the case of the Fukushima Daiichi corium or spent fuel storage). Monte Carlo simulations then appear as a natural alternative, but current simulations rely on variance reduction techniques that fail to correctly preserve stochastic fluctuations.

This thesis proposes to address this scientific challenge by adapting a relatively recent variance reduction method known as Adaptive Multilevel Splitting (AMS), originally developed to efficiently sample rare events while preserving their statistical properties. The goal is to extend this method to neutron transport in multiplying media and to make it a tool capable of faithfully simulating the temporal correlations characteristic of neutron noise. Following the theoretical developments, the algorithm will be implemented in Geant4, compared to analytical benchmark solutions, and experimentally validated through in situ measurements (using neutron sources or research reactors). In the long term, this work may lead to direct applications in nuclear monitoring, safety diagnostics, and detector physics, while also opening perspectives in fundamental physics and medical physics.

Designing a hybrid CPU-GPU estimator for neutron transport: Advancing eco-efficient Monte Carlo simulations

Digital twins incorporating Monte Carlo simulation models are currently being developed for the design, operation, and decommissioning of nuclear facilities. These twins are capable of predicting physical quantities such as particle fluxes, gamma/neutron heating, and dose equivalent rates. However, the Monte Carlo method presents a major drawback: high computational time to achieve acceptable variance levels.
To enhance simulation efficiency, the eTLE estimator has been developed and integrated into the TRIPOLI-4® Monte Carlo code. Compared to the conventional TLE (Track Length Estimator), eTLE offers lower theoretical variance, particularly in highly absorbing media, by contributing to the detector response even when particles do not physically reach it. Nevertheless, its computational cost remains significant, especially when evaluating multiple detectors.
Two recent PhD works have proposed variants to overcome this limitation. The Forced Detection eTLE- (Guadagni, EPJ Plus 2021) employs preferential sampling that directs pseudo-particles toward the detector at each collision. It is particularly effective for small detectors and configurations with moderate shielding, especially for fast neutrons. The Split Exponential TLE (Hutinet & Antonsanti, EPJ Web 2024) is based on an asynchronous GPU approach, offloading straight-line particle transport to the graphics processor. Through multiple sampling, it maximizes GPU utilization and enables more efficient exploration of phase space.
The proposed thesis aims to combine these two approaches into a hybrid estimator named seTLE-DF. This new estimator could be used either directly or to generate importance maps without relying on auxiliary deterministic calculations. Its implementation will require dedicated GPU developments, particularly to optimize the geometry library and memory management in complex geometries.
This research topic aligns with green computing objectives, aiming to reduce the carbon footprint of high-performance computing. It relies on a hybrid CPU-GPU strategy, avoiding full porting of the Monte Carlo code to GPU. Solutions such as half-precision formats will be considered, and an energy impact assessment will be conducted before and after implementation. The future PhD student will be welcomed with the IRESNE Institute (CEA Cadarache)and will acquire strong expertise in neutron transport simulation, facilitating integration into major research institutions or companies within the nuclear sector.

Characterization and calibration of cryogenic detectors at the 100 eV scale for the detection of coherent neutrino scattering (CEvNS)

DESCRIPTIONS:

The NUCLEUS experiment [1] aims to detect reactor neutrinos via coherent elastic neutrino–nucleus scattering (CEvNS). Predicted in 1974 and first observed in 2017, this process provides a unique opportunity to test the Standard Model at low energies. Because the scattering is coherent over the entire nucleus, the cross section is enhanced by several orders of magnitude, making CEvNS also promising for reactor monitoring using neutrinos.

