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94f. / Submitted by Suelen Reis (suziy.ellen@gmail.com) on 2013-05-08T15:49:37Z No. of bitstreams: 1 Dissertacao Leandro-Cerqueira.pdf: 1078120 bytes, checksum: 5957a3c4eabaf662c579b6c328a3293a (MD5) / Rejected by Alda Lima da Silva(sivalda@ufba.br), reason: Documento de Física on 2013-05-08T18:27:35Z (GMT) / Submitted by Suelen Reis (suziy.ellen@gmail.com) on 2013-05-09T16:49:07Z No. of bitstreams: 1 Dissertacao Leandro-Cerqueira.pdf: 1078120 bytes, checksum: 5957a3c4eabaf662c579b6c328a3293a (MD5) / Approved for entry into archive by Rodrigo Meirelles(rodrigomei@ufba.br) on 2013-05-09T17:06:14Z (GMT) No. of bitstreams: 1 Dissertacao Leandro-Cerqueira.pdf: 1078120 bytes, checksum: 5957a3c4eabaf662c579b6c328a3293a (MD5) / Made available in DSpace on 2013-05-09T17:06:14Z (GMT). No. of bitstreams: 1 Dissertacao Leandro-Cerqueira.pdf: 1078120 bytes, checksum: 5957a3c4eabaf662c579b6c328a3293a (MD5) Previous issue date: 2009 / Neste trabalho apresentamos um método analítico para estudar o átomo de Hélio e íons de dois elétrons confinados. O potencial de Yukawa é utilizado como um potencial confinante. Usamos pela primeira vez, como função base, um produto de soluções analíticas aproximadas do estado fundamental e do estado excitado do Hidrogênio de Yukawa. O princípio variacional é usado e a energia do estado fundamental de sistemas confinados de dois elétrons, com Z variando de 2 a 18, é determinada. Este estudo permite analisar o comportamento dos níveis de energia e outras propriedades de sistemas atômicos imersos em um meio material neutro, que pode ser tratado como um plasma. Uma discussão sobre a carga dual e outras propriedades do sistema estudado é realizada para diferentes parâmetros de blindagem correspondendoa diferentes confinamentos do tipo plasma. / Salvador

This works examines the question whether a vapour-liquid phase transition exists in systems of particles with purely dipolar interactions, a topic which has been the subject of a longstanding debate. Monte Carlo simulation results for two modi operandi to tackle this issue are presented. One approach examines the phase behaviour of fluids of charged hard dumbbells (CHD), each made up of two oppositely charged hard spheres with diameters σ and separation d. In the limit d/σ → 0, and with the temperature scaled accordingly, the system corresponds to dipolar hard spheres (DHS) while for larger values of d ionic interactions are dominant. The crossover between ionic and dipolar regimes is examined and a linear variation of the critical temperature T*c in dipolar reduced units as a function of d is observed, giving rise to an extrapolated T*cDHS ≈ 0:15. The second approach focuses on the dipolar Yukawa hard sphere (DYHS)fluid, which is given by a dipolar hard sphere and an attractive isotropic interaction Y of the Yukawa tail form. In this case, the DHS limit is obtained for Y → 0. It is found that T*c depends linearly on the isotropic interaction strength Y over a wide range, coinciding with the results for the CHD model and extrapolating to a similar value of T*c;DHS. However, with the use of specially adapted biased Monte Carlo techniques which are highly efficient, it is shown that the linear variation of T*c is violated for very small values of the Yukawa interaction strength, almost two orders of magnitude smaller than the characteristic dipolar interaction energy. It is found that phase separation is not observable beyond a critical value of the Yukawa energy parameter, even though in thermodynamic and structural terms, the DYHS and DHS systems are very similar. It is suggested that either some very subtle physics distinguishes the DYHS and DHS systems, or the observation of a phase transition in DHSs is precluded by finite-size effects. In the context of phase separation in highly correlated fluids, new flat-histogram Monte Carlo simulation techniques based on the Wang-Landau algorithm are evaluated and shown to be useful tools. This work presents a general and unifying framework for deriving Monte Carlo acceptance rules which facilitate flat histogram sampling. The framework yields uniform sampling rules for thermodynamic states given either by the mechanically extensive variables appearing in the Hamiltonian or, equivalently, uniformly sample the thermodynamic fields which are conjugate to these mechanical variables.

