Precise time tagging and tracking of leptons in Enhanced Neutrino Beams with large area PICOSEC-Micromegas detectors

The ENUBET (Enhanced NeUtrino BEams from kaon Tagging) project aims to develop a monitored neutrino beam with a precisely known flux and flavor composition, enabling percent-level precision in neutrino cross-section measurements. This is achieved by instrumenting the decay tunnel to detect and identify charged leptons from kaon decays.
The PICOSEC Micromegas detector is a fast, double-stage micro-pattern gaseous detector that combines a Cherenkov radiator, a photocathode, and a Micromegas amplification structure. Unlike standard Micromegas, it operates with amplification also occurring in the drift region, where the electric field is even stronger than in the amplification gap. This configuration enables exceptional timing performance, with measured resolutions of about 12 ps for muons and ~45 ps for single photoelectrons, making it one of the fastest gaseous detectors ever developed.
Integrating large-area PICOSEC Micromegas modules in the ENUBET decay tunnel would provide sub-100 ps timing for lepton tagging, improving particle identification, reducing pile-up, and enhancing the association between detected leptons and their parent kaon decays — a key step toward precision-controlled neutrino beams.
Within the framework of this PhD work, the candidate will optimize and characterize 10 × 10 cm² PICOSEC Micromegas prototypes, and contribute to the design and development of larger-area detectors for the nuSCOPE experiment and the ENUBET hadron dump instrumentation.

Machine Learning-Based Algorithms for Real-Time Standalone Tracking in the Upstream Pixel Detector at LHCb

This PhD aims to develop and optimize next-generation track reconstruction capabilities for the LHCb experiment at the Large Hadron Collider (LHC) through the exploration of advanced machine learning (ML) algorithms. The newly installed Upstream Pixel (UP) detector, located upstream of the LHCb magnet, will play a crucial role from Run 5 onward by rapidly identifying track candidates and reducing fake tracks at the earliest stages of reconstruction, particularly in high-occupancy environments.

Achieving fast and highly efficient tracking is essential to fulfill LHCb’s rich physics program, which spans rare decays, CP-violation studies in the Standard Model, and the characterization of the quark–gluon plasma in nucleus–nucleus collisions. However, the increasing event rates and data complexity expected for future data-taking phases will impose major constraints on current tracking algorithms, especially in heavy-ion collisions where thousands of charged particles may be produced per event.

In this context, we will investigate modern ML-based approaches for standalone tracking in the UP detector. Successful applications in the LHCb VELO tracking system already demonstrate the potential of such methods. In particular, Graph Neural Networks (GNNs) are a promising solution for exploiting the geometric correlations between detector hits, allowing for improved tracking efficiency and fake-rate suppression, while maintaining scalability at high multiplicity.

The PhD program will first focus on the development of a realistic GEANT4 simulation of the UP detector to generate ML-suitable datasets and study detector performance. The next phase will consist in designing, training, and benchmarking advanced ML algorithms for standalone tracking, followed by their optimization for real-time GPU-based execution within the Allen trigger and reconstruction framework. The most efficient solutions will be integrated and validated inside the official LHCb software stack, ensuring compatibility with existing data pipelines and direct applicability to Run-5 operation.

Overall, the thesis will provide a major contribution to the real-time reconstruction performance of LHCb, preparing the experiment for the challenges of future high-luminosity and heavy-ion running.

Mining LEP data for fragmentation: A TMD-oriented analysis of pi+pi- pairs in e+e- collisions

This project aims to advance our understanding of quark and gluon fragmentation by performing the first-ever extraction of Transverse-Momentum-Dependent Fragmentation Functions (TMDFFs) for charged pions using archived data from LEP experiments like DELPHI or ALEPH.
Fragmentation Functions, which describe how partons form detectable hadrons, are non-perturbative and must be determined from experimental data. TMDFFs provide more detailed information about the transverse momentum of these hadrons. An ideal process to study them is the production of back-to-back pi+pi- pairs in electron-positron annihilations, a measurement surprisingly absent from both past and current experiments.
The project will leverage CERN OpenData initiative to access this historical data. The work is structured in three key steps: first, overcoming the technical challenge of accessing the data using potentially obsolete software; second, extracting relevant physical distributions, such as the transverse momentum of the pion pairs; and third, using Monte Carlo simulations (e.g., Pythia8) to interpret the results.
A crucial part of the analysis will be to identify the observables most sensitive to TMDFFs through simulations. The final data analysis will employ modern techniques to ensure a robust estimate of all uncertainties. Once completed, this pioneering measurement will be incorporated into a global analysis of TMD data, significantly improving the accuracy of TMDFFs and pushing the boundaries of our knowledge of non-perturbative QCD.

