PhD Position (M/F) – Probing New Physics in High-Energy Diboson Production through Polarization Measurements and Effective Field Theory with ATLAS Data

Job Description

Thesis Subject

The discovery of the Higgs boson by the ATLAS and CMS experiments in 2012 completed the Standard Model (SM) of particle physics by confirming the mechanism of electroweak symmetry breaking (EWSB). Within the Standard Model, this mechanism gives rise to the masses of the electroweak gauge bosons and their longitudinal polarization, making polarization measurements a direct probe of the electroweak symmetry-breaking mechanism and a powerful tool to search for physics beyond the Standard Model (BSM). The Higgs boson plays a crucial role in regulating the scattering of longitudinally polarized vector bosons at high energies, thereby preserving perturbative unitarity. Any deviation of the Higgs boson couplings from their Standard Model predictions would require the existence of new interactions or resonances to restore unitarity. If these new particles are too heavy to be produced directly at the LHC, their effects may nevertheless be observed indirectly in proton-proton collisions through modifications of kinematic distributions and the polarization of vector bosons at high energies.

This PhD project focuses on the study of vector-boson pair production in association with two forward hadronic jets, a characteristic signature of Vector Boson Scattering (VBS). The analysis will concentrate on semi-leptonic final states, where one vector boson decays hadronically and the other leptonically. Since potential BSM effects are expected to become most pronounced at high energies, the event selection will target boosted hadronically decaying bosons, whose decay products are reconstructed as a single large-radius jet.

Possible BSM effects will be interpreted within the framework of Effective Field Theory (EFT). In this approach, the low-energy effects of heavy particles that cannot be produced directly at the LHC are described by higher-dimensional operators added to the Standard Model Lagrangian. The measured distributions can then be used to constrain the corresponding EFT coefficients in a largely model-independent way. Since EFT contributions typically grow with energy, measurements of diboson production in the high-energy regime provide excellent sensitivity to these effects. Particular emphasis will be placed on EFT operators that modify the polarization composition of vector bosons. The PhD student will develop an innovative method to constrain EFT parameters using a multivariate classifier combining several kinematic and angular observables.

The project also aims to measure the polarization fractions of vector bosons in the high-energy VBS phase space. Previous polarization measurements have mainly relied on leptonic boson decays, where the polarization information can be directly reconstructed from the decay leptons, but these analyses are statistically limited in the high-energy regime. By contrast, measuring polarization in boosted hadronic boson decays will provide access to larger event samples and higher energies, albeit in a more challenging jet environment. Information from jet substructure, combined with modern machine-learning techniques, will be exploited to improve the discrimination between the different polarization states.

The analysis will use the complete Run 2 and Run 3 datasets collected by the ATLAS experiment. Prospective studies will also be performed to assess the expected sensitivity of these measurements at the High-Luminosity LHC (HL-LHC).

The PhD project will be carried out in close collaboration with the ILANCE laboratory, which is actively involved in this research area. The University of Tokyo has internationally recognized expertise in jet reconstruction and plays a leading role in semi-leptonic VBS analyses. Strengthening this collaboration will provide an excellent opportunity to further develop these studies in the context of Run 3 while preparing for the HL-LHC physics programme.

Your Work Environment

The PhD project will be carried out at the Laboratoire des 2 Infinis – Toulouse (L2IT), within the ATLAS group, in close collaboration with the French-Japanese ILANCE laboratory and the University of Tokyo. The successful candidate will become part of a large international collaboration and actively contribute to the ATLAS experiment at CERN.
The research will focus on the analysis of the LHC Run 2 and Run 3 datasets, with prospects for extending the studies to the High-Luminosity LHC (HL-LHC) programme.

Constraints and risks

The project requires a solid background in scientific programming (Python, C++), statistical data analysis, and the processing of large datasets. The PhD candidate will work within a distributed computing environment (Computing Grid) and will become proficient in the software framework and analysis tools developed by the ATLAS Collaboration.

The research will follow the timeline of the ATLAS Collaboration, with milestones associated with physics analyses and publications. Regular visits to CERN, as well as travel to international workshops and conferences, are expected. No specific occupational risks have been identified beyond those associated with computer-based work and the use of standard computing equipment.

Compensation and benefits

Compensation

2300 € gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

CNRS

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