This chapter presents the core of the analysis strategy and is structured into five main parts. First, the \(\text {b}\text {b}\tau \tau \) decay channel is presented, focusing on its experimental signature and associated challenges. These considerations are the bases for the analysis procedure described in the subsequent two sections, covering the selection of physics objects based on the Particle Flow (PF) approach and the full event categorization. The discussion then moves to the modelling of signal and background processes with Monte Carlo (MC) techniques and their correction based on data-MC comparison. The last section is finally devoted to the treatment of systematic uncertainties. The search for Higgs boson pair (HH) production in the \(\text {b}\text {b}\tau \tau \) final state is one of the most challenging and yet most sensitive channels in our exploration of the Higgs boson (H) self-coupling ( \(\lambda _{{\text {H}}{\text {H}}{\text {H}}} \) ). Although initially designed as an explorative analysis meant to cover additional phase space in the study of HH production, the \({\text {H}}{\text {H}} \rightarrow \text {b}\text {b}\tau \tau \) is now the second most sensitive analysis in the Gluon Fusion ( \({\text {g}}{\text {g}}\text {F}\) ) production channel and the most sensitive in the Vector Boson Fusion (VBF) production channel. Investigating this decay channel requires the experimental capability to identify and reconstruct several different types of final state objects and to use them to select signal-like events. This chapter is devoted to the presentation of the analysis strategy, the selection procedures, and the identification techniques, which are the foundation of the \({\text {H}}{\text {H}} \rightarrow \text {b}\text {b}\tau \tau \) search. The \(\tau \) lepton is an unstable particle whose decays contain neutrinos, thus rendering impossible the complete reconstruction of the event. At the same time, hadronic final states must be distinguished from instrumental backgrounds caused by the misidentification of quark and gluon jet. The complexity and heterogeneity of the final states demand the use of all the physics objects’ information and require the exploitation of the excellent particle identification capabilities of the CMS detector. Furthermore, the presence of neutrinos in the final state, together with the tiny \({\text {g}}{\text {g}}\text {F}\) cross section of \(31.05\,\mathrm{{fb}}\) , hinders the possibility of a simple bump hunt but demands the use of novel and more complex analysis approaches to identify the signal on top of a huge background. Moreover, the need for the use of the event kinematic properties to reduce background contamination requires a thorough understanding of the process. For all these reasons, the \(\text {b}\text {b}\tau \tau \) decay channel is probably one of the most challenging at the LHC. The presented physics analysis is performed with proton-proton collision data recorded at a centre-of-mass energy \(\sqrt{s}=13\,\text {TeV} \) by the CMS experiment during Run-2. The analyzed dataset has been collected during 2016, 2017, and 2018, corresponding to integrated luminosities of 35.9, 41.5, and \(59.7{\,\text {fb}^{-1}} \) respectively, yielding a total of \(137{\,\text {fb}^{-1}} \) , an unprecedented dataset size in high energy physics. This search is based on the previous publication of 2016 [1] and improves it by introducing several upgrades to the analysis flow, from better trigger selection to improved signal extraction, from enhanced signal modelling to strengthened limit setting. The physics search presented in this Thesis is the result of the collective effort between scientists from several universities and institutes within the CMS Collaboration, which culminated in a publication in the Physics Letters B journal [2]. Within this team, I have had the chance to take part in most of the steps of the analysis. Most notably, I have been in charge of the evaluation of the event selection performance, the modelling of the top quark-antiquark pair ( \(\mathrm{{t}}\bar{\textrm{t}}\) ) background and of the Effective Field Theory (EFT) benchmarks; I have contributed to the optimization of the discriminating variable used for the statistical interpretation and to the validation of the analysis statistical model, and I have been responsible for the production of the EFT interpretation of the results.

The Search for \({\text {H}}{\text {H}} \rightarrow \text {b}\bar{\text {b}} {\tau }^{+} {\tau }^{-} \)

章节
发表日期：

2025年

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Jona Motta

摘要

This chapter presents the core of the analysis strategy and is structured into five main parts. First, the \(\text {b}\text {b}\tau \tau \) decay channel is presented, focusing on its experimental signature and associated challenges. These considerations are the bases for the analysis procedure described in the subsequent two sections, covering the selection of physics objects based on the Particle Flow (PF) approach and the full event categorization. The discussion then moves to the modelling of signal and background processes with Monte Carlo (MC) techniques and their correction based on data-MC comparison. The last section is finally devoted to the treatment of systematic uncertainties. The search for Higgs boson pair (HH) production in the \(\text {b}\text {b}\tau \tau \) final state is one of the most challenging and yet most sensitive channels in our exploration of the Higgs boson (H) self-coupling ( \(\lambda _{{\text {H}}{\text {H}}{\text {H}}} \) ). Although initially designed as an explorative analysis meant to cover additional phase space in the study of HH production, the \({\text {H}}{\text {H}} \rightarrow \text {b}\text {b}\tau \tau \) is now the second most sensitive analysis in the Gluon Fusion ( \({\text {g}}{\text {g}}\text {F}\) ) production channel and the most sensitive in the Vector Boson Fusion (VBF) production channel. Investigating this decay channel requires the experimental capability to identify and reconstruct several different types of final state objects and to use them to select signal-like events. This chapter is devoted to the presentation of the analysis strategy, the selection procedures, and the identification techniques, which are the foundation of the \({\text {H}}{\text {H}} \rightarrow \text {b}\text {b}\tau \tau \) search. The \(\tau \) lepton is an unstable particle whose decays contain neutrinos, thus rendering impossible the complete reconstruction of the event. At the same time, hadronic final states must be distinguished from instrumental backgrounds caused by the misidentification of quark and gluon jet. The complexity and heterogeneity of the final states demand the use of all the physics objects’ information and require the exploitation of the excellent particle identification capabilities of the CMS detector. Furthermore, the presence of neutrinos in the final state, together with the tiny \({\text {g}}{\text {g}}\text {F}\) cross section of \(31.05\,\mathrm{{fb}}\) , hinders the possibility of a simple bump hunt but demands the use of novel and more complex analysis approaches to identify the signal on top of a huge background. Moreover, the need for the use of the event kinematic properties to reduce background contamination requires a thorough understanding of the process. For all these reasons, the \(\text {b}\text {b}\tau \tau \) decay channel is probably one of the most challenging at the LHC. The presented physics analysis is performed with proton-proton collision data recorded at a centre-of-mass energy \(\sqrt{s}=13\,\text {TeV} \) by the CMS experiment during Run-2. The analyzed dataset has been collected during 2016, 2017, and 2018, corresponding to integrated luminosities of 35.9, 41.5, and \(59.7{\,\text {fb}^{-1}} \) respectively, yielding a total of \(137{\,\text {fb}^{-1}} \) , an unprecedented dataset size in high energy physics. This search is based on the previous publication of 2016 [1] and improves it by introducing several upgrades to the analysis flow, from better trigger selection to improved signal extraction, from enhanced signal modelling to strengthened limit setting. The physics search presented in this Thesis is the result of the collective effort between scientists from several universities and institutes within the CMS Collaboration, which culminated in a publication in the Physics Letters B journal [2]. Within this team, I have had the chance to take part in most of the steps of the analysis. Most notably, I have been in charge of the evaluation of the event selection performance, the modelling of the top quark-antiquark pair ( \(\mathrm{{t}}\bar{\textrm{t}}\) ) background and of the Effective Field Theory (EFT) benchmarks; I have contributed to the optimization of the discriminating variable used for the statistical interpretation and to the validation of the analysis statistical model, and I have been responsible for the production of the EFT interpretation of the results.