Thèses sur le sujet « Bosone di Higg »

Questa tesi di dottorato presenta lo studio della produzione associata di un bosone di Higgs con un bosone vettore (VH) in collisioni protone-protone, utilizzando i dati raccolti ad un'energia nel centro di massa delle collisioni pari a 13 TeV con l'esperimento CMS nel 2016. Questo studio è ristretto allo stato finale dove il boson di Higgs decade in due fotoni. Dopo un'introduzione riguardante il contesto teorico e sperimentale che motiva questo studio (Capitolo 1) e una descrizione generale dell'esperimento CMS (Capitolo 2), il lavoro sperimental necessario per ottenere exxellenti performance dal calorimetro eletromagnetico di CMS sono descritti in dettaglio (Capitolo 4). Le utilizzate nell'analisi dei dati e i risultati sono descritti nel capitolo 4, includendo la discussione dell'ottimizzazione dell'analisi che sono stati ritenuti benefici per gli sviluppi futuri. In particolare, il canale in cui il bosone vettore decade in due quark a quello in cui il bosone W decade leptonicamente sono descritti. I risultati dell'analisi sono inclusi in un articolo recentemente sottomesso per la pubblicazione su una rivista internazionale, rappresentando i primi risultati della misura della produzione VH nel canale H→ γγ con dati raccolti nel Run 2 di LHC.
This Ph.D. thesis presents a study of the associated production of the Higgs boson with a vector boson (VH) in proton-proton collisions, using data collected at a center-of-mass energy of 13 TeV with the CMS experiment in 2016 for an integrated luminosity of 35.9 fb −1 . The study is restricted to final states were the Higgs boson decays in two photons (H→ γγ). After an introduction on the experimental and theoretical landscape that motivates this study (Chapter 1) and an overview of the CMS experiment (Chapter 2), the exper- imental work performed to ensure the best quality of the CMS ECAL reconstruction is described in detail (Chapter 3). The analysis method and the results are then described (Chapter 4), including the discussion of optimization steps that were identified and will be relevant for future developments. In particular, the channels with the vector boson decaying into a pair of quarks and the W boson decaying into a lepton and a neutrino are described This analysis, included in a paper recently submitted for publication, represents the first result on the measurement of the VH production process in the H→ γγ final state with the LHC Run 2 data.

Une particule compatible avec le boson de Higgs du modèle standard de la physique des particules a été découverte au grand collisionneur de hadrons (LHC) en juillet 2012 par les expériences Atlas et CMS. A ce jour, l'ensemble des mesures des couplages de ce boson de Higgs sont compatibles avec ceux prédits par le modèle standard. Toutefois le modèle standard possède des limitations et d'autres théories, telles la supersymétrie, ont été bâties afin de pallier à ces manques. Certaines de ces théories prédisent en particulier un secteur scalaire enrichi avec la présence de bosons de Higgs additionnels.D'un point de vue expérimental, le canal de désintégration du boson de Higgs en deux photons offre une signature claire et permet d'explorer le secteur de Higgs avec une grande précision. Les travaux présentés dans cette thèse ont été effectués dans le cadre de l'expérience CMS au LHC.La première partie est dédiée à l'étude de la reconstruction expérimentale des photons. Tout d'abord, une méthode de correction en énergie des photons est exposée. Puis des prospectives concernant l'évolution de la qualité de leur reconstruction avec le vieillissement du détecteur sont présentées.La deuxième partie fait l'objet de la recherche d'un second boson de Higgs de basse masse(en dessous de 110 GeV) dans le canal di-photon. Les résultats complets obtenus avec le jeu de données collecté en 2012, ayant fait l'objet d'une publication officielle, sont tout d'abord présentés. Les premiers résultats obtenus avec le jeu de données collecté en 2015 sont enfin introduits
A particle compatible with the Standard Model Higgs boson has been discovered at the Large Hadron Collider (LHC) in July 2012 by Atlas and CMS experiments. All the couplings measurements are so far compatible with those predicted by the Standard Model. However, this model is not able to address several fundamental physics issues and theories beyond the Standard Model, such as supersymmetry, have been proposed to address these questions. Some of them predict an extended Higgs sector with additional scalar or pseudoscalars at low mass.On the experimental point of view, the diphoton decay channel provides a clear signature and allows us to explore the Higgs sector with high precision.The studies presented in this thesis have been performed using the data collected by CMS at the LHC.The first part of the document is dedicated to photon reconstruction studies. First, a photon energy correction method is presented. Then, photon reconstruction performance is assessed in the context of the Phase I and Phase II detector upgrades, in high luminosity conditions.The second part of the document is dedicated to the search for an additional light Higgs boson (with a mass below 110 GeV) in the diphoton channel. First, the official results corresponding to the full 2012 dataset are exposed. Finally, results corresponding to 2015 data are presented

Cette thèse présente une recherche d'événements avec paires de bosons de Higgs (HH) en collisions proton-proton à 13 TeV, fournies par le Large Hadron Collider, au sein de l'expérience CMS (Compact Muon Solenoid) du CERN (Genève). L'étude de la production de paires de bosons de Higgs permet la mesure de la constante d’auto-couplage trilinéaire. En plus, la production HH par fusion de bosons vecteurs (Vector Boson Fusion ou VBF) donne accès à la mesure de la constante de couplage entre deux bosons de Higgs et deux bosons vecteurs. La valeur de ces deux paramètres est particulièrement sensible à l'existence de physique au-delà du Modèle Standard : même des faibles variations par rapport aux valeurs des couplages prévus par la théorie peuvent induire un changement important de la section efficace. Cette thèse cible le cas où un des bosons de Higgs se désintègre dans deux leptons tau et l'autre dans deux jets de particules engendrés par des quarks de type beau : cet état final permet de conjuguer une statistique importante garantie par la désintégration en quarks et la pureté de la désintégration en leptons tau.Cependant, la production de HH au LHC est un processus très rare. La production par le mécanisme principal, de fusion de gluon, a une section efficace d'environ 30 fb, suivi par le processus de VBF, lequel est environ 10 fois moins probable. Ainsi, optimiser l'efficacité de la sélection du signal est essentiel. Par conséquent, la première partie du travail de thèse a été dédiée à l'étude d'algorithmes pour le premier niveau du système de déclenchement de CMS, en implémentant des sélections similaires à celles qui sont appliquées au niveau de l'analyse finale, et un algorithme dédié au processus VBF a été mis au point en ciblant des possibles améliorations pour la recherche d'événements HHbbtt. Sa topologie, caractérisée par la présence de deux jets reconstruits dans des régions opposées du détecteur, est un levier puissant pour le distinguer des autres processus. L’algorithme sélectionne les événements en exploitant les propriétés cinématiques des jets, indépendamment des caractéristiques de la désintégration du boson de Higgs : d'ailleurs, utilisé en complément des algorithmes classiques, ciblant les produits de désintégration du boson de Higgs, l’algorithme VBF permet d’étendre la couverture de l'espace des phases de manière significative. Il s’agit du premier algorithme VBF pour le système de déclanchement et il a été inclus dans la prise de données à partir de l'été 2017. D'ailleurs, les données ainsi sélectionnées sont accessibles pour les recherches du boson de Higgs en cours.La suite du travail de thèse a été consacrée à l'analyse d'événements HH->bbtautau avec les données collectées en 2017. L'extraction du signal depuis les données requiert une bonne connaissance des bruits de fond, dont les principaux sont la production de ttbar qui est irréductible dans la mesure où il produit une paire de quark b et une paire de leptons taus ; le bruit de fond d'événements multi-jet génériques, où les jets peuvent être identifiés à tort comme leptons taus ; et le bruit de fond de Z->tautau où des jets additionnels peuvent être identifiés comme des jets de b. Ce dernier bruit de fond a été étudié en détail pour en améliorer la modélisation. En plus de l'étude inclusive des événements de type HH->bbtautau, une catégorie d’événements dédiée à la production par VBF a été incluse, conçue à partir de l'espace des phases couvert par l’algorithme VBF du système de déclanchement et de la topologie typique du processus, en utilisant aussi des techniques d’apprentissage automatique. L'introduction de telle catégorie est la première mesure dédiée à ce mécanisme de production dans le cadre des analyses de HH->bbtautau. Ces améliorations et les interprétations additionnelles sont très importantes pour la collaboration CMS et ils seront inclus dans la publication relative aux données collectées pendant le Run 2
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

