A search for Higgs boson pairs produced in the decay of high mass exotic resonances is presented. The search uses the bb̄bb̄ final state, analyzing 139 fb⁻¹ of proton–proton collision data at √s = 13 TeV collected with the ATLAS detector. Spin-0 and spin-2 benchmark signal models are considered and no significant deviation from the Standard Model prediction is observed. The search is combined with a complementary analysis for lower mass resonances to set upper limits on the production cross-section times HH branching ratio of new resonances in the mass range of 251 GeV to 5 TeV. In addition, the methodology for the in-situ calibration of a novel double-b-tagging algorithm (Xbb2020) using gluon to bb̄ decays is presented. Preliminary scale factors for Monte-Carlo simulation are computed using 139 fb⁻¹ of √s = 13 TeV pp collision data collected with the ATLAS detector. The completed calibration will allow the Xbb2020 algorithm to be used in future ATLAS searches for H → bb̄ decays.

This work presents the search for Dark Matter particles associated with the Higgs Boson decaying into a b b-bar quark pair. The dark matter search result is based on proton-proton collision data collected at a center-of-mass energy of 13 TeV by the ATLAS detector during Run II. The results are interpreted in the context of a simplified model (Z’-2HDM) which describes the interaction of dark matter and standard model particles via new heavy mediator particles. The new powerful Higgs tagging techniques, which exploit the jet substructure and heavy flavor information to a large extent, are developed to improve the search sensitivity of the search. The target physics signals are signature with an optimized search region and interpreted with background estimation result statistically.

The Standard Model of particle physics constitutes the most accurate and comprehensive known description of the fundamental building blocks of the universe. However, overwhelming evidence suggests that the theory is incomplete and that new physics may be hiding at the TeV-scale. The Large Hadron Collider (LHC) at CERN probes these high-energy scales, opening a potential gateway to access physics beyond the Standard Model (BSM). Long-lived particles (LLPs) arise in many promising BSM theories, but they remain weakly constrained at the LHC. This thesis presents a novel search for displaced and delayed diphoton and dielectron vertices originating from the decay of a neutral LLP. The analysis uses the full LHC Run 2 dataset of pp collisions at a center-of-mass energy of √s = 13 TeV recorded by the ATLAS detector, corresponding to an integrated luminosity of 139 fb-1. The search harnesses the capabilities of the ATLAS Liquid Argon calorimeter to precisely measure the displacement and delay of the final state electromagnetic objects. The results are interpreted in a gauge-mediated supersymmetry breaking model that features the pair-production of LLPs, with each LLP subsequently decaying into either a Higgs boson or a Z boson. Since no significant excess is observed above the background expectation, the results are used to set upper limits on the cross section of LLP pair-production for signals with an LLP mass between 100 and 725 GeV and lifetime between 0.25 ns and 1 μs. A model-independent limit is also set on the production of pairs of photons or electrons with a significantly delayed arrival at the calorimeter.

Orientador: Alex Gomes Dias / Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Física, 2009