From Detector to Discovery: Constructing the ATLAS Inner Tracker and Probing Higgs Physics at the HL-LHC

This PhD project combines work on the construction of the new Inner Tracker (ITk) for the ATLAS experiment and an analysis of ATLAS sensitivity at the High-Luminosity LHC (HL-LHC) to key processes related to Higgs boson physics using ITk. The candidate will take part in the development, operation, and optimization of the test benches for ITk pixel modules at CEA. Together with two other partner laboratories in the Paris region, CEA will assemble and test about 20% of the ITk pixel modules. The student will contribute to the commissioning of the detector at CERN. The candidate will also carry out HL-LHC sensitivity studies of the interactions between the Higgs boson and the top quark, including for instance a CP-violation analysis in the ttH channel and an analysis of tH production, a process particularly sensitive to the Higgs–top and Higgs–W couplings. The first two years of the PhD are expected to be based at CEA Saclay, while the last year will be based at CERN.

Cosmology with the Lyman-alpha forest from the DESI cosmological survey.

We use the large-scale distribution of matter in the universe to test our cosmological models. This is primarily done using baryon acoustic oscillations (BAO), which are measured in the two-point correlation function of this distribution. However, the entire matter field contains information at various scales, allowing us to better constrain our models than BAO alone. At redshifts greater than 2, the Lyman-alpha forest is the best probe of this matter distribution. The Lyman-alpha forest is a set of absorption lines measured in the spectra of distant sources. The large DESI spectroscopic survey has collected approximately one million of these spectra. Using the partial data set "DR2," we measured the BAO with an accuracy of 0.7%, which strongly constrains the expansion rate of the universe during the first billion years of its evolution.

This thesis aims to exploit the full set of large-scale Lyman-alpha data from DESI to obtain the strongest constraints on cosmological models possible. First, the student will apply a method known as reconstruction to improve the accuracy of BAO measurements by exploiting information from the matter density field. For the remainder of the thesis, the student will implement a new method known as simulation-based inference. Similar efforts have been carried out in our group with DESI galaxies. In this approach, the entire matter field is used directly to estimate cosmological parameters, particularly dark energy. Thus, the student will make an important contribution to DESI's final cosmological measurements with Lyman-alpha.

An internship is preferred before beginning this thesis.

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)

Axion searches in the SuperDAWA experiment with superconducting magnets and microwave radiometry

Axions are hypothetical particles that could both explain a fundamental problem in strong interactions (the conservation of CP symmetry in QCD) and account for a significant fraction of dark matter. Their direct detection is therefore a key challenge in both particle physics and cosmology.

The SuperDAWA experiment, currently under construction at CEA Saclay, uses superconducting magnets and a microwave radiometer placed inside a cryogenic cryostat. This setup aims to convert potential axions into measurable radio waves, with frequencies directly linked to the axion mass.

The proposed PhD will combine numerical modeling with hands-on experimental work. The student will develop a detailed model of the experiment, including magnetic fields, radio signal propagation, and detector electronics, validated step by step with real measurements. Once the experiment is running, the PhD candidate will participate in data-taking campaigns and their analysis.

This project provides a unique opportunity to contribute to a state-of-the-art experiment in experimental physics, with direct implications for the global search for dark matter.

