Description du sujet de thèse

Ten years after the Higgs (H) boson discovery by the ATLAS and CMS experiment, Higgs physics play a central role in the vast majority of the remaining open questions of particle physics, offering a unique access to different aspects of the Standard Model (SM) and to possible new physics.
The study of the Higgs boson properties remains one of the priorities for the Large Hadron Collider (LHC) physics program and for the ATLAS experiment: among these properties, the Higgs self-coupling is particularly interesting due to its direct influence on the shape of the Higgs potential.
Direct production of Higgs boson pairs (HH), while very rare and still not observed, offers the most sensitive determination of the Higgs self-coupling.
This process is characterised by a low production cross-section, requiring advanced analysis strategies in order to improve the sensitivity to such a rare signal.
This thesis will focus on the search for HH production using the LHC Run 3 dataset collected by the ATLAS experiment: the project will focus on the most sensitive decay channel for the search of HH, in the final state with two bottom quarks and two tau leptons. This decay channel provides a compromise between the large branching ratio of the hadronic channel, and the clean experimental signature of tau leptons.
The improvement of the tau lepton reconstruction and identification algorithms with innovative techniques will constitute one of the main axes of the thesis project, as well as the calibration of these new algorithms with data collected by ATLAS during the LHC Run 3.
An aspect of the thesis work will consist in the development of analysis techniques in order to optimise the sensitivity to HH production, and of statistical methods to improve the signal extraction.
Another aspect of the project may focus on the interpretation of the results in term of self-coupling parameters, as well as the development of the statistical framework to combine the main decay channels for the HH processes.
The ATLAS LPSC group has a strong commitment in the construction of the upgrade for the ATLAS inner tracker detector (ITk) for the High-Luminosity LHC phase.
The candidate will have the opportunity to contribute to the production of this new detector by integrating the ongoing group activity.

Bibliography:
The ATLAS Collaboration, Search for the nonresonant production of Higgs boson pairs via gluon fusion and vector-boson fusion in the bbtautau final state in proton-proton collisions at \sqrt{s}=13TeV with the ATLAS detector, Phys. Rev. D 110, 032012, DOI: https://doi.org/10.1103/PhysRevD.110.032012

Contexte de travail

The Laboratoire de Physique Subatomique et de Cosmologie (LPSC) in Grenoble (http://lpsc.in2p3.fr), is a joint research unit of CNRS-IN2P3, Université Grenoble Alpes (UGA), and Grenoble INP, with an average staff of around 230 people.
The selected candidate will join the ATLAS group, composed by 9 staff members of the LPSC, and under the coordination of the ATLAS group leader.
The thesis director will be Jean Baptiste de Vivie de Régie. The thesis co-director will be Carlo Pandini.
The successful candidate should have a Master-level knowledge of particle physics, and familiarity with the concept of electroweak symmetry breaking, the Higgs potential, and the general principles of the Standard Model. During the thesis, they will work on the development of algorithms and code in C++ and python, thus experience with these programming languages is required.
The successful candidate will work in collaboration with the ATLAS LPSC group, as well as with the ATLAS Collaboration at large outside of the LPSC. The ATLAS Collaboration counts more than 3000 scientists from 182 institutes and from 42 countries. The thesis will be carried out in collaboration with different international teams, therefore strong communication skills in English, and the ability to work within a team are required.
Regular trips to CERN are foreseen in the context of the thesis, to participate to Collaboration meetings and possibly to detector operations.
One or more publications are foreseen as a result of the thesis project. Communication via posters or public talks at conferences is also foreseen.
All results obtained in the context of the ATLAS Collaboration are required to be in open access once published, as well as the developed algorithms.

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