Con l’avvento della fase ad Alta Luminosità di LHC (HL-LHC), la luminosità istantanea del Large Hadron Collider del CERN aumenterà di un fattore 10, oltre il valore di progettazione. La Fisica delle Alte Energie si dovrà quindi confrontare con un notevole aumento della quantità di eventi raccolti, consentendo l'indagine di scale energetiche ancora inesplorate. In questo contesto si colloca la ricerca di bosoni di Higgs neutri aggiuntivi, previsti dalle estensioni del Modello Standard, quali ad esempio quella del Modello Supersimmetrico Minimale, teoria che suppone l’esistenza di altri bosoni di Higgs, più massivi di quello rilevato nel 2012. Alcune precedenti ricerche sono state condotte dai diversi esperimenti di LHC; in particolare per questa tesi è stata presa come riferimento l’analisi pubblicata nel 2019 dalla Collaborazione CMS, ricercando i bosoni di Higgs in uno stato di decadimento finale mu^{+} mu^{-} in un range di massa tra i 130 e i 1000 GeV, prodotti dalla collisione di protoni ad una energia del centro di massa di 13 TeV. Con il nuovo upgrade di LHC sarà possibile estendere il range di massa in cui ricercare i bosoni fino ad oltre 1 TeV può quindi dimostrarsi conveniente l’utilizzo di nuove tecniche e strumenti di analisi che utilizzano algoritmi di Machine Learning. In questo modo sarebbe infatti possibile incrementare il livello di complessità dell’analisi, includendo eventualmente tra le variabili di ingresso anche l’ipotesi di massa iniziale del bosone di Higgs supersimmetrico ed ottenendo un modello sempre più generalizzato. In questa tesi vengono confrontati i risultati della classificazione tra eventi di segnale ed eventi di fondo di un’analisi "tradizionale" cut-based, sulla base di quella di riferimento, e un' analisi multivariata condotta utilizzando una BDT implementata in ROOT. I risultati confermano un possibile miglioramento nella classificazione tra segnale e fondo, utilizzando un’analisi multivariata.

Durante il mio percorso di dottorato mi sono occupato principalmente dello sviluppo di nuovi sensori a pixel in Silicio per l’upgrade dell’esperimento CMS in vista della fase ad alta luminosità del collisionatore LHC (HL – LHC o LHC fase 2) al CERN, un’attività che avevo già intrapreso per il mio lavoro di tesi magistrale. Questi sensori devono tollerare fluenze di radiazione di alcuni 10^16 neq/cm^2 ad una distanza di circa 3 cm dal punto di interazione. Il programma di R&D portato avanti da un gruppo INFN ATLAS-CMS, in collaborazione con Fondazione Bruno Kessler, riguarda lo sviluppo di sensori planari sottili e 3D su substrati prodotti con la tecnica del Direct Wafer Bonding. L'impiego di questa tecnica permette di attuare tutti i processi di fabbricazione su una sola superficie del wafer, riducendo notevolmente i costi del processo. Lo spessore attivo dei prototipi è di 100 o 130 um per i sensori planari e 130 um per quelli 3D. I primi sensori prodotti hanno pixel delle stesse dimensioni di quelli attualmente in uso in CMS, 100 x 150 um^2. Le dimensioni sono state poi ridotte a 25 x 100 um^2 e 50 x 50 um^2 in modo da aumentare il numero di canali di lettura in vista della maggiore molteplicità di tracce attesa ad HL-LHC. Sono state effettuate numerose campagne di prova su fascio per caratterizzare i sensori planari e quelli 3D, prima e dopo l’irraggiamento a fluenze fino a 1 x 10^16 neq/cm^2. I miei contributi alla caratterizzazione dei prototipi sono riportati in questa tesi e includono anche lo sviluppo di algoritmi dedicati. Dopo circa due anni di lavoro sulla caratterizzazione dei nuovi prototipi ho sviluppato un interesse anche per l’impatto del rivelatore a pixel sul programma di fisica dell’esperimento. All’inizio del mio secondo anno di dottorato mi sono messo in cerca di un’analisi in cui le informazioni del rivelatore a pixel fossero largamente sfruttate. Ho scelto di lavorare alla ricerca della produzione di coppie di bosoni di Higgs nello stato finale con 2 b-jets e 2 leptoni tau. Ho scelto questa analisi sia perchè la ricostruzione delle particelle nello stato finale fa ampio uso delle informazioni di tracce e vertici sia perchè l’analisi trarrà grande benificio dell'incremento della statistica previsto ad HL-LHC, che potrebbe portare ad avere la prima evidenza sperimentale di produzione HH. La ricerca di coppie di bosoni di Higgs ha un ruolo fondamentale nella caratterizzazione di questa particella in quanto rappresenta il canale migliore per la misura della costante di accoppiamento trilineare. Ogni deviazione dalle previsioni teoriche del Modello Standard porterebbe a cambiamenti importanti nella cinematica e nel rateo di produzione di coppie di bosoni di Higgs, il che rende queste ricerche sensibili a effetti di nuova fisica. Lo stato finale bbtau^+tau^- rappresenta uno dei canali più interessanti in questo studio visti il branching ratio elevato e la piccola contaminazione di eventi di fondo. In questa tesi viene descritta la strategia sviluppata per l'analisi dei dati raccolti dall’esperimento CMS durante il Run 2 di LHC (2016 + 2017 + 2018), corrispondenti ad una luminosità integrata di 137 fb^-1. Viene inoltre riportata la sensitività attesa dell'analisi, che include lo studio di due meccanismi di produzione di coppie di bosoni di Higgs: Gluon-Gluon Fusion (GGF) e Vector Boson Fusion (VBF).
My research activity during the PhD has focused mainly on the development of new silicon pixel sensors for the upgrade of the CMS experiment inner tracker in view of the CERN LHC High Luminosity phase (HL-LHC), an activity I started to work on during my master thesis. These sensors must be capable of surviving irradiation fluences up to a few 10^16 neq/cm^2 at ~ 3 cm from the interaction point. The R&D program carried out by an INFN ATLAS-CMS group, in collaboration with Fondazione Bruno Kessler, covers both planar and 3D pixel devices made on substrates obtained by the Direct Wafer Bonding technique. Using this technology, every fabrication process takes place on one side only of the wafer, with consequent cost savings. The active thickness of the planar sensors studied in this thesis is 100 um or 130 um, that of 3D sensors 130 um. Sensors belonging to the first batches have pixel cells with dimensions of 100 x 150 um^2, same as the sensors currently installed in CMS. These dimensions have been reduced to 25 x 100 um^2 and 50 x 50 um^2 in order to cope with the higher track multiplicity expected at HL-LHC. Prototypes of hybrid modules, bump-bonded to different readout chips, have been characterized in beam tests. Results on their performance before and after irradiation up to maximum fluence of ∼ 1 x 10^16 neq/cm^2 are reported in this thesis. After a couple of years working on the characterization of the new prototypes I became interested also on the impact of the pixel detector on the physics program of the experiment. At the beginning of my second year of PhD I started to look for a physics analysis where the information of the pixel detector is largely exploited. I chose to work on the search for double Higgs boson production in final states with 2 b-jets and 2 tau leptons since the reconstruction of the b-jets and tau leptons makes large use of vertexes and tracks information. Another reason why I chose this analysis is that it will benefit from the incremented statistics foreseen at HL-LHC, possibly leading to the first experimental evidence of HH production. Double Higgs searches play a fundamental role in the characterization of the Higgs boson as they represent the favorite channel to measure the Higgs boson trilinear self coupling. Any deviation from the theoretical predictions of the Standard Model would lead to sizable changes in both the kinematics and production rate of HH events, thus making double Higgs searches sensitive to new physics effects. The bbtau^+tau^-final state represents one of the most interesting channels to explore double Higgs production, because of the high branching ratio and the relatively small background contamination. The strategy developed to analyze data collected by the CMS experiment during the LHC Run 2 (2016 + 2017 + 2018), corresponding to an integrated luminosity of 137 fb^-1, is described in this thesis. The expected sensitivity of the analysis, targeting both the Gluon Fusion (GGF) and Vector Boson Fusion (VBF) production channels, is also reported.

