Questa tesi presenta la misura della produzione del bosone di Higgs all’interno del Modello Standard nel decadimento H → ττ con particolare enfasi allo stato finale in cui uno dei due τ decade adronicamente e l’altro decade leptonicamente (canale H → ττ → τlepτhad). L’analisi utilizza un campione di dati corrispondenti a 36.1 fb−1 registrati dal rivelatore ATLAS durante il Run 2 nel periodo di presa dati 2015 e 2016 ad un’energia nel centro di massa √s = 13 TeV. Il decadimento del bosone di Higgs in una coppia di leptoni τ `e attualmente l’unico modo possibile per misurare l’accoppiamento di Yukawa tra il bosone di Higgs ed i lep- toni. In questo contesto, lo stato finale H → ττ → τlepτhad ha un ruolo importante dovuto all’alta frazione di decadimento e ad un fondo moderato. L’analisi `e basata su una selezione ottimizzata per aumentare la sensitivit`a del segnale considerando le caratteristiche cinematiche dei principali canali di produzione del bosone di Higgs: la fusione di gluoni e la fusione di bosoni vettori. I risultati sono stati estratti da un profile likelihood fit sulla massa della coppia di τ e saranno presentati considerando prima solamente lo stato finale τlepτhad e poi con- siderando tutti i possibili stati finali derivanti dai decadimenti dei due τ. Considerando il fit combinato, sono stati misurati la signal strength μ = 1.08+0.175(stat.)+0.265(syst.) ed il −0.171 −0.222 prodottodellesezionid’urtoperlefrazionididecadimentoσHττ(VBF) = 0.28+0.09(stat.) −0.09 +0.10(syst.) pb, σHττ(ggH) = 2.97+1.03(stat.)+1.67(syst.) pb. I risultati sono in buon ac- −0.09 −1.01 −1.23 cordo con le previsioni del Modello Standard all’interno delle incertezze stimate. La significativit`a osservata (attesa), considerando solo i dati raccolti nel Run 2, `e 4.36 (4.13) deviazioni standard rispetto all’ipotesi di solo fondo. Dalla combinazione dei dati raccolti nel Run 1 e nel Run 2 si ottiene una significativita` di 6.37 (5.43), che `e superiore alla soglia di 5 deviazioni standard stabilita per l’osservazione di un nuovo processo.

Understanding the origin or Electroweak symmetry breaking within the Standard Model was a key motivation for the construction of the Large Hadron Collider (LHC) experiment at CERN. This thesis presents a search for evidence of Higgs boson production in the 4.7 fb−1 of collision data recorded at a centre-of-mass energy of 7 TeV at the ATLAS detector during 2011. This search is focused on signal events in which a Higgs boson is produced in the mass range 100 < mH < 180 GeV/c2 and subsequently decays to a pair of W bosons or a pair of tau leptons to final states with one hadronically decaying tau lepton and one light lepton. After an event selection criteria has been applied, the number of events in this data sample is consistent with the total background estimate and an upper limit is placed on the SM Higgs boson production rate at 95% confidence level. In addition, the prospects for measuring the SM Higgs coupling strength to tau leptons with the associated Higgs production channels and the full LHC dataset are also presented.

This thesis presents a cross-section measurement of the Z -+ TeTh process performed using the ATLAS detector at the Large Hadron Collider at CERN. The data used were collected at a centre-of-mass energy of 7 TeV during 2010 and corresponds to an integrated luminosity of 36 pb-I. This measurement uses the final state where one T lepton decays leptonically, to an electron or muon, plus neutrinos, and the other T lepton decays hadronically. The "(* / Z -+ a, It and di-boson background processes are estimated using Monte Carlo simulation, the W -+ TV Monte Carlo background prediction is estimated using a semi data-driven method. The multijet background is estimated with a fully data-driven 'ABCD' method using three exclusive control regions built by inverting uncorrelated requirements on the isolation of the electron or muon and the product of the electric charge of the identified electron or muon and hadronically decayed T lepton. The measured cross-sections in each channel are corrected for the individual branching ratios and yield the total cross-section a(Z -+ TT, minv 66 - 116 GeV) = 1142 ± 135.5(stat.) ± 206.2(syst.) ± 40. 19(1umi.) ± 3.6(theo.) pb for the electron channel and a(Z -+ TT, minv 66 - 116 GeV) = 857.6 ± 81.4(stat.) ± 132.5(syst.) ± 30.19(lumi.) ± 2.8(theo.) pb for the muon channel, where the invariant mass of the Z boson is between 66 and 116 GeV. These are in good agreement with the theoretical prediction of960 ± 49.5 pb. Performing a measurement of a well-known Standard Model process using the ATLAS de- tector in this new high energy regime is essential for validating the T lepton identification in particular, which is essential for any new physics search such as H -+ TeTh' A cut-based analysis for the Vector Boson Fusion produced Higgs boson decaying to two T leptons channel is introduced and the possibility of using the more complex analysis technique, the Matrix Element method, to increase sensitivity is discussed.

