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Vector Boson Fusion trigger and study of events featuring di-tau pairs and b jets in the CMS experiment

Abstract : This thesis describes a search for events with a pair of Higgs bosons (HH) in proton-proton collisions at 13 TeV, provided by the Large Hadron Collider, with the CMS (Compact Muon Solenoid) experiment at CERN (Geneva).The study of the Higgs boson pair production allows its trilinear self-coupling to be measured. Moreover, the HH production through Vector Boson Fusion (VBF) gives access to the measurement of the coupling between two Higgs bosons and two vector bosons. The values of these parameters are particularly sensitive to the existence of physics beyond the Standard Model: even small variations from the values of the couplings predicted by the theory can lead to a large modification of the cross section. This thesis is focused on the scenario where one of the Higgs bosons decays in two tau leptons and the other in two jets of particles generated by b-quarks: the study of this final state benefits from the high statistics of the decay in quarks and from the purity of the decay in tau leptons.However, the HH production at the LHC is a very rare process. The production through the main mechanism, by gluon fusion, has a cross section of about 30 fb, followed by the VBF process, which is about 10 times less likely. Therefore, the optimisation of the signal selection is essential. Hence, the first part of the thesis work was devoted to the study of algorithms for the Level-1 (L1) trigger system of CMS, implementing selections similar to those applied at the final analysis stage, and a dedicated algorithm for the VBF process was optimised, targeting possible improvements for the search for HH->bbtautau events. The VBF topology, characterised by two jets reconstructed in opposite regions of the detector, is a powerful handle to discriminate it from other processes. The algorithm selects the events exploiting the kinematic features of the jets, independently of the properties of the decay of the Higgs boson: used as a complement of the classic algorithms that target the decay products of the Higgs boson, the VBF algorithm allows the coverage of the phase-space to be significantly expanded. This is the first VBF algorithm for the L1 trigger system and it was included for the data-taking starting from summer 2017. Hence, the events thus selected are available for the ongoing Higgs boson searches.The rest of the thesis work was dedicated to the analysis of events HHbbtt with the data collected in 2017. The signal extraction from data requires a good knowledge of the background processes. Among them, the most important are the ttbar production, which is irreducible if its final state has a pair of tau leptons in addition to the two b quarks; the generic QCD multi-jet background, since the jets can be misidentified as tau leptons; and the Z->tautau background, where additional jets can be identified as b-jets. The latter was studied in detail to improve its modelling. In addition to the inclusive study of the HH->bbtautau events, an event category for the VBF production was included, designed starting from the phase-space covered by the L1 trigger VBF algorithm and from the typical topology of the process, also using machine learning techniques. The introduction of the VBF category gives the first dedicated measurement for this production mechanism in the context of the HH->bbtautau analyses. These improvements and the additional interpretations are important for the CMS collaboration and they will be included in the publication corresponding to the data collected during the Run 2.
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Submitted on : Monday, May 25, 2020 - 9:40:54 AM
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  • HAL Id : tel-02614330, version 2

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Chiara Amendola. Vector Boson Fusion trigger and study of events featuring di-tau pairs and b jets in the CMS experiment. High Energy Physics - Experiment [hep-ex]. Université Paris-Saclay, 2019. English. ⟨NNT : 2019SACLX103⟩. ⟨tel-02614330v2⟩

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