La recherche d'un boson de Higgs léger dans le canal H --> gamma gamma dans l'expérience ATLAS nécessite d'excellentes performances du calorimètre électromagnétique et une bonne compréhension des bruits de fond. Ce travail de thèse s'articule donc autour de deux points. Le premier concerne le fonctionnement et les performances du calorimètre électromagnétique. D'abord, des développements sont apportés au code de reconstruction online des calorimètres à Argon liquide, et ses algorithmes sont validés, ce qui inclut un contrôle des données en ligne. Ensuite, deux mesures indépendantes de l'uniformité du calorimètre électromagnétique tonneau sont réalisées, l'une avec les muons cosmiques et portant sur une couverture effective d'environ 20% du calorimètre, et l'autre avec les événements pi0 --> gamma gamma dans les premières collisions à sqrt(s) = 7 TeV, avec une luminosité intégrée de L = 414.8 μb−1. La mesure sur les muons montre que les non-uniformités dans la direction eta sont inférieures à 1.0% dans le compartiment milieu et 1.7% dans le compartiment avant, à 95% de niveau de confiance. Avec les pi0, ces résultats sont améliorés et aboutissent à une dispersion de la réponse entre données et simulation inférieure à 0.7% dans les trois cryostats (tonneaux et bouchons), ce qui est au niveau des performances attendues au démarrage. Le deuxième axe de cette thèse concerne l'extraction de paires non-résonnantes de photons. Deux méthodes sont présentées. La première est une double mesure de pureté utilisant une extrapolation du bruit de fond à partir de régions enrichies en bruit de fond vers une région de signal. La seconde méthode est basée sur une matrice d'efficacités de taille 4x4, entièrement déterminée par la simulation et appliquée événement par événement. Ces méthodes sont appliquées aux données de collision, correspondant à une luminosité intégrée de L = 2.82 pb−1. Un signal di-photons direct de 108.6 ± 19.5 (stat.) ± 34.5 (syst.) avec une pureté de (65.1 ± 8.6 (stat.) ± 8.7 (syst.))% est extrait.

Uno dei cardini nel programma di ricerca attuale del Large Hadron Collider (LHC) al CERN è l’approfondimento della conoscenza relativa al bosone di Higgs e agli accoppiamenti di questa particella, di recente scoperta, con le altre del Modello Standard. Il prossimo Run di LHC sarà caratterizzato da collisioni di fasci di protoni con un'energia di 6.5 TeV ciascuno e renderà possibile l’acquisizione di grandi campioni di dati nei quali si prevede un aumento della statistica per tipologie di eventi che fino a questo momento è stato problematico studiare. Tra questi la produzione per Higgs-strahlung del bosone di Higgs associato al bosone vettore Z, che, essendo caratterizzata da una bassa sezione d’urto, è sempre stata considerata un processo molto difficile da investigare. Questa tesi fornisce uno studio preliminare della fattibilità di recuperare in modo efficiente questo canale, con l’obiettivo di individuare alcuni tagli che permettano di ripulire il grande fondo adronico prodotto nelle collisioni protone-protone a LHC. La presente analisi è stata effettuata su campioni di dati ottenuti tramite una generazione Monte Carlo e una simulazione parametrica del rivelatore ATLAS. Sebbene la statistica dei campioni MC sia ancora limitata e la simulazione della risposta del detector non sia dettagliata, le tecniche e i tagli utilizzati in questo lavoro di tesi potranno dare utili indicazioni per futuri studi più dettagliati e per l'investigazione di questo processo una volta che i dati del prossimo Run di LHC a √s=13 TeV saranno disponibili.

In questa tesi viene presentato il contributo personale all’interno della misura dell'accoppiamento del bosone di Higgs con i quark b condotta da ATLAS sui dati raccolti nel 2015-2018 (Run2), ad una luminosità di 140 fb-1. Tale contributo si sviluppa all’interno dell’analisi ZH-> ll ̅bb ̅, dove mi sono occupato delle incertezze sistematiche relative alla modellizzazione dei processi di fondo Z+jets. Ho confrontato le distribuzioni nominali prodotte dal generatore SHERPA2.2.1 per alcune variabili cinematiche con le distribuzioni ottenute variando le PDF prima, e con quelle ottenute variando i coefficienti di fattorizzazione \mu_F e di rinormalizzazione \mu_R poi. Ho infine stimato l’incertezza sistematica dovuta a queste variazioni in due regioni di impulso trasverso del bosone vettore Z e per diverse molteplicità di b-jets. Si è ottenuto che le distribuzioni ottenute variando le PDF si discostano da quella nominale per valori oscillanti tra l’1% ed il 3% sia per molteplicità di 2 che >= 3 jets. Le distribuzioni ottenute variando i fattori di scala presentano variazioni del 15% per molteplicità di 2 jets e maggiori del 50% per un numero >= 3 di jets.