Thesis advisor: Gabor Kalman / Complex plasma systems have been studied for a long time. In this thesis we focus on a quasi-2D layer system. In fact, most experimental studies of complex plasmas are based on 2D systems, because it is easy to use camera to record the in-plane movement of particles. Unfortunately, due to the finite confining strength, the system is not a strictly 2D layer, it is a quasi-2D layer. We firstly studied the density profile of such a quasi-2D system by density functional theory(DFT). From the density profile research result, we found that the system can form a trilayer structure with proper parameters. Then we studied the dynamical properties of a trilayer system, and for simplicity, we only studied an ideal three layer model, both in liquid and lattice case. In lattice case, we firstly searched the stable lattice structure at different inter-layer distance. Then we used lattice sites summation to construct the dynamical matrix and solve the dispersion relation. For liquid case, we did the theoretical prediction for the collective dispersion by quasi localized charge approximation(QLCA), then we extracted the collective mode information from the molecular dynamics(MD) simulation. The QLCA and MD results were compared and discussed. The reason for the previous gap discrepancy problem is discovered. / Thesis (PhD) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

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Estudamos a série de Mayer de um gás de Yukawa em duas dimensões. Introduzimos brevemente a noção de série de Mayer de um gás não-ideal através do uso de expansões em clusters e apresentamos motivações pelas quais o seu uso pode apresentar dificuldades. Introduzimos um formalismo algébrico que servirá de ferramenta alternativa para o estudo da convergência dessa série através do método dos majorantes via equações diferenciais parciais. Uma vez que o gás de Yukawa não é regular na origem, é preciso regulariza-lo para utilizar o método dos majorantes. Acredita-se que esta série (não regularizada) apresenta divergência do coeficiente de ordem 2k no regime de temperatura (recíproca) > k:= 8(2k-1)/2k por conta do colapso dos multipolos neutros contido nos clusters dessa ordem, e que neste regime, a retirada desses termos é suficiente para garantir a convergência da série remanescente. Por fim, estudamos a convergência da série de Mayer para um gás de Yukawa bidimensional quando a regularizacão é removida no intervalo < 16/3 e apresentamos uma extensão desse resultado para o regime < 6. Mostramos que haverá de fato divergência do coeficiente de ordem 2 da série, que pode ser regularizado enquanto < 6, garantindo a convergência da série com termo 2 omitido. / We study the Mayer series for the bidimensional Yukawa gas. We briefly introduce the notion of a Mayer series of a non-ideal gas using cluster expansions and present some motivations on the dificulties this formalism may present. An algebraic formalism is introduced in order to develop an alternative tool to study this series using a majorant method via partial diferential equations. As the Yukawa gas is not regular at the origin, it should be regularized for the majorant method to be applicable. It is believed that the non-regularized Yukawa series presents divergence on its coeficients of order 2k when the inverse temperature > k := 8(2k - 1)/2k due to the collapse of neutral multipoles belonging to clusters of this order and if we omit these coficients, the convergence is obtained. Finally we study the convergence of the Mayer series of a bidimensional Yukawa gas when the regularization is removed when < 16/3 and we extend these results to < 6. Its is shown explicitly that the coeficient of order 2 diverges but can be regularized if < 6, assuring the convergence of the remaining series when the coeficient of order 2 is omitted.