The NUCLEUS experimental setup is currently being installed near the EDF nuclear reactors in Chooz (Ardennes, France), which constitute an intense neutrino source. The only physical signal of a CEvNS event is the tiny recoil of the target nucleus, with an energy below 1 keV. To detect this, NUCLEUS uses CaWO4 crystals of about 1 g, placed in a cryostat cooled to 15 mK. The nuclear recoil produces vibrations in the crystal lattice, equivalent to a temperature rise of about 100 µK, measured with a Transition Edge Sensor (TES) deposited on the crystal. These detectors achieve excellent energy resolutions of only a few eV and detection thresholds on the order of ~10 eV [2]. The NUCLEUS setup was successfully tested and validated in 2024 at TU Munich [3], and data taking at Chooz is scheduled to start in summer 2026, simultaneously with the beginning of the PhD. An initial contribution will involve data acquisition and analysis at the reactor site. More specifically, the PhD student will be responsible for the characterization of the deployed cryogenic CaWO4 detectors — stability, energy resolution, calibration, and intrinsic background of the crystal.

Calibration at the sub-keV scale is a crucial challenge for CEvNS (and dark matter) experiments. Until recently, it was extremely difficult to generate nuclear recoils of known energy to characterize detector responses. The CRAB method [4, 5] addresses this issue by using thermal neutron capture (25 meV) on nuclei that constitute the cryogenic detector. The resulting compound nucleus has a well-known excitation energy — the neutron separation energy — between 5 and 8 MeV, depending on the isotope. When it de-excites by emitting a single gamma photon, the nucleus recoils with a precisely determined energy given by two-body kinematics. A calibration peak in the desired energy range of a few hundred eV then appears in the detector’s energy spectrum. A first measurement in 2022, using a NUCLEUS CaWO4 detector and a commercial ²5²Cf neutron source, validated this method [6].

The second part of the PhD will take place within the “high-precision” phase of the project, which consists in performing measurements with a pure thermal neutron beam from the TRIGA-Mark-II reactor in Vienna (TU Wien, Austria). The calibration setup was successfully installed and characterized in 2025 [7]. It consists of a cryostat housing the cryogenic detectors to be characterized, surrounded by large BaF2 crystals for coincidence detection of the nuclear recoil and the gamma ray that induced it. The whole setup is placed directly on the neutron beam axis, which provides a flux of about 450 n/cm²/s. This coincidence technique will significantly reduce background and extend the CRAB method to a wider energy range and to materials used in most cryogenic detectors. These measurements are expected to provide a unique characterization of the response of cryogenic detectors in the energy region of interest for light dark matter searches and coherent neutrino scattering. In parallel with the measurement of nuclear recoils, the installation of a low-energy X-ray source in the cryostat will generate electronic recoils, enabling a direct comparison between the detector responses to sub-keV energy deposits produced by nuclear and electronic recoils.

The arrival of the PhD student will coincide with the completion of the measurement program on CaWO4 and Al2O3 detectors of NUCLEUS and with the start of the measurement programs on Ge (TESSERACT project) and Si (BULLKID project) detectors.
The high-precision measurements will also open a new sensitivity window to subtle effects coupling nuclear physics(nuclear de-excitation times) and solid-state physics (nuclear recoil times in matter, and the creation of crystal defects induced by nuclear recoils) [8].

The PhD student will be deeply involved in all aspects of the experiment: simulation, data analysis, and interpretation of the obtained results.

WORK PLAN:

