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Accueil > Thèses, Stages, Formation et Enseignement > Propositions de thèses antérieures > Propositions de thèses 2022 > Development of the new silicon pixel tracker for the upgrade of the ATLAS detector for the High Luminosity LHC (HL-LHC) phase and study of the impact of the new tracking system on the research of new physics and precision Higgs measurements at HL-LHC starting from the present detector

Development of the new silicon pixel tracker for the upgrade of the ATLAS detector for the High Luminosity LHC (HL-LHC) phase and study of the impact of the new tracking system on the research of new physics and precision Higgs measurements at HL-LHC starting from the present detector

Title: Development of the new silicon pixel tracker for the upgrade of the ATLAS detector for the High Luminosity LHC (HL-LHC) phase and study of the impact of the new tracking system on the research of new physics and precision Higgs measurements at HL-LHC starting from the present detector

Supervisor: Giovanni Calderini

Team: Masses and fondamental interactions; ATLAS experiment

Founded PhD: Yes, already funded

Description:

ATLAS is one of the four large detectors installed at the LHC proton collider of CERN at Geneve. The analysis of data collected by LHC at an energy of 7 and 8 TeV (Run1: 2011-2012) led to the discovery by ATLAS and CMS of a Higgs boson of mass 125 GeV. After this first phase, the accelerator has moved on to a second phase called Run2 in 2015-2018, characterized by proton-proton collisions at 13 TeV and an integrated luminosity more important with respect to Run1. A third phase “Run3” will start in a few months in Spring 2022 with the goal of recording more than 150 fb-1 of data at an energy between 13 and 14 TeV in three years. In the meantime, a large upgrade of the accelerator and the detectors is under way for the so-called high-luminosity phase of LHC, or HL-LHC. 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 4 inverse attobarns of proton-proton collisions. The density of events (“pile-up”) will be of the order of 200 per bunch crossing, 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 design and construction of totally new systems has been started, to be ready to put them on the beamline in 2026.

The LPNHE group of ATLAS has developed in the years a large experience in silicon pixel detectors, with a significant contribution to the design and construction of the system presently installed in ATLAS in particular for the development of the new insertable B-layer installed in 2014. Since then, a large effort has been devoted to the design and development of the pixel sensors which will be used in the upgraded system. Thin sensors, of the order of 150um, 100um and even smaller thickness, have been tested showing a large improvement in term of radiation hardness and tracking performance, due to the reduced material. Much smaller pixels with respect to the present ones are also used, to face the increased occupancy given by the higher instantaneous luminosity and number of tracks. 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 in France 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.

In parallel, the extrapolation of the expected performance of the new system to the HL-LHC data-taking conditions will be studied starting from the present Run3 analysis of precision measurements and search for new physics, in particular in the field of Higgs boson physics. One of the Higgs decay modes for which the detector tracking capability is critical is the decay to bb states, given the track jet reconstruction and the needed b-quark tagging. The H -> bb channel represents about 58% of all the Higgs decays, but cannot be reconstructed in inclusive way given the overwhelming QCD background. Evidence of H->bb decays has been observed exploiting more complex event topologies, such as the Higgs production associated with a Z of a W bosons decaying to leptons (electrons or muons).

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.
Run3 data will be crucial to further probe the properties of the Higgs boson through its decay to b-quark pairs, and to better test the hypothesis that it corresponds or not to what predicted by the Standard Model. All this will be then extrapolated to the new HL-LHC detector to study how the improved performance and the increase in luminosity 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.

Working Place: LPNHE

Possible trips:
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.
Internship foreseen before the PhD, for a period of 4-5 months

Contacts: Giovanni Calderini, 33 (0)1 44 27 23 25

Documentations:

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