Das Higgs-Teilchen ist essentiell für die Erzeugung von Massen für Fermionen und Eich- bosonen der schwachen Wechselwirkung. Ziel dieser Arbeit ist es, die Masse und die Zerfallsbreite des Higgs-Teilchens einzugrenzen. Grundlage für die Berechnung physikalischer Größen ist dabei das Pfadintegral, welches mittels Monte-Carlo Simulationen bestimmt wird. Ein polynomieller Hybrid-Monte- Carlo-Algorithmus berücksichtigt dabei alle dynamischen Freiheitsgrade der Fermionen. Die chirale Natur der Fermionen werden mit Hilfe des Neuberger- Overlap-Operators beschrieben. In dieser Arbeit wird das Standardmodell auf den Higgs-Yukawa-Sektor eingegrenzt, welcher keine Eichbosonen enthält und lediglich ein degeneriertes Quark-Doublet berücksichtigt. Anhand des Higgs-Teilchen-Propagators werden die Ergebnisse aus der Git- terstörungsrechnung bis zu einer Schleife mit denen aus der Monte-Carlo- Simulation verglichen. Für die untersuchten Parameter, stimmen die Ergeb- nisse aus der Störungstheorie mit den Monte-Carlo-Daten sehr gut überein. Ein Schwerpunkt dieser Arbeit ist die Analyse der Resonanzparameter des Higgs-Teilchens. Die Resonanzmasse und die Resonanzbreite werden bei schwachen als auch bei starken quartischen Kopplungen untersucht. Das Higgs-Teilchen erscheint nicht als asymptotisch stabiles Teilchen, sondern als Resonanz. In allen Fällen liegt die Resonanzbreite unter 10% der Resonanzmasse. Die Resonanzmasse wird sodann mit der Propagatormasse verglichen. Für alle betrachteten Kopplun- gen gibt es eine hervorragende Übereinstimmung beider Größen. Zuletzt gilt es, den Einfluss einer schweren vierten Generation von Quarks auf die obere und untere Massenschranke des Higgs-Teilchens zu untersuchen. Alle numerischen Resultate involvieren eine umfassende Analyse der Volu- menabhängigkeit und erfordern zwingend eine Extrapolation ins unendliche Volumen. / The Higgs boson is a central part of the electroweak theory and is crucial to generate masses for fermions and the weak gauge bosons. The goal of this work is to set limits on the mass and the decay width of the Higgs boson. The basis to compute the physical quantities is the path integral which is here evaluated by means of Monte Carlo simulations thus allowing for fully non perturbative calculations. A polynomial hybrid Monte Carlo algo- rithm is used to incorporate dynamical fermions. The chiral symmetry of the electroweak model is incorporated by using the Neuberger overlap operator. Here, the standard model is considered in the limit of a Higgs-Yukawa sector which does not contain the weak gauge bosons and only a degenerate doublet of top- and bottom quarks are incorporated. Results from lattice perturbation theory up to one loop of the Higgs boson propagator are compared with those obtained from Monte Carlo simulations. At all values of the investigated couplings, the perturbative results agree very well with the Monte Carlo data. A main focus of this work is the investigation of the resonance parameters of the Higgs boson. The resonance width and the resonance mass are investigated at weak and at large quartic couplings. The Higgs boson does not appear as an asymptotic stable state but as a resonance. In all considered cases the Higgs boson resonance width lies below 10% of the resonance mass. The obtained resonance mass is compared with the mass obtained from the Higgs boson propagator. The results agree perfectly at all values of the quartic coupling considered. Finally, the effect of a heavy fourth generation of fermions on the upper and lower Higgs boson mass bound is studied. All numerical results presented in this work involve extensive finite volume analysis and an extrapolation to infinite volume is inevitable.