A search for the neutral Higgs bosons predicted by the Minimal Supersymmetric Standard Model (MSSM) is presented. The analysis is performed on data from proton-proton collisions at a centre-of-mass energy of sqrt{s}=8 TeV at the Large Hadron Collider recorded by the ATLAS detector. The data were collected in 2012, correspond to an integrated luminosity of 20.3 fb-1 and represent the full 8 TeV dataset. This search is performed in the tau-tau final state, with one tau lepton decaying leptonically and the other hadronically. The analysis is optimized in three categories, addressing low-mass Higgs bosons which are accompanied by a bottom quark induced jet or not and high-mass Higgs bosons. No deviations from Standard Model predictions are observed, and therefore no evidences of new Higgs bosons are found. Exclusion limits are set on the cross-section times branching fraction of the Higgs bosons and for parameters m_A and tan{beta} of MSSM benchmark scenarios. / In dieser Arbeit wird die Suche nach neutralen Higgs-Bosonen im Rahmen des Minimalen Supersymmetrischen Standardmodells (MSSM) vorgestellt. Die Analyse basiert auf dem kompletten Datensatz von Proton-Proton-Kollisionen des Large Hadron Colliders, LHC, die mit dem ATLAS-Detektor bei einer Schwerpunktsenergie von sqrt{s}=8 TeV im Jahr 2012 aufgenommen wurden. Die Daten entsprechen einer Luminosität 20.3 fb-1. Die Suche fokussiert auf den Tau-Tau Endzustand, wobei ein Tau-Lepton hadronisch zerfällt und das andere leptonisch, entweder in ein Elektron oder in ein Muon. Die Analyse wurde in drei Kategorien optimiert, um sowohl für Higgs-Bosonen im niedrigen Massenbereich von 90-200 GeV als auch im hohen Massenbereich von 200 GeV bis 1 TeV sensitiv zu sein. Der niedrige Massenbereich wurde dabei in zwei Unterkategorien geteilt, entsprechend der Anwesenheit bzw. Abwesenheit von zusätzlichen, durch b-Quarks induzierten Jets, um die Empfindlichkeit für spezielle Higgs-Boson-Produktionsmodi zu erhöhen. Im Rahmen dieser Analyse wurden keine Hinweise auf neue Higgs-Bosonen gefunden und daher Ausschlussgrenzen auf den Wirkungsquerschnitt x Verzweigungsverhältnis gesetzt. Ebenfalls wurden die Resultate in MSSM-Benchmark-Szenarien interpretiert.

There are experimental and theoretical indications that the Standard Model of particle physics, although tremendously successful, is not sufficient to describe the universe, even at energies well below the Planck scale. One of the most promising new theories to resolve major open questions, the Minimal Supersymmetric Standard Model, predicts additional neutral and charged Higgs bosons, among other new particles. For the search of the new heavy neutral bosons, the decay into two hadronically decaying tau leptons is especially interesting, as in large parts of the search parameter space it has the second largest branching ratio while allowing for a considerably better background rejection than the leading decay into b-quark pairs. This search, based on proton-proton collisions recorded at sqrt(s) = 13 TeV in 2015 and early 2016 by the ATLAS experiment at the Large Hadron Collider at CERN, is presented in this thesis. No significant deviation from the Standard Model expectation is observed and CLs exclusion limits are determined, both model-independent and in various MSSM benchmark scenarios. The MSSM exclusion limits are significantly stronger compared to previous searches, due to the increased collision energy and improvements of the event selection and background estimation techniques. The upper limit on tan beta at 95% confidence level in the mhmod+ MSSM benchmark scenario ranges from 10 at mA = 300 GeV to 48 at mA = 1.2 TeV.

The search for additional heavy neutral Higgs bosons predicted in Minimal Supersymmetric Extensions of the Standard Model is presented, using the direct decay channel into two tau leptons which themselves decay hadronically. The study is based on proton-proton collisions recorded in 2011 at a center-of-mass energy of 7 TeV with the ATLAS detector at the Large Hadron Collider at CERN. With a sample size corresponding to an integrated luminosity of 4.5 fb−1, no significant excess above the expected Standard Model background prediction is observed and CLs exclusion limits at a 95% confidence level are evaluated for values of the CP-odd Higgs boson mass mA between 140 GeV to 800 GeV within the context of the mhmax and mhmod± benchmark scenarios. The results are combined with searches for neutral Higgs bosons performed using proton-proton collisions at a center-of-mass energy of 8 TeV recorded with the ATLAS detector in 2012, with a corresponding integrated luminosity of 19.5 fb−1. The combination allowed an improvement of the exclusion limit at the order of 1 to 3 units in tan β. Within the context of this study, the structure of additional interactions during a single proton-proton collision (the “underlying event”) in di-jet final states is analyzed using collision data at a center-of-mass energy of 7 TeV recorded with the ATLAS detector in 2010, with a corresponding integrated luminosity of 37 pb−1. The contribution of the underlying event is measured up to an energy scale of 800 GeV and compared to the predictions of various models. For several models, significant deviations compared to the measurements are found and the results are provided for the optimization of simulation algorithms.