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.
Higgs boson decay into a Z boson and a photon and time resolution of the CMS electromagnetic calorimeter
The thesis focuses on Higgs boson physics, specifically one of its rare and yet unobserved decay channels: the decay into a Z boson and a photon (Zgamma channel). This decay not only complements our understanding of the Higgs boson but also uniquely involves all currently known neutral bosons (Higgs, Z, photon) and is sensitive to potential processes beyond the Standard Model. The final state of the analysis consists of the two lepton decay products from the Z boson (muons or electrons for this study) and a photon. Background events produced by other Standard Model processes that contain two leptons and a photon (or misidentified particles) form the background of the analysis. With all data gathered during LHC Run 2 (2015-2018) and Run 3 (2021-2026), it is possible to have evidence of this decay, that is to observe it with a statistical significance exceeding three standard deviations.
In addition, the thesis includes an instrumental part focused on optimizing the time resolution of the CMS electromagnetic calorimeter (ECAL). Although designed for precise energy measurements, the ECAL also shows excellent timing resolution for photons and electrons (approximately 150 ps in collisions, 70 ps in test beam conditions). In a final state populated by photons from multiple overlapping events (pileup), the arrival time of a photon helps to verify its compatibility with the Higgs boson decay vertex. This will be crucial during the high-luminosity phase of the LHC (2029 onward), when the number of overlapping events is expected to be about three times greater than today. A new readout electronics for the ECAL is being developed and will be installed in the ECAL and CMS during the duration of the thesis. The new electronics achieves a timing resolution of 30 ps for high-energy photons and electrons. This performance was tested in ideal beam conditions (no magnetic fields, no tracker material in front of ECAL, no pileup). The thesis aims to develop algorithms to maintain this performance within CMS.
The thesis work is a continuation of the ongoing Z? analysis within the CMS group at CEA Saclay and the timing performance analysis of the ECAL, where the Saclay group is a leader. Simple, robust, and efficient analysis tools written in modern C++ and leveraging the ROOT analysis framework allow to understand and contribute to every stage of the analysis, from raw data to published results. The CMS Saclay group has leading responsibilities in CMS since its construction, including deep expertise in Higgs physics, electron and photon reconstruction, detector simulation, and machine learning and artificial intelligence techniques.
Regular trips to CERN are proposed for presenting the results of this work to the CMS collaboration and for participating in laboratory tests planned for the new ECAL electronics, as well as for participating to its installation.
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.
Multi-Probe Cosmological Mega-Analysis of the DESI Survey: Standard and Field-Level Bayesian Inference
The large-scale structure (LSS) of the Universe is probed through multiple observables: the distribution of galaxies, weak lensing of galaxies, and the cosmic microwave background (CMB). Each probe tests gravity on large scales and the effects of dark energy, but their joint analysis provides the best control over nuisance parameters and yields the most precise cosmological constraints.
The DESI spectroscopic survey maps the 3D distribution of galaxies. By the end of its 5-year nominal survey this year, it will have observed 40 million galaxies and quasars — ten times more than previous surveys — over one third of the sky, up to a redshift of z = 4.2. Combining DESI data with CMB and supernova measurements, the collaboration has revealed a potential deviation of dark energy from a cosmological constant.
To fully exploit these data, DESI has launched a “mega-analysis” combining galaxies, weak lensing of galaxies (Euclid, UNIONS, DES, HSC, KIDS) and the CMB (Planck, ACT, SPT), aiming to deliver the most precise constraints ever obtained on dark energy and gravity. The student will play a key role in developing and implementing this multi-probe analysis pipeline.
The standard analysis compresses observations into a power spectrum for cosmological inference, but this approach remains suboptimal. The student will develop an alternative, called field-level analysis, which directly fits the observed density and lensing field, simulated from the initial conditions of the Universe. This constitutes a very high-dimensional Bayesian inference problem, which will be tackled using recent gradient-based samplers and GPU libraries with automatic differentiation. This state-of-the-art method will be validated alongside the standard approach, paving the way for a maximal exploitation of DESI data.
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.
Measurement of low lying dipole excitations using neutron inelastic scattering
The pygmy dipole resonance is a vibration mode observed in neutron-rich nuclei and which has initially been described as the oscillation of a neutron skin against a symmetric core in term of proton and neutron numbers. But experimental studies have revealed a more complex structure. Few years ago, we have proposed to take benefit of the high intensity neutron flux from SPIRAL2-NFS to study the pygmy resonance with an original approach: the neutron inelastic scattering. Following the success of the first experiment carried out in 2022, we propose to continue our program in a new region of the nuclear chart. The objective of the thesis is to study the pygmy dipole resonance in 88Sr by inelastic neutron scattering. The thesis will consist of: i) participation in the experiment, ii) data analysis, and iii) interpretation of the results in collaboration with theorists.
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.
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.