The search for the Higgs boson will be one of the primary tasks of the LHC. One of the most promising channels in the search for a low mass Higgs boson will be the scenario where the Higgs is produced via Vector Boson Fusion and subsequently decays to a pair of tau leptons. One of the dominant (and most complex) backgrounds to this analysis is expected to come from tt decays. Monte Carlo samples with a centre-of-mass energy of 14 TeV are used to investigate the contributions from different tt decay modes and to develop methods to reduce the contribution from this background process and improve the overall sensitivity of the analysis. A technique to estimate contributions from background processes when there are insufficient Monte Carlo samples available to calculate this directly is also presented. A low mass Standard Model Higgs boson will require at least 10 inverse femtobarns of data before a discovery (or exclusion) can be claimed, but extensions to the Standard Model that realise Supersymmetry predict that there should be three neutral Higgs bosons. The production cross-section is strongly enhanced and it is there- fore expected that competitive limits on an MSSM Higgs boson can be made using data from the 2010-2011 LHC data-taking period (1 inverse femtobarn expected with 7 TeV centre-of-mass collisions). A search strategy for an MSSM Higgs boson decaying to two taus is presented, focusing on ensuring analysis robustness during this early period. Comparisons with the first ATLAS data are also made.

The Higgs self-coupling has a vital role by giving a deeper understanding of the Higgs particle. Furthermore, the way it opens to physics beyond the SM, encourages us to do MC simulationstudies for varying λ_hhh . In this project, we investigate the effects of choosing different values for λ_hhh on the kinematics of all particles involved in the hh → τ τ τ τ decay channel and the resultsare compared with what we get from the SM prediction of λ_hhh .The data show that λ_hhh more close to the SM trilinear Higgs self-coupling results in generatingthe Higgs particles with the higher masses and higher momenta. On the other hand, for the moremassive Higgs bosons we have more energetic neutrinos in the final states which escape from thedetector without being detected.

This document, prepared to obtain the "Habilitation à Diriger des Recherches", is a compendium of the photon-related analysis activities I carried on within ATLAS in the past four years and a half. The activities I will describe can be broadly classified into three categories: optimization and/or in situ measurement of photon-related performance, measurements of the cross sections of Standard Model processes producing prompt photons, and searches (leading to discovery!) of a Standard Model Higgs boson decaying to final states containing photons.

Lo studio descritto in questa tesi è volto a migliorare la selezione degli eventi per la ricerca del bosone di Higgs del Modello Standard nel canale di decadimento in due muoni con il rivelatore CMS ad LHC. Sono stati analizzati i dati raccolti durante il Run del 2017, corrispondenti ad una luminosità integrata di fb-1 . La selezione degli eventi studiata consiste in una categorizzazione del segnale basata sul meccanismo di produzione, che separi gli eventi prodotti tramite il meccanismo di Gluon Fusion (ggH) da quelli prodotti tramite il meccanismo di Vector Boson Fusion (VBF). La topologia di questi ultimi è caratterizzata dalla presenza di due jet, a grande angolo ed elevata massa invariante, prodotti insieme al bosone di Higgs. Questo consente una più efficace reiezione del fondo rispetto al caso inclusivo. A tutti gli eventi non VBF-like è stata applicata un'ulteriore categorizzazione basata sulla pseudorapidità del muone più in "avanti’’, che consente di sfruttare la diversa risoluzione del momento E' stata confrontata la significatività ottenuta combinando le quattro categorie con quella del caso inclusivo,ottenendo un guadagno di circa l'8\%.

The performance of the HH->bbZZ (l) (l = e, mu) analysis is presented considering the possible High Energy upgrade of the LHC collider, at a center-of-mass energy of √s = 27 TeV and with an integrated luminosity of 15000 fb^{-1}; a comparison of the results with the same study performed in the context of the High Luminosity LHC, at a center-of-mass energy of √s =14 TeV and with an integrated luminosity of 3000 fb^{-1}, is also reported. The Phase-II upgraded CMS detector, expected in 2024, is assumed in both the scenarios. The analysis is fully based on Monte Carlo simulations: the parton-level generation of signal and backgrounds is performed with MADGRAPH5_AMC@NLO, while parton showering and hadronization are achieved with PYTHIA8; the detector response is then obtained in the parametric framework provided by DELPHES. A limit on the di-Higgs signal strength of r =1.45 is estimated at 95% CL in the inclusive bb4l channel, with a corresponding signal significance of 1.43 σ. A study on the prospects for the measurements of the Higgs self-coupling (λ_3) is also performed: the projected confidence interval on k_λ = λ_3/λ_3^{SM} is found to be [-0.7,7.2] at the 95% CL.

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.