The PhD student will actively participate in data taking and in the analysis of the first results from the NUCLEUS cryogenic CaWO4 detectors at Chooz. This work will be carried out in collaboration with the Nuclear Physics Department (DPhN), the Particle Physics Department (DPhP) of CEA-Saclay, and the TU Munich team. It will begin with familiarization with the CAIT analysis framework used for cryogenic detectors. The student will focus in particular on detector calibration, studying the detector response to electronic recoils induced by optical photon pulses injected through fibers and by X-ray fluorescence generated by cosmic rays. Once this calibration is established, two types of backgrounds will be investigated: Nuclear recoils in the keV range induced by cosmogenic fast neutrons, and a low-energy background, known as the Low Energy Excess (LEE), intrinsic to the detector.
The comparison between the experimental and simulated fast neutron background spectra will be analyzed in light of the differences between nuclear and electronic recoil responses measured in the CRAB project. The long data-taking periods at the Chooz site will also be used to study the time evolution of the LEE background. This work will be conducted in collaboration with solid-state physics experts from the Institute for Applied Sciences and Simulation (CEA/ISAS) to better understand the origin of the LEE, which remains a major open question in the cryogenic detector community.
The analysis skills acquired on NUCLEUS will then be applied to the high-precision CRAB measurement campaigns planned for 2027 at the TRIGA reactor (TU Wien) with Ge and Si detectors. The student will be deeply involved in the setup, data acquisition, and analysis of results. The planned measurements on germanium, using both phonon and ionization channels, have the potential to resolve the current ambiguity in the ionization yield of low-energy nuclear recoils, a key factor for the sensitivity of future experiments.
The high calibration precision will also be exploited to study fine effects in nuclear and solid-state physics, such as timing effects and crystal defect formation induced by nuclear recoils in the detector. This study will be conducted in synergy with teams from CEA/IRESNE and CEA/ISAS, who provide detailed simulations of nuclear de-excitation gamma cascades and molecular dynamics simulations of nuclear recoil propagation in matter.

Through this work, the student will receive comprehensive training as an experimental physicist, including strong components in simulation and data analysis, as well as hands-on experience with cryogenic techniques during the commissioning of the NUCLEUS and CRAB detectors. The proposed contributions are expected to lead to several publications during the PhD, with high visibility in the CEvNS and dark matter communities. Within the CEA, the student will also benefit from the exceptionally cross-disciplinary nature of this project, which already
fosters regular interaction among the communities of nuclear physics, particle physics and condensed matter physics.

COLLABORATIONS:

NUCLEUS: Germany (TU-Munich, MPP), Austria (HEPHY, TU-Wien), Italy (INFN), France (CEA-Saclay).
CRAB: Germany (TU-Munich, MPP), Austria (HEPHY, TU-Wien), Italy (INFN), France (CEA-Saclay, CNRS-IJCLab, CNRS-IP2I, CNRS-LPSC).

BIBLIOGRAPHY:

[1] NUCLEUS Collaboration, Exploring CE?NS with NUCLEUS at the Chooz nuclear power plant, The European Physical Journal C 79 (2019) 1018.
15, 48, 160, 174
[2] R. Strauss et al., Gram-scale cryogenic calorimeters for rare-event searches, Phys. Rev. D 96 (2017) 022009. 16, 18, 78, 174
[3] H. Abele et al., Particle background characterization and prediction for the NUCLEUS reactor CE?NS experiment, https://arxiv.org/abs/2509.03559
[4] L. Thulliez, D. Lhuillier et al. Calibration of nuclear recoils at the 100 eV scale using neutron capture, JINST 16 (2021) 07, P07032
(https://arxiv.org/abs/2011.13803)
[5]https://irfu.cea.fr/dphp/Phocea/Vie_des_labos/Ast/ast.php?id_ast=4970
[6] H. Abele et al., Observation of a nuclear recoil peak at the 100 eV scale induced by neutron capture, Phys. Rev. Lett. 130, 211802 (2023) (https://arxiv.org/abs/2211.03631)
[7] H.Abele et al., The CRAB facility at the TUWien TRIGA reactor: status and related physics program, (https://arxiv.org/abs/2505.15227)
[8] G. Soum-Sidikov et al., Study of collision and ?-cascade times following neutron-capture processes in cryogenic detectors Phys. Rev. D
108, 072009 (2023) (https://arxiv.org/abs/2305.10139)

Dimensionality reduction method applied to the deformed coupled cluster ab initio many-body method

The theoretical description from first principles, i.e. in a so-called ab initio manner, of atomic nuclei containing more than 12 nucleons has only recently become possible thanks to the crucial developments in many-body theory and the availability of increasingly powerful high-performance computers. These ab initio techniques are successfully applied to study the structure of nuclei, starting from the lightest isotopes and now reaching all medium-mass nuclei containing up to about 80 nucleons. The extension to even heavier systems requires decisive advances in terms of storage cost and computation time induced by available many-body methods. In this context, the objective of the thesis is to develop the dimensionality reduction method based on the factorization of tensors involved in the non-perturbative many-body theory known as deformed coupled cluster (dCC). The proposed work will exploit the latest advances in nuclear theory, including the use of nuclear potentials from chiral effective field theory and renormalization group techniques, as well as high-performance computing resources and codes.