Estudamos a série de Mayer de um gás de Yukawa em duas dimensões. Introduzimos brevemente a noção de série de Mayer de um gás não-ideal através do uso de expansões em clusters e apresentamos motivações pelas quais o seu uso pode apresentar dificuldades. Introduzimos um formalismo algébrico que servirá de ferramenta alternativa para o estudo da convergência dessa série através do método dos majorantes via equações diferenciais parciais. Uma vez que o gás de Yukawa não é regular na origem, é preciso regulariza-lo para utilizar o método dos majorantes. Acredita-se que esta série (não regularizada) apresenta divergência do coeficiente de ordem 2k no regime de temperatura (recíproca) > k:= 8(2k-1)/2k por conta do colapso dos multipolos neutros contido nos clusters dessa ordem, e que neste regime, a retirada desses termos é suficiente para garantir a convergência da série remanescente. Por fim, estudamos a convergência da série de Mayer para um gás de Yukawa bidimensional quando a regularizacão é removida no intervalo < 16/3 e apresentamos uma extensão desse resultado para o regime < 6. Mostramos que haverá de fato divergência do coeficiente de ordem 2 da série, que pode ser regularizado enquanto < 6, garantindo a convergência da série com termo 2 omitido. / We study the Mayer series for the bidimensional Yukawa gas. We briefly introduce the notion of a Mayer series of a non-ideal gas using cluster expansions and present some motivations on the dificulties this formalism may present. An algebraic formalism is introduced in order to develop an alternative tool to study this series using a majorant method via partial diferential equations. As the Yukawa gas is not regular at the origin, it should be regularized for the majorant method to be applicable. It is believed that the non-regularized Yukawa series presents divergence on its coeficients of order 2k when the inverse temperature > k := 8(2k - 1)/2k due to the collapse of neutral multipoles belonging to clusters of this order and if we omit these coficients, the convergence is obtained. Finally we study the convergence of the Mayer series of a bidimensional Yukawa gas when the regularization is removed when < 16/3 and we extend these results to < 6. Its is shown explicitly that the coeficient of order 2 diverges but can be regularized if < 6, assuring the convergence of the remaining series when the coeficient of order 2 is omitted.

Le Grand Collisionneur de Hadrons (Large Hadron Collider, LHC) du CERN a redémarré au printemps 2015 pour trois ans (Run 2) avec une énergie dans le centre de masse de 13 TeV.Une mesure précise de l’efficacité de reconstruction des électrons avec le détecteur ATLAS au LHC, est présente dans la première partie de cette thèse en utilisant les données récoltées au cours de l’année 2015. Cela a permis d’extraire les rapports d’efficacité de reconstruction des électrons, lesquels sont utilisés par toutes les analyses d’ATLAS impliquant des électrons. Les résultats montrent d'une part une grande efficacité de reconstruction des électrons avec le détecteur ATLAS, et une bonne compréhension de ses performances d’autre part.La seconde partie de ce manuscrit est dédiée à la recherche de la production associée du boson de Higgs avec une paire de quarks top (bar), qui pourrait permettre une première mesure directe du couplage de Yukawa entre le boson de Higgs et le quark top. Une déviation dans la mesure par rapport aux prédictions du Modèle Standard serait une preuve manifeste de Nouvelle Physique. La signature de l’état final avec deux leptons de même charge électrique électrons ou muons) est examinée en utilisant les premiers 10\% du total de données attendues pour le Run 2. Une méthode améliorant l'estimation des événements avec de faux (non-prompt) leptons a développé}e et est discutée en détails dans ce rapport. / The Large Hadron Collider (LHC) at CERN restarted in spring 2015 for three years (Run2) at an unexplored center-of-mass energy of 13 TeV. A precise measurement of electron reconstruction efficiency in ATLAS, one of the two general purpose experiments of the LHC, is presented in the first part of this thesis with $Zee$ data sample recorded in 2015. This allows to extract scale factors between data and simulation that are used by all ATLAS physics analyses involving electrons. The results show the high ability of the ATLAS detector to reconstruct electrons from one hand and the good understanding of its performance on the other hand. The second part of the thesis is dedicated to a search for the Higgs boson production in association with a top quark pair (ttH), which could allow a first direct measurement of the top quark Yukawa couplingand could reveal new physics. The signature with two same-charge light leptons (electron or muon) without a hadronically decaying tau lepton final state is examined using the first 10% of the total expected Run2 dataset. Events with fake (non-prompt) leptons represent the main reducible background of this signature. An improved method to estimate it has been developped and is discussed in details in this thesis. Driving the total error, fake leptons background is found to be 1.5 to 3.6 times higher than in simulation and represent between 32 and 48% of the total background. The best-fit value of the ratio of observed and Standard Model cross sections of ttH production process, combining with other multilepton channels, is 2.5±0.7(stat)+1.1-0.9(syst), and an upper limit on this ratio of 4.9 (2.3 expected) is found at 95% confidence level.