The present thesis work has been carried out in the framework of the CMS collaboration, one of the experiment designed to study the physics of the proton-proton collisions at the Large Hadron Collider (LHC) at CERN. Experimentation at CMS (and at ATLAS) led to the discovery of a new particle in 2012 which has been identified as the Higgs boson, the missing brick of the Standard Model of the fundamental interactions. All the experiments at LHC are upgrading their detectors in order to fulfill the continuous increment of the LHC luminosity and the consequent increment of the per collision event rate. The CMS upgrade project foresees, inter alia, the production of a new pixel detector (CMS Phase 1 Pixel Upgrade) to be commissioned at the beginning of 2017. Crucial part of the upgrade is the new readout chip (ROC) for the silicon sensor, psi46digV2respin, designed at the Paul Scherrer Institute (PSI) with a 250 nm CMOS technology. This chip represents the state of the art in the readout electronics for the silicon detectors. The thesis concerns the study and the development of test procedures for this new readout chip. Thanks to a long stay at PSI, I could provide an important contribution to the debug phases of the first version of the ROC and TBM, the chip that handles the various ROCs in the pixel module, and to the development of the software used by the whole collaboration for the ROC and module testing. This experience allowed me to be the expert for the installation and commissioning of the ROC readout system in all the production centres in Italy. Furthermore, I managed the ROC wafers test from the early project phases. The ROCs are produced on silicon wafers and undergo various processes before being assembled on the modules, e.g., metal deposition on the pixel pads, thinning and dicing. These processes lead mechanical and thermal stresses that can damage the chips. The ROC wafers test has thus been performed following the same procedure before and after the processing. In order to minimize the failing ROCs fraction mounted on the modules. It has been measured that the processing introduces a 5.2 % reduction of the number of perfectly working ROCs. The pixel detector production line, the module qualification process and the ROC wafers test results are reported in this thesis. The modifications performed on the ROC-sensor connection technology are also described. The new pixel detector installation will allow an increase of the tracks reconstruction efficiency and a 10-15 um resolution to be maintained in the interaction vertices reconstruction, independently from the increment of the mean number of events per p-p interaction from the current 15 to 50-60 in 2017. The excellent performances of the new pixel vertex detector plus the planned increment of the luminosity (a factor 35 between 2017 and the current value) could allow access to physical processes with a low cross section and a high number of b quarks in the final states. For this purpose, a preliminary study on the non-resonant Higgs bosons pair production in the fully hadronic decay channel bbbb is here presented. The study has been performed analysing the data collected by the CMS experiment in 2015, equal to 2.19 fb-1. This process has a low cross section, accordingly to the SM, and its measurement in p-p collision at centre of mass energies of 13-14 TeV is forecast only with a high amount of data (ab-1). The data collected in 2015 do not allow to set a reasonable exclusion limit on the cross section of this process and the analysis will be completely developed in 2016 and following years. The study of the Higgs bosons pair production is relevant because the pairs can be produced also after couplings which are not allowed by the Standard Model (anomalous), such as the higgs-gloun contact interaction. These anomalous couplings lead to an increment of the cross section of the process and to differences in the kinematics of the final states. The process of Higgs bosons pair production via anomalous couplings is described by a Lagrangian with five free parameters. This implies a difficulty in the identification of parameters space point to be experimentally investigated. I developed an analysis technique which allows, by studying simulated samples, to divide the parameters space in kinematically similar regions and to identify a benchmark in each of them. The distance among different points of the parameters space has been defined through a binned likelihood ratio and an iterative algorithm has been developed to group them together. Twelve regions which are kinematically equivalent have been identified in a 5-D space. The results of this study, described in this thesis, are collected in an article which is under publication on JHEP and they will be considered as guideline for the searches of non-resonant Higgs bosons pair production at CMS.
Questo lavoro di tesi è stato svolto nell'ambito della collaborazione CMS, uno degli esperimenti progettati per studiare la fisica delle collisioni protone-protone al Large Hadron Collider (LHC) presso il CERN. La sperimentazione a CMS (ed a ATLAS) ha portato nel 2012 alla scoperta di una nuova particella che è stata in seguito identificata come il bosone di Higgs, l'ultimo tassello mancante dello Standard Model delle interazioni fondamentali. Tutti gli esperimenti a LHC sono in una fase di miglioramento degli apparati in vista del continuo aumento di luminosità di LHC e del conseguente aumento del rate di eventi per collisione. Il progetto di upgrade di CMS prevede tra l'altro la produzione di un nuovo rivelatore a pixel (CMS Phase 1 Pixel Upgrade) da inserire nell'apparato all'inizio del 2017. Parte cruciale dell'upgrade è il nuovo chip di lettura (ROC) dei sensori al silicio, psi46digV2respin, disegnato al Paul Scherrer Institute (PSI) in tecnologia CMOS 250 nm. Tale chip rappresenta lo stato dell'arte nell'elettronica di lettura di rivelatori al silicio. La tesi riguarda lo studio e lo sviluppo di procedure di test di questo nuovo chip di lettura. Grazie ad una lunga permanenza al PSI ho potuto fornire un importante contributo alle fasi di debug delle prime versioni del ROC e del TBM, il chip che gestisce i diversi ROCs all'interno di un pixel module, ed allo sviluppo del software utilizzato da tutta la collaborazione per il test del ROC e dei moduli. Questa esperienza mi ha permesso essere l'esperto per l'installazione e messa a punto dei sistemi di lettura dei ROC nei centri di produzione di moduli in Italia. Inoltre ho gestito il test dei ROC wafers fin dalle prime fasi di sviluppo del progetto. I ROC sono prodotti su wafers di silicio e subiscono diversi processi di lavorazione prima dell'assemblaggio sui moduli, e.g., la deposizione di metalli sulle pixel pads, l'assottigliamento ed il taglio. Queste lavorazioni comportano stress meccanici e termici che possono danneggiare i chips. Il test dei ROC wafers è stato quindi effettuato, con la stessa procedura, prima e dopo la lavorazione per ridurre al minimo la frazione di ROCs non funzionanti montati sui moduli. Si è misurato che la lavorazione dei wafers introduce una riduzione del 5.2 % del numero di ROCs perfettamente funzionanti. Nella tesi sono riportati la catena di produzione del rivelatore a pixel, il processo di qualifica dei moduli ed il dettaglio dei test su ROC wafers. Vengono inoltre descritte le modifiche apportate alla tecnologia utilizzata per la connessione ROC-sensore. L'installazione del nuovo rivelatore a pixel permetterà di aumentare l'efficienza di ricostruzione delle tracce e di mantenere una risoluzione di 10-15 um nella ricostruzione dei vertici d'interazione anche all'aumentare del numero medio di eventi per interazione p-p dagli attuali 15 a 50-60 nel 2017. Le ottime prestazioni del nuovo rivelatore di vertice a pixel sommate al previsto aumento di luminosità (un fattore 35 tra il valore attuale e quello previsto per il 2017) potranno dare accesso a processi fisici con bassa sezione d'urto ed alto numero di b quarks negli stati finali. Viene a tal fine presentato uno studio preliminare della produzione non risonante di coppie di bosoni di Higgs nel canale di decadimento completamente adronico (bbbb) analizzando i dati raccolti dall'esperimento CMS nel 2015, pari a 2.19 fb-1. Questo processo presenta una ridotta sezione d'urto, secondo il Modello Standard, e la sua misura in collisioni p-p ad energie del centro di massa di 13-14 TeV è prevista solo con un'elevata quantità di dati (ab-1). I dati raccolti nel 2015 non permettono di ottenere un limite ragionevole sulla sezione d'urto di produzione di tale processo e l'analisi verrà sviluppata pienamente nel 2016 e nei successivi anni. Lo studio della produzione di coppie di bosoni di Higgs è rilevante poiché esse possono essere prodotte anche in seguito ad accoppiamenti non previsti dal Modello Standard (anomali), come l'interazione di contatto tra bosoni di Higgs e gluoni. Tali accoppiamenti anomali danno luogo ad un incremento della sezione d'urto del processo e ad una differente cinematica degli stati finali. Il processo di produzione di coppie di bosoni di Higgs per mezzo di accoppiamenti anomali è descritto da una Lagrangiana con cinque parametri liberi. Questo comporta una difficoltà nell'individuare i punti dello spazio dei parametri da indagare sperimentalmente. Ho sviluppato una tecnica di analisi che permette, attraverso lo studio di campioni simulati, di suddividere tale spazio in regioni cinematicamente simili ed identificare per ognuna di esse il punto maggiormente significativo. La distanza tra i diversi punti dello spazio dei parametri è stata definita tramite un ”binned likelihood ratio” ed un algoritmo iterativo è stato sviluppato per raggruppare tali punti. Sono state quindi individuate dodici regioni cinematicamente simili in uno spazio 5-D. I risultati di tale studio, descritti in questa tesi, sono raccolti in un articolo in fase di pubblicazione su JHEP e verranno considerati come linee guida per le ricerche di produzione non risonante di coppie di bosoni di Higgs a CMS.

Questa tesi si svolge nell’ambito della ricerca del bosone di Higgs supersimmetrico all’esperimento CMS del Cern, sfruttando le collisioni protone-protone dell’acceleratore LHC. Il lavoro da me svolto riguarda lo studio della parametrizzazione dell’insieme dei dati che costituiscono il fondo dell’eventuale presenza di un segnale del bosone di Higgs nel modello supersimmetrico minimale. Ho eseguito dei fit al fondo aspettato tramite il package Root, utilizzando diverse funzioni per parametrizzarne analiticamente l’andamento. Nel lavoro di tesi mostro come tali funzioni approssimano i dati e ne discuto le loro caratteristiche e differenze.

