L'étude du diagramme des phases de la Chromodynamique Quantique (QCD) et des transitons associées (déconfinement et restoration de la symétrie chirale) représentent des défis majeurs de la Physique moderne et nombreuses sont les approches théoriques qui visent à en sonder les multiples facettes. Du fait de l'intensité de l'interaction forte dans les régimes d’énergie pertinents pour les transitions susmentionnées, ces approches sont en général de nature non-perturbative, la théorie des perturbations étant réputée inapplicable à ces échelles. Il est, cependant, bien établi que le point de départ de la théorie usuelle des perturbations, basée sur la procédure de fixation de jauge de Faddeev-Popov, est ambigu à ces échelles (ambiguïté de Gribov). Dans ce contexte, une approche perturbative modifiée, basée sur le Lagrangien de Curci et Ferrari, a été proposée, via l’ajout phénoménologique d'un terme de masse effectif pour le gluon en jauge de Landau. Cette approche a été testée avec succès, notamment dans sa capacité à reproduire les fonctions de corrélation de la théorie Yang-Mills (et QCD dans la limite de quarks lourds) et la thermodynamique à temperature et potentiel chimique non nuls.Dans cette thèse, nous avons testé la robustesse de ces résultats en évaluant la structure de phase de la QCD avec quarks lourds au deuxième ordre de la théorie des perturbations dans le modèle de Curci-Ferrari et en comparant nos résultats à ceux d'approches nonperturbatives. Nos résultats indiquent que, dans ce régime de quarks lourds, le diagramme de phases est contrôlée perturbativement. Nous avons égalementétendu notre étude au cas de la QCD avec quarks légers en utilisant un schéma de resommation qui exploite la présence de petits paramètres dans le régime infrarouge de la QCD. Dans le secteur des quarks, cette démarche donne lieu à la resommation des fameux diagrammes dits "arc-en-ciel”. Ici, nous généralisons ce formalisme à temperature et densité non nulles et en presence d'un champ de fond gluonique. Nous réalisons une toute première étude qualitative des prédictions du modèle CF concernant l’existence possible d’un point critique dans le diagramme de phases de QCD sur la base d’une version simplifiée des équations générales ainsi obtenues. / Unravelling the structure of the QCD phase diagram and its many aspects such as (de)confinementand chiral symmetry breaking, is one of the big challenges of modern theoretical physics, and manyapproaches have been devised to this aim. Since perturbation theory is believed to cease feasibilityat low energy scales, these approaches treat the relevant order parameters, the quark condensate andthe Polyakov loop, non-perturbatively. However, it is also well-established that the starting point forperturbation theory, the Fadeev-Popov gauge-fixing procedure, is inherently ill-defined in the infrareddue to the presence of Gribov ambiguities. In this context, a modified perturbative approach based onthe Curci-Ferrari Lagrangian has been introduced, where a phenomenologically motivated effective gluonmass term is added to the Landau gauge-fixed action. Prior to the beginning of the thesis, this approach hasproven extremely fruitful in its descriptions of (unquenched) Yang-Mills correlation functions and thermodynamics at (non)zero temperature and density.Throughout the thesis we extend this analysis to the entire phase structure of QCD and QCD-liketheories and test the validity of the model in various regimes of interest. For instance, to further aprevious one-loop study in the regime of heavy quark masses, we have computed the two-loop quarksunset diagram in the presence of a non-trivial gluon background in a finite temperature and densitysetting. We come to the conclusion that the physics underlying center symmetry is well-described by our perturbative model with a seemingly robust weak-coupling expansion scheme. Furthermore, we study the regime of light quarks by means of a recently proposed resummation scheme which exploits the presence of actual small parameters in the Curci-Ferrari description of infrared QCD. In the quark sector, this leads to the renown rainbow equations. We extend this first-principle setup to nonzero temperature, chemical potential, and gluon background. We perform a first qualitative analysis of the prediction of the model concerning the possible existence of a critical endpoint in the QCD phase diagram by using a simplified version of these general equations.