Exotic shape of the nucleus: decay spectroscopy of neutron-deficient actinides with the detector SEASON

The question of the limit of stability of nuclei, both in terms of proton/neutron asymmetry and in terms of mass, is an important open question in modern nuclear physics. In the region of heavy nuclei, the neutron-deficient actinides present a great interest. Indeed, strong octupolar deformation, giving a pear shape to the nuclei, are predicted and have event been already observed in some isotopes. These deformations seem to play a key role for nuclear stability, for nuclear decay modes, and may also be related to physics beyond the standard model. The main goal oh this thesis will be to pursue the systematic study of these deformations by making use of the brand-new SEASON detector, whose first experiment will take place at the University of Jyväskylä (Finland) in February 2026. The thesis will focus on the analysis of data from the experimental campaign that will occur in summer 2026. Several experiments are foreseen, making use of different beam-target combinations to produce actinides by fusion-evaporation reaction. These actinides will then be sent inside SEASON to perform their decay spectroscopy. Depending on the plannings, another campaign could be scheduled at Jyväskylä in 2027. Finally, the return of the instrument in France to be set up at GANIL-Spiral2 (Caen) coupled to the S3 spectrometer will certainly take place this the thesis period.
The thesis can be co-directed by the university of Jyväskylä.

Monitoring criticality risk through neutron noise in degraded nuclear environments

Our team at CEA/Irfu is working with ASNR to study the possibility of using neutron noise measurements, i.e., stochastic variations in neutron flux, to estimate the reactivity of subcritical nuclear systems. The aim is to propose this technique for online measurement of the reactivity of the corium at Fukushima Daiichi during future decommissioning operations. The thesis work will focus on evaluating a solution based on Micromegas-type neutron detectors (nBLM detectors) developed by IRFU, which are adapted to the extreme gamma radiation expected in the vicinity of the Fukushima Daiichi corium. The student will participate in experiments at nuclear research facilities in Europe and the United States to test this technical solution and measure neutron noise for a wide range of reactivities. He/she will be responsible for analyzing the data and evaluating the various inversion methods used to estimate reactivity from neutron noise measurements.

Development of the Micromegas CyMBaL Detector and study of gluon saturation for the future electron-ion collider

The future Electron-Ion Collider (EIC), to be constructed at Brookhaven National Laboratory (NY, USA) is a next-generation facility designed to explore the inner structure of protons and nuclei with unprecedented precision. It will explore how quarks and gluons generate the mass, spin, and structure of visible matter, and study the increase of gluon density at small Bjorken-x. To meet its ambitious physics goals, innovative detectors are being developed — including the Micromegas CyMBaL system, a gaseous tracker for the central region of the first EIC experimental apparatus ePIC.
This PhD project combines experimental detector R&D and physics simulations:
* Prototype characterization: build and test full-scale Micromegas detectors; measure efficiency, gain uniformity, and spatial resolution in laboratory and beam environments. Test and validate the prototypes with the new ASIC SALSA developed at CEA for gasesous detectors at ePIC.
* Detector simulations: integrate the CyMBaL geometry into the EIC framework and assess global tracking and performance requirements.
* Physics studies: simulate key processes sensitive to gluon saturation (e.g. final-state di-hadron correlations) to understand QCD at small-x and evaluate how detector performance influences physics sensitivity.
The PhD student will have opportunities to participate in the development of state-of-the-art gaseous detectors and to work within an international community of hadronic physicists on topics at the forefront of the field, with trips to Brookhaven National Laboratory (NY, USA) and opportunities for test-beam campaigns at accelerator facilities.