Cette thèse a traité trois différents aspects ayant trait à la recherche du boson de Higgs dans le canal de désintégration en deux photons avec le détecteur ATLAS. Le premier est une étude sur l'uniformité intrinsèque du calorimètre électromagnétique (EM) d'ATLAS en utilisant la mesure du temps de dérive des électrons d'ionisation. Environs 500 000 signaux provenant de rayons cosmiques sont enregistrés et sont comparés à la forme du signal prédite en utilisant la méthode FPM (pour First Principle Method). L'ajustement de la forme prédite à celle mesurée permet de mesurer le temps de dérive des électrons d'ionisation de l'argon liquide. L'uniformité du temps de dérive dans le compartiment Middle calculée par blocs de taille 0.1 x 0.1 dans le plan (η,φ) est estimée à 1.27 ± 0.03%. L'impact de la variation du temps de dérive sur l'uniformité de la réponse du calorimètre est estimée à 0.29 ± 0.01%. En tenant compte de la variation de l'épaisseur des plaques de plomb qui constituent le milieu passif et qui ont été mesurées par ailleurs, l'uniformité intrinsèque du calorimètre est estimée à 0.34%. La vitesse de dérive des électrons mesurée dans les différents compartiments du calorimètre est en moyenne V_{D(E = 1 kV/mm) = 4.58 ± 0.07 mm.μ s^-1 en bon accord avec des mesures antérieures de la vitesse de dérive des électrons dans l'argon liquide à la température de 88.5K. Le second aborde la question de l'estimation de la matière en amont du calorimètre, utilisant une cartographie du détecteur interne. Cette mesure est faite relativement au nombre de conversions dans le tube à vide (beam pipe) de manière à être indépendante du flux initial de photons. Une connaissance précise de la matière en amont du calorimètre est essentielle à la calibration en énergie des électrons et photons reconstruits dans le calorimètre. Puisqu'une large fraction des photons proviennent de mésons π^0, le flux initial de photons qui intervient dans le taux de conversions dans le beam pipe est normalisé aux désintégrations Dalitz du π^0. L'observable qui permet de comparer les données aux simulations Monte-Carlo (MC) est le rapport du nombre de conversions reconstruites dans le beam pipe et du nombre de Dalitz reconstruits. Ce rapport estimé avec les données de collision à une énergie de 7 TeV dans le centre de masse donne un bon accord sur la mesure de la matière dans le beam pipe avec la prédiction du MC : X/X_0 = 0.669 ± 0.033(stat) ^+0.013_-0.080(syst)%, X/X_0 = 0.709 ± 0.028(syst) %, et X/X_0 = 0.709 ± 0.028(syst) %respectivement. Le dernier est l'étude de la normalisation du signal et du bruit de fond ainsi que l'analyse du potentiel discriminant dans l'analyse inclusive du canal H → γγ. Différentes simulations MC sont utilisées pour estimer une normalisation à l'ordre NLO du signal et du bruit de fond irréductible (γγ) ainsi que les erreurs systématiques associées. Celles-ci représentent respectivement 16% et 26% de la section efficace totale. Le pouvoir discriminant de l'impulsion transverse du système diphoton et du cosθ* est aussi étudié. En particulier, une forte corrélation entre le cosθ* et la masse invariante diphoton est observée. Le bruit de fond semi-réductible (γ-jet) est normalisé à NLO avec 27% d'erreurs systématiques. Une nouvelle normalisation qui s'applique après l'identification des photons est proposée. Elle prend en compte la différente réjection des jets provenant de quark ou de gluon et est estimée à 1.9 ± 0.6. Les perspectives d'exclusion du boson de Higgs dans le canal diphoton à 10 TeV et l'extrapolation à 7 TeV pour un luminosité intégrée de 1 f b^−1 sont présentées. Dans le dernier scénario, environ 5 fois le Modèle Standard est exclu à 95% de niveau de confiance pour une masse du boson de Higgs autour de 120 GeV/c^2 .

This thesis is concerned with the study of cosmic strings. We studied the values for the Higgs mass and string coupling for which the gravitational effect of an infinite cosmic string in the context of the Einstein theory is not only locally but also globally weak. We conclude this happens for strings formed at scales less or equal to the Planck one with Higgs mass being less or equal to the boson vectorial mass. Then we examined the metric of an isolated self-gravitating abelian-Higgs vortex in dilatonic gravity for arbitrary coupling of the vortex fields to the dilaton. We looked for solutions in both massless and massive dilaton gravity. We compared our results to existing metrics for strings in Einstein and .Jordan-Brans-Dicke theories. We explored the generalisation of Bogomolnyi arguments for our vortices and commented on the effects on test particles. We then included the presence of an axion field and examined the metric of an isolated self-gravitating axionic-dilatonic string. Finally we studied dilatonic strings through black hole solutions in string theory. We concluded that the horizon of non-extreme charged black holes supports the long-range fields of the Nielsen-Olesen string that can be considered as black hole hair and whose gravitational effect is in general the production of a conical deficit into the metric of the black hole background. We also concluded that the effect of the dilaton on the horizon of these black holes is to generate an additional charge.

Made available in DSpace on 2014-06-11T19:25:30Z (GMT). No. of bitstreams: 0 Previous issue date: 2005-03Bitstream added on 2014-06-13T18:53:34Z : No. of bitstreams: 1 cardona_sah_me_ift.pdf: 677122 bytes, checksum: ef4eda2c094c5339bde81ff781b3d4cd (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / A supersimetria é um tópico importante na física teórica atual. Em particular, tem-se dedicado grande esforço no estudo das extensões supersimétricas do Modelo Padrão (SM) desde a década de 80. A incorporação da supersimetria no SM resulta em uma grande quantidade de modelos. O modelo com o conteúdo mínimo de partículas assim como de interações é chamado o Modelo Mínimo Supersimétrico (MSSM). DEvido à supersimetria, todos os modelos supersimétricos apresentam diferenças com relação ao SM. A principal delas, além do conteúdo de partículas, está no setor de Higgs. Em particular, o setor de Higgs do modelo MSSM contem cinco graus de liberdade (cinco bósons de Higgs), diferentemente do SM, que contem apenas um bóson de Higgs. Outra diferença importante no caso do MSSM deve-se à mistura dos estados associados pela supersimetria aos bósons de Gauge e aos bósons de Higgs, chamados gauginos e higgsinos respectivamente, cujos autoestados de massa são conhecidos como charginos e neutralinos. Estas partículas desempenham um papel fundamental na possível descoberta da supersimetria na escala de energia de TeV's. / Supersymmetry is a fundamental topic in the actual theoretical physics. In particular, since the 80's, huge efforts have been done studying the supersymmetric extensions of the Standard Model (SM). Including supersymmetry in the SM generates a great amount of models. Among all of these, there is one that involves the minimum number of particles and interactions. This model is known as the Minimal Supersymmetric Standard Model (MSSM). Due to the incorporation of supersymmetry, all the extensions have differences in relation with the SM. The most remarkable one, beyond the particles content, lies in the Higgs sector. Particularly, in the MSSM Higg's sector there are five degrees of freedom (five Higgs bosons), in contrast with the SM (just one). Another difference is related wit the higgsino and gaugino mixture. This result in the presence of mass eigenstates known as charginos and neutralinos. The later particles play a fundamental role in the possible test of supersymmetry at the TeV's scales.

