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Measurement of the Higgs boson production in association with two b-jets, in view of the di-Higgs HH → bbγγ measurement with Run 3 data

The measurement of the Higgs self-coupling at the LHC is of fundamental importance since it is the only way to experimentally assess the properties of the Higgs potential in a model-independent way. The most promising channel to perform such a measurement is HH → bbγγ process, that retains one of the largest sensitivities to the di-Higgs production, and will be the focus of the ATLAS Higgs physics program at the LHC Run 3. One of the other largest uncertainty affecting the search for the HH → bbγγ process is the imperfect knowledge of the gluon fusion single-Higgs plus 2 b-jets cross-section (ggH/H → γγ + 2b), a process constituting a partially-resonant background very difficult to disentangle from the HH → bbγγ signal. In recent publications, a conservative 100% uncertainty was assigned by ATLAS to the theoretical predictions of the ggH/H → γγ + 2b process. On the other hand, it has been recently shown that the uncertainty on the ggH/H → γγ + 2b process would become the dominant theoretical systematic uncertainty affecting the HH → bbγγ measurement at the High Luminosity run of the LHC (HL-LHC), and it needs therefore to be addressed with dedicated measurements. The stage will focus on the measurement of the H → γγ process in association with an increasing num- ber of b-jets - especially focusing on the 2 b-jet topology. The measurement will be performed with the increasing Run 3 dataset, in the context of the single Higgs boson fiducial cross-section measurement. A study of the kinematical properties of the associated b-jets will be performed, to evaluate the feasibility properties of performing the measurement in a segmented phase space for a given number of b-jets. The analysis will also explore the possibility to disentangle the ggH/H → γγ+2b signal from the bbH/H → γγ one, by exploiting the different kinematical properties of the b-jets in the two processes. The internship will be within the focus of to the current activities of the ATLAS LAPP Group dedicated to the Higgs to diphoton studies. It will require a regular presence at CERN and could potentially evolve on a PhD thesis project.

Optional information on a possible PhD subject : The M2 internship can be expanded into a PhD project focused on the measurement and interpretation of the HH → bbγγ signal exploiting the full LHC Run 3 dataset. Model analysis techniques based on Machine Learning approaches will be used to increase the sepa- ration between signal and background, as well as a full exploitation of the sensitivity brought by sepa- rately considering the various HH production modes. The sensitivity of the analysis will be further im- proved by trying to recover signal events failing the current analysis selections, e.g. by developing a more efficient b-tagging algorithm for boosted topologies, or a photon identification algorithm recov- ering low-momentum photons that characterizes the H → γγ decay in the HH → bbγγ signal for val- ues of the Higgs trilinear coupling than those predicted by the Standard Model (SM). The analysis will be designed in such a way that the used classifiers will be robust against any deviation from the SM prediction, so that it would retains strong sensitivity to a wide range of Beyond the SM (BSM) scenarios, and its results could be interpreted in the context of Effective Field Theories (EFTs), such as the HEFT model that has already been explored in di-Higgs searches, or the SMEFT framework used in the interpretation of single-Higgs results and other SM processes. The analysis work will take place in the broader context of the ATLAS search for di-Higgs production in various decay channels, and a participation to Run 3 di-Higgs combination as well as of that of the re- sults based on Run 2 and Run 3 data is foreseen. In parallel, the student will contribute to the improvement of the photon performance in ATLAS, with a strong connection to the understanding of the performance of the ATLAS Liquid Argon electromag- netic calorimeter with Run 3 data. The duration of the PhD thesis is three years. The geographical proximity of LAPP and CERN, about 50 km and 35’ drive, will allow a strong involvement both with the data taking and detector operation and with the participation in person to the working meetings of the ATLAS (di)Higgs Group. A period of several months of continuous presence at CERN is foreseen.

o Level : Master 2 internship, can be followed by a PhD

o Date of the internship : spring 2023

o Supervisor : Marco Delmastro, Nicolas Berger

o Phone : 04 50 09 17 85

o e-mail : marco.delmastro@cern.ch