Testing the Standard Model in the Higgs-top sector in a new inclusive way with multiple leptons using the ATLAS detector at the LHC

The LHC collides protons at 13.6 TeV, producing a massive dataset to study rare processes and search for new physics. The production of a Higgs boson in association with a single top quark (tH) in the multi-lepton final state (2 same-sign leptons or 3 charged leptons) is particularly promising, but challenging to analyze due to undetected neutrinos and fake leptons. The tH process is especially interesting because its small Standard Model cross section originates from a subtle destructive interference between diagrams including the Higgs coupling to the W boson and the Higgs coupling to the top quark. This makes tH uniquely sensitive: even small deviations from the Standard Model can strongly enhance its production rate. The measurement of the tH cross section is delicate because the ttH and ttW processes have similar topologies and much larger cross sections, requiring a simultaneous extraction to obtain a reliable result and properly account for correlations between signals. ATLAS observed a moderate excess of tH using the Run 2 dataset (2.8 s), making the analysis of Run 3 data including these correlations crucial. The thesis will first exploit AI algorithms based on Transformer architectures to reconstruct event kinematics and extract observables sensitive to the CP nature of the Higgs-top coupling. In a second phase, a global approach will be adopted to analyze simultaneously the ttW, ttZ, ttH, tH, and 4-top processes, searching for anomalous couplings, including those violating CP symmetry, within the framework of the Standard Model Effective Field Theory (SMEFT). This study will provide the first complete measurement of tH in the multi-lepton channel with Run 3 data and will pave the way for a global analysis of rare processes and anomalous couplings at the LHC in this channel.

Precision measurements of neutrino oscillations and search for CP violation with the T2K and Hyper-Kamiokande experiments

The study of neutrino oscillations has entered a precision era, driven by long-baseline experiments like T2K, which compare neutrino signals at near and far detectors to probe key parameters, including possible Charge-Parity Violation (CPV). Detecting CPV in neutrinos could help explain the Universe’s matter–antimatter asymmetry. T2K’s 2020 results gave first hints of CPV but remain limited by statistics. To improve sensitivity, T2K has undergone major upgrades: replacing the most upstream part of its near detector with a new target, increased accelerator power (up to 800 kW by 2025, aiming for 1.3 MW by 2030). The next-generation Hyper-Kamiokande (Hyper-K) experiment, starting in 2028, will reuse the T2K beam and near detector but with new far detector 8.4 times larger than Super-Kamiokande greatly boosting the statistics. The IRFU group has key role in the near detector upgrade and is now focusing on analysis, crucial for controlling systematic uncertainties crucial for the Hyper-K high statistics time. The proposed PhD work centers on analyzing the new near detector data: designing new sample selections taking into account for the low-momentum protons and neutrons from neutrinos, and refining neutrino–nucleus interaction models to improve energy reconstruction. The second goal is to propagate these improvements to Hyper-K, guiding future oscillation analyses. The student will also contribute to Hyper-K construction and calibration (electronics testing at CERN, installation in Japan).

Search for di-Higgs production in the multilepton channel with the ATLAS detector using 13.6 TeV data

In the Standard Model (SM), the Higgs field is responsible for the breaking of the electroweak symmetry, thereby giving mass to the W and Z bosons. The discovery of the Higgs boson in 2012 at the LHC provided experimental confirmation of the existence of this field. Despite extensive studies, the self-coupling of the Higgs boson remains unmeasured, yet it is crucial for understanding the shape of the Higgs potential and the stability of the universe’s vacuum. Studying Higgs pair production (di-Higgs) is the only direct way to access this parameter, providing key insights into the electroweak phase transition after the Big Bang. Di-Higgs production is extremely rare (cross-section ~40 fb for proton-proton collisions at a centre-of-mass energy of 13.6 TeV), and among its possible final states, the multilepton channel is promising due to its distinctive kinematics, though complex due to diverse topologies and backgrounds. Recent advances in artificial intelligence, particularly transformer-based architectures respecting physical symmetries, have recently significantly improved event reconstruction in complex Higgs channels such as HH?4b or HH?bbtt. Applying these techniques to the multilepton channel offers strong potential to enhance sensitivity. This PhD project will focus on searching for di-Higgs production in the multilepton final state with the full ATLAS Run 3 dataset at 13.6 TeV, leveraging the group’s ongoing ttH multilepton work to develop advanced AI-based reconstruction and analysis methods. The projet aims to approach SM sensitivity for the Higgs self-coupling.