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Accueil > Thèses, Stages, Formation et Enseignement > Propositions de thèses 2025 > Development of the new silicon pixel tracker for the upgrade of the ATLAS detector at the High Luminosity LHC (HL-LHC), study of the performance of the new design, and application to the research of new physics and precision Higgs measurements at HL-LHC

Development of the new silicon pixel tracker for the upgrade of the ATLAS detector at the High Luminosity LHC (HL-LHC), study of the performance of the new design, and application to the research of new physics and precision Higgs measurements at HL-LHC

par Tristan Beau - 29 octobre

Titre : Development of the new silicon pixel tracker for the upgrade of the ATLAS detector at the High Luminosity LHC (HL-LHC), study of the performance of the new design, and application to the research of new physics and precision Higgs measurements at HL-LHC.

Directrice/directeur de thèse : Giovanni Calderini

Groupe d’accueil :ATLAS

Webpage du projet :
The ATLAS group at LPNHE

Webpages :
ATLAS at CERN : https://atlas.cern/
The ATLAS group at LPNHE : https://lpnhe.in2p3.fr/spip.php?rub...

Collaboration : ITk/Higgs

Description :

ATLAS is one of the four large detectors installed at the LHC proton collider of CERN at Geneve. A large upgrade of the accelerator and the detectors is under way for the so-called high-luminosity phase of LHC, or HL-LHC which will start in 2029. During this phase, which will last for about ten years, an instantaneous luminosity between 5 and 7 times larger than the nominal LHC luminosity will be delivered, providing a dataset of 3-4 inverse attobarns of proton-proton collisions. Each bunch crossing will produce about 200 collisions, so more than 5 times the design LHC density, and the level of radiation in the interaction region will 20 times larger than the nominal one. Under these conditions, not only the design of the present detectors, in particular the trackers, but the technology itself which has been used to build them will be totally inadequate. For this reason, the conception and construction of totally new systems has been started, to be ready to put them on the LHC beamline in 2029.

The LPNHE group of ATLAS has a longstanding expertise in silicon detectors. After having contributed to the new innermost pixel layer added in 2014 to the present ATLAS tracker, the group has participated to the design and validation of the pixel sensors for the HL-LHC tracker upgrade (ITk). The new sensors are significantly thinner than the previous ones and feature a much smaller pixel size to master the high number of expected tracks and the increase in radiation.

Electrical characterization of the new sensors, which will be produced for the detector upgrade, will be done in the LPNHE cleanrooms and at CERN test beams after assembly with the front-end electronics into detector modules. Three laboratories of the Paris area, the LPNHE, IJCLab and IRFU will be involved in this activity working in close connection, which represents one of the largest module assembly and test clusters of all the ATLAS tracker upgrade project. Participation to test beams in which the new modules will be evaluated and analysis of test beam data will complete this part of the thesis work, which will be of primary importance for the characterization of the pixel technology which is being used. Semiconductor device simulations tools will also be used in the interpretation of results, in line with a long-standing experience of the ATLAS LPNHE group in this field.

In parallel, the extrapolation of the expected performance of the new system to the HL-LHC data-taking conditions will be studied, in particular in the field of the Higgs boson physics. The decay mode of Higgs to pairs of b-quarks (H->bb), which represents about 58% of all the Higgs decays, is one of the channels for which the detector tracking capability is more critical, given the need for a performant track jet reconstruction and b-quark tagging. The H -> bb cannot be reconstructed in inclusive way given the overwhelming QCD background. This mode has been studied exploiting more complex event topologies, such as the Higgs production associated with a Z of a W bosons decaying to leptons (electrons or muons), and more recently in hadronic decays of the vector boson.

Our LPNHE groups has been strongly involved in these analyses and also in the performance studies for b-jet tagging and for transverse missing energy reconstruction, which are essential ingredients in these measurements.

The present performance will be extrapolated to the new HL-LHC detector to study how the improved tracking capability will increase the sensitivity of the analysis and will pave the way also to new measurements such as the Higgs HH self-coupling, which are out of reach with the present statistics.

Lieu(x) de travail : LPNHE, with frequent travel to CERN for working group meetings, Collaboration meetings, data taking shifts, work for qualification task, test-beams.
A period of short-term or middle-term permanence at CERN can be organized.
Stage foreseen before the thesis, for a period of 4-5 months

Déplacements éventuels : CERN / DESY Hambourg

Stage proposé avant la thèse : Oui

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