Made available in DSpace on 2015-07-13T12:10:09Z (GMT). No. of bitstreams: 0 Previous issue date: 2015-03-09. Added 1 bitstream(s) on 2015-07-13T12:25:43Z : No. of bitstreams: 1 000833427.pdf: 449220 bytes, checksum: 758ad7ad483cbe265ba29cfb939252ad (MD5) Bitstreams deleted on 2016-05-04T13:08:32Z: 000833427_.pdf,. Added 1 bitstream(s) on 2016-05-04T13:09:34Z : No. of bitstreams: 1 000833427.pdf: 27828302 bytes, checksum: f9bffd8775ba9e024fb2915e6e0a6087 (MD5) / O objetivo deste trabalho é estudar os aspectos gerais do setor escalar do Modelo Padrão, a partir princípio de gauge, que dá consistência teórica à formulação do Modelo Eletrofraco, assim como a quebra espontânea de simetria e o mecanismo de Higgs que são fundamentais no contexto do modelo para a geração de massa das partículas elementares e, tem como consequência o surgimento de uma nova partícula escalar massiva, o bóson de Higgs do Modelo Padrão, cuja massa não está definida no modelo. Apresentamos um estudo dos principais modos de produção e decaimento do bóson de Higgs bem como a sua busca em aceleradores. Apresentamos um conjunto de resultados experimentais que foram publicados ao longo dos anos nas principais colaborações como LEP, Tevatron e LHC, que contribuíram na longa trajetória da busca, até sua identificação em 2012 / The objective of this work is to study the general aspects of the scale sector of the Standard Model the principle of gauge, which gives theoretical consistency to the electroweak model formulation, as well as spontaneous symmetry breaking and the Higgs mechanism, that are fundamental for the mass generation of the elementary particles, and gives rise to a new massive particle scale, the Higgs boson of whose value is not defined in the model. We present a study of the major modes of production and decay of Higgs boson as well as search of such particle in accelerators. We present a set of experimental results that have been published over the years in the major collaborations as LEP, Tevatron and LHC, which contributed in the long history of the Higgs boson search, until its identification in 2012

Orientador: Fernando Luis de Campos carvalho / Banca: Julio Marny Hoff da Silva / Banca: José Abdalla Helayël-Neto / Resumo: O objetivo deste trabalho é estudar os aspectos gerais do setor escalar do Modelo Padrão, a partir princípio de gauge, que dá consistência teórica à formulação do Modelo Eletrofraco, assim como a quebra espontânea de simetria e o mecanismo de Higgs que são fundamentais no contexto do modelo para a geração de massa das partículas elementares e, tem como consequência o surgimento de uma nova partícula escalar massiva, o bóson de Higgs do Modelo Padrão, cuja massa não está definida no modelo. Apresentamos um estudo dos principais modos de produção e decaimento do bóson de Higgs bem como a sua busca em aceleradores. Apresentamos um conjunto de resultados experimentais que foram publicados ao longo dos anos nas principais colaborações como LEP, Tevatron e LHC, que contribuíram na longa trajetória da busca, até sua identificação em 2012 / Abstract: The objective of this work is to study the general aspects of the scale sector of the Standard Model the principle of gauge, which gives theoretical consistency to the electroweak model formulation, as well as spontaneous symmetry breaking and the Higgs mechanism, that are fundamental for the mass generation of the elementary particles, and gives rise to a new massive particle scale, the Higgs boson of whose value is not defined in the model. We present a study of the major modes of production and decay of Higgs boson as well as search of such particle in accelerators. We present a set of experimental results that have been published over the years in the major collaborations as LEP, Tevatron and LHC, which contributed in the long history of the Higgs boson search, until its identification in 2012 / Mestre

A search for a heavy new particle Y decaying to a Standard Model Higgs boson H and another new particle X is presented. The search is performed using 139 fb−1 of p−p collision data at √s = 13 TeV recorded by the ATLAS detector. The H boson is identified through its decays to bb, with the only assumption applied to X being that it decays hadronically. The X is identified through a novel anomaly detection method via the use of a Variational Recurrent Neural Network trained directly on data collected by the ATLAS detector. This effort marks the first application of a fully unsupervised machine learning method to an ATLAS analysis. An additional benchmark based on interpreting the Y → XH process in the context of a heavy vector triplet model in which the X decays to two quarks defines an additional signal region in which upper limits on the HVT process cross section are reported at 95% confidence level.