Nel modello standard (SM), le masse dei fermioni sono generate attraverso gli accoppiamenti di Yukawa tra i campi di Higgs e fermionico. La misura di questi accoppiamenti e' fondamentale per corroborare la natura del bosone con massa intorno a 125 GeV scoperto dalle Collaborazioni ATLAS e CMS nel 2012. Il canale tautau e' particolarmente promettente, grazie al branching ratio relativamente alto e alla contenuta contaminazione da parte dei processi di fondo. In questa tesi vengono riportati i risultati della ricerca di un bosone di Higgs SM nello stato finale comprendente due leptoni tau. Sono stati analizzati gli interi dataset di collisioni p-p raccolti da CMS nel 2011 e nel 2012, corrispondenti alla luminosita' di 4.9 fb-1 a sqrt(s)=7 TeV e 19.7 fb-1 a sqrt(s)=8 TeV. Sono stati studiati tutti i sei possibili stati finali di-tau: mutau, etau, tautau, emu, mumu e ee. Questa tesi, in particolare, descrive l'analisi condotta nello stato finale doppio adronico tautau. Nei dati e' stato osservato un eccesso di eventi rispetto alla predizione per i soli fondi. La significanza locale eccede le 3 deviazioni standard per valori di mH compresi tra 115 e 130 GeV. Il prodotto della sezione d'urto per branching ratio, misurato per il segnale a mH=125 GeV, corrisponde a 0.78+/-0.27 volte il valore predetto dallo SM. Questo costituisce l'evidenza dei decadimenti in coppie di leptoni tau del bosone di Higgs a 125 GeV. Nello SM, la massa del bosone di Higgs non è protetta da divergenze date dalle correzioni a loop e la cancellazione di queste divergenze avviene tramite il cosiddetto fine tuning. In modelli oltre lo SM, ad esempio il Minimal Supersymmetric Standard Model (MSSM), la cancellazione avviene in maniera naturale grazie all'introduzione di un bosone (fermione) per ogni fermione (bosone) presente nello SM. In questa tesi viene anche descritta la ricerca di un bosone di Higgs neutro MSSM nel canale tautau. Questo canale e' particolarmente interessante poiche' i decadimenti in tau sono favoriti in buona parte dello spazio dei parametri. Non viene osservato nessun eccesso e si procede quindi a fissare limiti di esclusione nello spazio dei parametri di diversi benchmark scenarios. Vengono inoltre forniti limiti sul prodotto di sezione d'urto per branching ratio per i due piu' rilevanti modi di produzione, gluon-gluon fusion e produzione associata con b-quark.

Ce travail de thèse a été effectué auprès du détecteur ATLAS. Trois méthodes indépendantes sont proposées pour mesurer l'efficacité du déclenchement basé sur la détection de photons. Elles sont d'abord évaluées à l'aide de simulations Monte-Carlo, puis appliquées sur les données enregistrées en 2010. Les deux méthodes utilisant des lots de photons montrent des résultats cohérents au niveau de quelques pourcents. Pour la méthode basée sur un lot d'électrons, la taille du lot est trop faible pour tirer des conclusions. Une mesure de la section efficace de production QCD de di-photons est effectuée sur les données 2010, correspondant à une luminosité de 37.2+/-1.2 pb-1. Elle se base sur une méthode d'ajustement bi-dimensionnel pour extraire le signal. Les sections efficaces en fonction des observables Mγγ, pT γγ et Δφγγ sont comparées avec les prédictions des générateurs DIPHOX et RESBOS. Un bon accord est trouvé pour la variable Mγγ, tandis que des écarts sont observés dans les distributions pTγγ et Δφγγ. Dans l'étude du canal H → γγ basée sur échantillon simulé, une détérioration de 4% dans la limite d'exclusion est observée après incorporation de l'incertitude systématique découlant de la résolution sur la masse invariante de la paire de photons. Dans l'analyse des données réelles, la méthode d'ajustement 2D est également appliquée pour décomposer le bruit de fond dans la gamme de masse invariante [100, 150] GeV. La sensibilité de la recherche du boson de Higgs dans ce canal est alors réévaluée. Dans la gamme de masse [110-140] GeV, une production de Higgs de 3.2 à 4.2 le Modèle standard devrait être exclue avec 1fb-1 de données à √ s = 7 TeV.

Le quark top a été découvert en 1995 par les collaborations CDF et D0 dans les collisions proton-antiproton du Tevatron. Aujourd'hui, la quantité de données collectées par chaque expérience est devenue suffisante pour étudier avec precision ce quark très massif. Cette thèse est consacrée à la mesure de la section efficace de production de paires de quarks top par interaction forte, dans un état final contenant deux électrons, deux jets de particules et de l'énergie transverse manquante. Elle utilise un échantillon de données de 1 fb-1 enregistré par l'expérience D0 entre 2002 et 2006.
La reconstruction et l'identification des électrons et des jets sont primordiales pour cette analyse, et ont été étudiées dans une topologie où un boson Z est produit en association avec un ou plusieurs jets. Le processus Z+jets constitue en effet le bruit de fond physique dominant pour la production top-antitop dans l'état final diélectron.
Le principal enjeu de la mesure de section efficace est la vérification des prédictions du Modèle Standard. Dans ce manuscrit, ce résultat est également interprété pour extraire de façon indirecte la masse du quark top. Par ailleurs, la mesure de la section efficace est sensible à une éventuelle manifestation de nouvelle physique telle que l'existence d'un boson de Higgs chargé. La sélection établie pour mesurer la section efficace de production top-antitop a été mise à profit pour rechercher un boson H+ plus léger que le quark top, ce dernier pouvant ainsi se désintégrer en un boson W+ ou H+ et un quark b. Dans le modèle étudié, le boson H+ se désintègre exclusivement en un lepton tau et un neutrino.

A test of CP invariance in Higgs boson production via Vector Boson Fusion using the optimal observable method is presented. The analysis exploits the decay mode of the Higgs boson into a pair of tau leptons in the decay channels H → TlTl and H → TlT_had and is based on 20.3 fb⁻¹ of proton-proton collision data at √s = 8 Tev collected by the ATLAS experiment at the LHC. CP-violating interactions between the Higgs boson and electroweak gauge bosons are described in an effective field theory framework, where the strength of CP violation is governed by a single parameter d̃. The mean values and distributions of CP-odd observables agree with the expectation in the Standard Model and show no sign of CP violation. The CP-mixing parameter d̃ is constrained to the interval [-0.11, 0.05] at the 68% confidence level, consistent with the Standard Model expectation of d̃ = 0.

ATLAS is one of the four main experiments at the LHC proton-proton accelerator at CERN. This thesis describes two correlated topics: the observation for the Higgs boson in the diphoton channel and the Monte Carlo calibration of electrons and photons. The Higgs boson is a particle predicted by the Standard Model to explain the mechanism for electroweak symmetry breaking, giving masses to the particles. A particle compatible with the SM Higgs boson has been discovered by the ATLAS and CMS experiments in 2012. If this new boson is the Higgs boson, all fundamental parameters of the SM are known and, for the first time, it is possible to overconstrain the SM at the electroweak scale and to evaluate its validity. The proton-proton collision datasets used for the diphoton analysis correspond to integrated luminosities of 4.8 fb-1 collected at sqrt(s) = 7 TeV and 13.0 fb-1 collected at sqrt(s) = 8 TeV. The results, for the first time, establish the observation in the diphoton channel alone. The observation has a local significance of 6.1 standard deviations with a measured mass of 126.6 +/- 0.3 (stat) +/- 0.7 (syst) GeV. The fitted number of signal events is found to be 1.80 +/- 0.30 (stat) +0.21 -0.15 (syst) +0.20 -0.14 (theory) times the value predicted by the Standard Model. Two energy calibrations of electromagnetic particles are presented, a simpler method already used by all the ATLAS analyses and a new method based on a multivariate technique. Single particle Monte Carlo simulations have been used for the optimization. Electrons, unconverted photons and converted photons have been optimized separately. To improve the calibration of converted photons a dedicated correction has been developed. The MVA calibration was developed to introduce more input variables. The improvement is visible in all the energy ranges and in all the pseudorapidity regions and for all the particle hypotheses. The improvement on the Higgs invariant mass resolution in the diphoton channel is about 4%.

