The analysis of the Higgs boson decay into the pairs of tau leptons H → ττ is presented. In the scope of this thesis, only the tau leptons decaying purely lep- tonically are considered. This analysis is performed using data recorded from the proton-proton collisions at a center-of-mass energy √ s = 13 TeV by the ATLAS detector at the Large Hadron Collider and correspond to the integrated lumi- nosity of 36.1 fb−1 . The Boosted Decision Trees (BDT) multivariate technique was used in order to provide a classification of events. Datasets considered in this analysis consisted of the triggered, reconstructed and calibrated events. In addition, events had to pass the pre-selection criteria and were separated into four categories. Our main contribution to this analysis is based on developing and incorporating techniques used in the process of BDT training and evaluation. The performed analysis presented in this thesis is blinded. Therefore, the Monte Carlo simulation was used instead of real data in the Maximum Likelihood Fit in order to determine the expected significance. For the H → ττ leptonic channel, the expected signal significance was measured to be 1.16 σ. 1

University, Diss., 2004--München.

The Standard Model predicts existence of one Higgs boson with combined parity CP = +1. In MSSM there exist Higgs boson with CP = −1 in addition. The work develops one method of Higgs boson CP determination on the basis of angular correlations of pions and ρ-mesons born in cascade decay H/A → τ− τ+ → ρ− /π− ντ ρ+ /π+ ¯ντ . The calculations are done in the leading order of perturbation theory. Further, the possibility of signal (Higgs boson decay) and background (Z boson decay) differentiation is studied. The processes in question are simulated using Monte Carlo generators Pythia and Tauola. Simulation outputs are compared with calculated theoretical results. 1

The Standard Model of elementary particles (SM) predicts the existence of a neutral scalar Higgs boson. However, there are also extensions of the SM (such as the MSSM) in which a number of Higgs bosons is predicted. Especially the additional presence of pseudoscalar and charged Higgs bosons represents one of the crucial differences between the SM and its exten- sions. This work develops a method for determination of the spin and parity of the Higgs boson in several H → ττ decays, namely: H→ τ+ τ− → (h+ ντ )(h− ¯ντ ), H→ τ+ τ− → ( + ντ ¯ν )(π− ¯ντ ) and H→ τ+ τ− → ( + ντ ¯ν )( − ¯ντ ν ), where h denotes π or ρ meson. The method is based on the angular and energy correlations of charged final products from the decays mentioned above. Additionally, the work studies the possibility of signal (Higgs boson decay) and background (Z → ττ decay) discrimination, when one considers a decaying boson with spin 1. All calcula- tions are done in the leading order of perturbation theory. 1

The ATLAS experiment is one of the two general-purpose detectors at the Large Hadron Collider (LHC) at the European Organisation for Nuclear Research (CERN) in Switzerland. ATLAS is designed for precision mea- surements of particle properties, the search of the Higgs boson and new physics beyond the Standard Model. The experiment got worldwide atten- tion in 2012, when after the collaborative efforts with the CMS experiment the Higgs boson discovery was announced. After the discovery, the preci- sion measurements of its properties became one of the main objectives of the LHC physics programme, since a potential observation of deviations from the Standard Model predictions might lead to the discovery of new physics. In this thesis, the measurements of the Higgs boson production cross-sections in the H → ττ decay channel are presented. Based on the proton-proton collision data collected at the centre-of-mass energy of 13 TeV in years 2015 and 2016, the signal over the expected background from the other Standard Model processes is established with the observed significance of 4.4σ. Com- bined with the data collected at 7 and 8 TeV, the observed signal significance amounts to 6.4σ, which constitutes a single experiment observation of the H → ττ decays by ATLAS. All presented results are found to be consistent with...

Le mode de désintégration en une paire de photons est la signature la plus claire permettant de rechercher un boson de Higgs de masse intermédiaire auprès du futur grand collisionneur de protons du CERN, le LHC. Puisque la largeur naturelle du Higgs est très faible dans cette zone, son observation dans ce canal repose sur les performances du calorimètre électromagnétique. Une étude complète a été effectuée pour évaluer le potentiel de découverte du détecteur ATLAS. Les paramètres et la géométrie du calorimètre électromagnétique à argon liquide ont été optimisés. Les résultats de la simulation utilisée pour ce travail sont confirmés par des tests sur faisceau d'un prototype. Cette simulation inclut différentes contributions comme le terme d'échantillonnage, les bruits d'électronique et d'empilement, le terme constant global du calorimètre et la mesure de direction de l'angle d'émission des photons de la paire. Le niveau de bruit de fond irréductible des paires de photons directs a été évalué, tout comme celui du bruit de fond réductible des jets de hadrons, et plus particulièrement celui des pions neutres isolés. Pour cela, la capacité du détecteur à rejeter ces bruits de fonds a été estimée. Le calcul de la résolution de masse invariante des paires de photons et l'estimation du taux d'événements de signal et de bruit de fond ont permis de calculer le potentiel de découverte du Higgs par le détecteur ATLAS pour ce canal. Un effet systématique à plus de cinq écarts types sera mis en évidence en une année de prise de données au LHC, si la masse du boson de Higgs est comprise entre 100 GeV/c2 et 150 GeV/c2.