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Accueil du site > Thèses, Stages, Formation et Enseignement > Propositions de thèses 2019 > Identification of boosted Higgs bosons for searches of new physics and improvement of the ATLAS tracker for the high luminosity phase of the LHC

Identification of boosted Higgs bosons for searches of new physics and improvement of the ATLAS tracker for the high luminosity phase of the LHC

by Julien Bolmont - 25 October 2018

Title: Identification of boosted Higgs bosons for searches of new physics and improvement of the ATLAS tracker for the high luminosity phase of the LHC

Advisors: Giovanni Calderini, Reina Camacho Toro

Team: Masses et Interactions Fondamentales ; ATLAS group


ATLAS is one of the experiments recording the products of proton-proton collisions created by the Large Hadron Collider (LHC) at CERN, Geneva. After a first period of data taking that lasted from 2010 to 2012, in 2015, collisions restarted at a higher energy (13 TeV) and larger instantaneous luminosity. This second data-taking period (Run-2) will end at the end of 2018 and a third one (Run-3) will start in 2021 and will last three years. Finally after an upgrade phase, the LHC will start the last data-taking phase in 2026, HL-LHC, with unprecedented collision rates. The Run-3 and the HL-LHC will address the heart of the New Physics research program at the LHC.

The analysis of data collected in 2010-2012 allowed ATLAS and CMS collaborations to discover a Higgs boson with mass of about 125 GeV. In 2018, after analysing more data from Run-2, the ATLAS collaboration reported the observation of the decay of the Higgs boson to two b-quarks, the first occasion in which observation of coupling of Higgs to quarks was reported. The LPNHE ATLAS group was deeply involved in this discovery. Now one of the main objectives of the LHC and ATLAS is to better understand the nature of the Higgs boson which is at the heart of the understanding of nature.

One of the key ingredients that made possible to observe the decay of Higgs boson into bquark pairs is the efficient identification of hadronic jets initiated by b-quarks (b-jets), which strongly depends on the performance of the tracking and vertexing detector. At the Large Hadron Collider (LHC) heavy particles like the Higgs boson are often highly boosted with large momentum, p, which will pull their decay products close together. Boosted Higgs boson are critical ingredients for many analyses in ATLAS to probe the Standard Model and in searches for new physics.

It is important to mention that the ATLAS tracking and vertexing detector will be replaced for the HL-LHC phase with a brand new silicon detector. The LPNHE ATLAS group is committed also to build pixel modules for the future ATLAS tracking detector, know as Inner Tracker (ITk). An R&D work on small pitch, radiation hard silicon pixel sensors is ongoing within ATLAS and in particular at LPNHE since many years. Thin small pitch sensors are needed to cope with the increase in event rate and pile-up events, and radiation fluences up to 1-2x1016 neq/cm2, a 10-fold increase with respect to today.

The thesis work will include a part dedicated to the development of the new boosted Higgs boson identification tools adapted to the future energy and luminosity conditions of the LHC at Run-3 and HL-LHC. We will explore simple combination of variables as well as machine learning methods to develop the identification algorithms. They will be validated using data collected by the ATLAS experiment. The other part of the PhD work will also focus on ITk and will integrate measurements of the electrical and physical properties of pixel sensors developed in the laboratory. Part of the activity will be in the clean room, another at the accelerators for measurements with test beams. The results obtained will be used to improve the design of the sensors to make them more resistant to radiation and more efficient. This activity is in collaboration with the group ATLAS ITk [1] and RD53 [2].

Frequent trips to CERN, but also to DESY (Hamburg), are planned for test beam studies data and regular trips, especially at CERN, are also expected to present to the collaboration the results of data analysis and instrumentation developments. This thesis will provide the student with an excellent overview of the world of particle physics as it combines data analysis, detector performance and detector development and will allow him/her to work in a collaborative and international environment.

[1] Atlas Inner Tracker
[2] RD53

Web site:

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

Possible trips: missions régulières au CERN à Genève et déplacements vers des autres laboratoires européens (surtout DESY à Hambourg)