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

The study of di-tau topologies is a very important part of the physics program of the ATLAS experiment, operating at the Large Hadron Collider (CERN). After the discovery of the Higgs boson in July 2012, the study of its decays to two tau leptons has turned out to be crucial for its characterisation, offering sensitivity to important aspects of the theoretical predictions, such as the coupling of the Higgs boson to fermions and scenarios beyond to the Standard Model. In this thesis several aspects of the study of di-tau topologies during LHC Run-I are presented. Preparatory studies include the measurement of the Z → τ+τ− process cross-section, which was measured with an uncertainty of ∼ 10% on 36 pb−1 of pp collision data collected at √s = 7 TeV. Studies crucial to the high performance of reconstruction and identification of hadronically decaying tau leptons at ATLAS are presented. They include contributions to the calibration and validation of the ATLAS Pixel detector clustering algorithms and the development of an in-situ method for the determination of the energy scale of hadronically decaying tau leptons. Their pseudorapidity intercalibration was cross-checked to 3% on a dataset corresponding to 4.26 fb−1 of pp collisions collected at √s = 7TeV. Validation and preliminary studies for the improvement of the ATLAS standard di-tau mass reconstruction algorithm are also presented. A cut-based analysis for the search of H → ττ → τlepτhad process on a dataset corresponding to 20.3 fb−1 of pp collisions collected at √s = 8TeV is presented. This analysis shares several points with and complements the ATLAS multi-variate result, which provided first evidence for H → τ+τ− decays in ATLAS.

Nous revisitons les équations de groupe de renormalisation (RGE) pour les théories de jauge générales renormalisables avec une précision à une et deux boucles. Nous identifions et corrigeons les diverses fautes dans la littérature pour les fonctions beta pour les paramètres du lagrangien avec et sans dimension de masse. Les contradictions résultent de l’hypothèse d’une renormalisation diagonale de la fonction d’onde, qui n’est pas appropriée pour les modèles avec mélange dans le secteur scalaire, et de l’utilisation inexacte de la méthode du ’champ fictif’, employée dans la littérature pour la dérivation de beta-fonctions pour les paramètres dimensionnels. Nous effectuons une contre-vérification indépendante en utilisant des RGE supersymétriques bien testées, qui confirme nos résultats. L’impact numérique des changements dans la fonction beta pour les masses de fermions est illustré à l’aide d’un toy-model avec une paire de fermions massifs de type vecteur, couplée à un scalaire singlet de jauge. Sans surprise, la correction pour la fonction beta pour les masses de fermions devient importante pour les gros couplages de Yukawa de l’ordre de O(1). De plus, nous démontrons l’importance de la correction des fonctions beta pour les couplages scalaires quartiques en utilisant un modèle général à deux Higgs-Doublet de type III. Nous fournissons également une discussion pédagogique détaillée de la méthode du ’champ factice’ et résumons toutes les expressions correctes pour les fonctions beta en un seul endroit.En tant que partie indépendante de la recherche, nous étudions la physique du Higgs au-delà du modèle standard, dans le modèle Randall-Sundrum (RS) avec une dimension supplémentaire, prédisant une nouvelle particule scalaire, le radion, dont la remarquable similitude avec le boson de Higgs a été remarquée. Le modèle RS est implémenté dans le package FeynRules pour la dérivation des règles de Feynman, ce qui est utile pour l’étude de la phénoménologie des collisionneurs du modèle RS avec madgraph, en particulier, la production de paires scalaires et les modifications de couplage du Higgs
We revisit the renormalisation group equations (RGE) for general renormalisable gaugetheories at one- and two-loop accuracy. We identify and correct various mistakes inthe literature for the beta functions for both dimensionless and dimensionful Lagrangianparameters. The discrepancies arise form the assumption of a diagonal wave-functionrenormalisation in the literature, which is not appropriate for models with mixing in thescalar sector, and due to inaccurate use of the dummy field method employed in the literature for the derivation of the beta functions for dimensionful parameters. We performan independent cross-check using well-tested supersymmetric RGEs which confirms ourresults. The numerical impact of the changes in the beta function for the fermion mass termsis illustrated using a toy model with a heavy vector-like fermion pair coupled to a scalargauge singlet. Unsurprisingly, the correction to the running of the fermion mass becomessizeable for large Yukawa couplings of the order of O(1). Furthermore, we demonstratethe importance of the correction to the beta functions of the scalar quartic couplings using a general type-III Two-Higgs-Doublet-Model. We also provide a detailed pedagogicdiscussion of the dummy field method and summarize all the correct expressions for thebeta functions in one place.As an independent part of the reserach, we study the BSM Higgs physics in theRandall-Sundrum (RS) model with one extra dimension, predicting a new scalar particle, the radion, the remarkable similarity of which to the Higgs boson has been noticed. The model is implemented in the FeynRules package for the derivation of theFeynman rules, which is helpful for the future study of the collider phenomenology of theRS model with MadGraph, in particular, the scalar pair production and Higgs couplingmodifications

The discovery of a heavy neutral particle would be a direct hint for new physics beyond the Standard Model. In this thesis searches for new heavy neutral particles decaying into two tau leptons, which further decay into hadrons, are presented. They cover neutral Higgs bosons in the context of the minimal supersymmetric extension of the Standard Model (MSSM) as well as Z′ bosons, predicted by various theories with an extended gauge sector. Both analyses are based on the full 2012 proton-proton collision dataset taken by the ATLAS experiment at the Large Hadron Collider (LHC). The extended Higgs sector in the MSSM suggests additional heavy neutral Higgs bosons which decay into tau leptons in about 10% of the time. Given that the dominant final state, φ → b¯b, suffers from tremendous QCD initiated backgrounds, the decay into two tau leptons is the most promising final state to discover such new resonances. The fully hadronic final state is the dominant one with a branching fraction of about 42%. It governs the sensitivity, in particular at high transverse momentum when the QCD multijet background becomes small. Other theoretical extensions of the Standard Model, which are mainly driven by the concept of gauge unification, predict additional heavy particles arising from an extended underlying gauge group. Some of them further predict an enhanced coupling to fermions of the third generation. This motivates the search for Z′ bosons in the fully hadronic di-tau final state. One major challenge in physics analyses involving tau leptons is to have an outstanding performance of trigger and identification algorithms suitable to select real tau leptons with high efficiency, while rejecting fake taus originating from quark or gluon initiated jets. In this work a new tau trigger concept based on multivariate classifiers has been developed and became the default tau trigger algorithm in 2012 data-taking. An updated tau identification technique based on the log-likelihood approach has been provided for 2011 data-taking. Furthermore, a new framework has been developed to perform the tuning of the tau identification algorithm and exploited for the optimisation for 2012 data-taking, accordingly. The search for new heavy neutral Higgs bosons in the context of the MSSM has been performed exploiting the full 2012 dataset corresponding to an integrated luminosity of 19.5 fb−1 taken at a centre-of-mass energy of √s = 8 TeV. Updated event selection criteria and novel data-driven background estimation techniques have been developed and are suitable to increase the sensitivity of the analysis significantly. No deviations from the Standard Model prediction are observed, and thus 95% C.L. exclusion limits on the production cross section times branching ratio, σ(pp → φ) × BR(φ → ττ), are derived exploiting the CLs method. The exclusion ranges from 13.0 pb at 150GeV to 7.0 fb at 1 TeV for Higgs boson production in association with b-quarks and from 23.6 pb at 150GeV to 7.5 fb at 1 TeV for Higgs bosons produced via gluon-gluon fusion. The obtained exclusion limit on σ(pp → φ) × BR(φ → ττ) can be related to an exclusion of the MSSM parameter space in the MA-tan β-plane. Various benchmark scenario are considered. The ”standard candle” is the mhmax scenario, for which tan β values between 13.3 and 55 can be excluded at 95% C.L. in the considered mass range. Updated benchmark scenarios designed to incorporate the recently discovered SM-like Higgs boson were suggested and analysed as well. In the mhmod+ (mhmod−) scenario tan β values between 13.5 (13.3 ) and 55 (52 ) can be excluded. Finally, a search for heavy neutral resonances in the context of Z′ bosons was performed. As in the search for new Higgs bosons, no deviation from the Standard Model prediction is observed, and hence exclusion limits on the production cross section times branching ratio, σ(pp → Z′) × BR(Z′ → ττ), and on the Z′ boson mass are derived exploiting the Bayesian approach. Z′ bosons with MZ′ < 1.9 TeV can be excluded at 95% credibility, and thus mark the strongest exclusion limit obtained in the di-tau final state by any collider experiment so far.

The work presented in this manuscript is based on the proton-proton collision data from the Large Hadron Collider at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector in 2015 and 2016. The research program of the ATLAS experiment includes the precise measurement of the parameters of the Standard Model (SM) and the search for signals of physics beyond the SM. Both these approaches are pursued in this thesis, which presents two different analyses. The first one is the measurement of the Higgs boson mass in the di-photon decay channel. The measured value of the mass is (125.11 ± 0.42) GeV. Its combination with a similar measurement in the four lepton Higgs boson decay final state is presented. The value of the Higgs boson mass obtained from the combined measurement is (124.98 ± 0.28) GeV. The second one is the search for production of supersymmetric particles (gluinos, squarks or winos) in a final state containing two photons and missing transverse momentum. No significant excess with respect to the SM background is observed and lower limits on the gluino, squark and wino masses are set in the context of a generalised model of gauge-mediated supersymmetry breaking with a bino-like next-to-lightest supersymmetric particle. Finally, ATLAS detector performance studies (electron and photon energy calibration and measurement of the photon->electron misidentification probability), which are key ingredients for the two analyses presented before, are also performed and described.

En 2011, le LHC a fourni près de 5 fb−1 de données aux expériences. Ces données ont été utilisées pour comprendre plus avant les détecteurs, leurs performances et effectuer des analyses de physique. Cette thèse est organisée en cinq chapitres. Le premier est une introduction théorique au Modèle Standard et à une de ses extensions possible : la TechniCouleur. Le deuxième chapitre donne un bref aperçu de l'accélérateur LHC et de l'expérience ATLAS. Dans le troisième chapitre, l'un des principaux sous-système de l'expérience ATLAS est présenté : le calorimètre à argon liquide. L'algorithme de contrôle de l'acquisition et de la qualité des données que j'ai développé au cours de ma thèse est également présenté. Le quatrième chapitre présente une étude des performances de la reconstruction des jets basée sur l'ensemble des données acquises en 2010. Cette étude a montré qu'en 2010, la résolution en énergie des jets dans le Monte-Carlo a été sous-estimée d'un facteur relatif d'environ 10% par rapport aux données. Cette étude a été ensuite reconduite pour évaluer l'impact de la réduction de la HV dans des zones du calorimètre sur la résolution en énergie des jets. Cet impact a été jugée négligeable. Pour des jets produits avec une rapidité |y| < 0.8, l'augmentation de la résolution en énergie due à la réduction de la HV, est évaluée à moins de 3 % pour un jet de pT = 30 GeV jet, et moins de 0,1 % pour un jet de pT = 500 GeV. Enfin, le dernier chapitre de cette thèse présente une étude de l'état final Wgamma. La contribution des différents processus du MS participant à cet état final a été estimée à partir du Monte Carlo et des données. Une recherche de résonances étroites a ensuite été effectuée en utilisant la distribution M(W,gamma) dans un intervalle [220,440] GeV, mais aucun écart significatif des prédictions du MS n'a été observé. Cette étude a permis de fixer des limites sur la production de particules TC correspondant à M(a_{T}) > 265 GeV ou M(\rho_{T}) > 243 GeV.

This thesis presents a search for neutral bosons, such as new Higgs and Z' bosons, predicted by theories extending the Standard Model of particle physics. The search is performed in the di-tau analysis channel, where both tau leptons decay hadronically. Promising candidates of such theories are the Minimal Supersymmetric Standard Model (MSSM) and the non-universal G(221) model, which predict large couplings to tau leptons in large regions of their parameter space. Proton–proton collisions produced by the Large Hadron Collider (LHC) at a center-of-mass energy of 13 TeV, recorded with the ATLAS detector in 2015 and 2016, are analyzed for this search. The data correspond to an integrated luminosity of 36.1 fb^-1. This search relies on sophisticated algorithms for the reconstruction of hadronic tau decays from their decay products. This thesis presents a novel approach employing multivariate techniques to significantly improve existing algorithms, which became the default for reconstruction of hadronic tau decays in ATLAS since 2017. Additionally, the new method can provide useful information for subsequent tau identification algorithms. The MSSM extends the Higgs sector of the Standard Model by four additional Higgs bosons. Of particular interest for this thesis are the neutral CP-even H and CP-odd A bosons. The search for these bosons is performed in the mass range of 0.2 TeV to 2.25 TeV in two orthogonal categories depending on the number of identified b-quarks, each preferring one of the two considered production modes via gluon–gluon fusion or b-associated production. The data are in good agreement with the Standard Model prediction. Upper limits are set on the cross-section times branching fraction using a confidence level (CL) of 95 % independently for both production modes. Most stringent observed limits are found for a resonance mass of 1.5 TeV to be 4.94 fb and 3.65 fb for gluon–gluon fusion and b-associated production, respectively. The results are further interpreted in the hMSSM, mhmax and mhmod scenarios. Observed upper limits in the hMSSM scenario on tanβ are found to be between 4.6 at mA = 0.25 TeV and 41.4 at mA = 1.5 TeV. The search for additional Z' bosons is performed independently on the number of identified b-quarks in the mass range of 0.2 TeV to 4 TeV. As for the search for additional Higgs bosons no significant hint for new physics has been observed. 95 % CL observed upper limits are set on the cross-section times branching fraction for Z' bosons in the Sequential Standard Model (SSM) benchmark scenario between 20.5 pb at mZ' = 0.2 TeV and 7.74 fb at mZ' = 1.75 TeV. The observed upper limit for the highest considered mass of mZ' = 4 TeV is found to be 16 fb. Z' bosons in the SSM and the non-universal G(221) model are excluded at 95 % CL for masses below 2353 GeV and 2232 GeV, respectively.