The aim of this project is to understand the structure of the Standard Model of the particle physics. Therefore quantum field theories (QFT) are studied in the both cases of abelian and non-abelian gauge theories i.e. quantum electrodynamics (QED), quantum chromodynamics (QCD) and electroweak interaction are reviewed. The solution to the mass problem arising in these theories i.e. spontaneous symmetry breaking is also studied. / Syftet med detta projekt är att förstå strukturen för partikelfysikens standardmodell. Därför studeras kvantfältsteorier (QFT) i båda fallen av abelska och icke-abelska gaugeteorier, dvs kvantelektrodynamik (QED), kvantkromodynamik (QCD) och elektrosvag växelverkan granskas. Lösningen på massproblemet som uppstår i dessa teorier, dvs. spontant symmetribrott studeras också.

No description available.

Hochenergiephysikanalysen zielen darauf ab, das Standardmodell—die gemeinhin akzeptierte Theorie—zu testen. Für überzeugende Schlüsse, sind Analysemethoden nötig, welche einen eindeutigen Vergleich zwischen Daten und Theorie ermöglichen und zuverlässige Abschätzung der Unsicherheiten erlauben. Die Matrixelement-Methode (MEM) ist eine Maximum-Likelihood-Methode, welche speziell auf Signalsuche und Parameterschätzung an Beschleunigern zugeschnitten ist. Die MEM hat sich durch optimale Nutzung vorhandener Information und sauberer statistischer Interpretation der Ergebnisse als vorteilhaft erwiesen. Sie hat jedoch einen großen Nachteil: In der Originalformulierung ist die Berechnung der Likelihood intrinsisch auf die erste störungstheoretische Ordnung in der Kopplung limitiert. Höhere Ordnungskorrekturen verbessern die Genauigkeit theoretischer Vorhersagen und erlauben eindeutige feldtheoretische Interpretation der gewonnen Informationen. In dieser Arbeit wird erstmalig die MEM unter Einbezug der Korrekturen der nächstführenden Ordnung (NLO) der QCD-Kopplung durch Definition von Ereignisgewichten für die Berechnung der Likelihood präsentiert. Diese Gewichte ermöglichen auch die Erzeugung ungewichteter Ereignisse, welche dem in NLO-Genauigkeit berechneten Wirkungsquerschnitt folgen. Der Methode wird anhand von Top-Quark-Ereignissen veranschaulicht. Die Top-Quark-Masse wird aus den erzeugten Ereignissen mithilfe der MEM in NLO-Genauigkeit bestimmt. Die erhaltenen Schätzer stimmen mit den Eingabewerten aus der Ereigniserzeugung überein. Wiederholung der Massenbestimmung aus denselben Ereignissen, ohne NLO-Korrekturen in den Vorhersagen, führt zu verfälschten Schätzern. Diese Verschiebungen werden nicht durch abgeschätzte theoretische Unsicherheiten berücksichtigt, was die Abschätzung der theoretischen Unsicherheiten der Analyse in führender Ordnung unzuverlässig macht. Die Resultate unterstreichen die Wichtigkeit der Berücksichtigung von NLO-Korrekturen in der MEM. / Analyses in high energy physics aim to put the Standard Model—the commonly accepted theory—to test. For convincing conclusions, analysis methods are needed which offer an unambiguous comparison between data and theory while allowing reliable estimates of uncertainties. The Matrix Element Method (MEM) is a Maximum Likelihood method which is especially tailored for signal searches and parameter estimation at colliders. The MEM has proven to be beneficial due to optimal use of the available information and a clean statistical interpretation of the results. But it has a big drawback: In its original formulation, the likelihood calculation is intrinsically limited to the leading perturbative order in the coupling. Higher-order corrections improve the accuracy of theoretical predictions and allow for unambiguous field-theoretical interpretation of the extracted information. In this work, the MEM incorporating corrections of next-to-leading order (NLO) in QCD by defining event weights suited for the likelihood calculation is presented for the first time. These weights also enable the generation of unweighted events following the cross section calculated at NLO accuracy. The method is demonstrated for top-quark events. The top-quark mass is determined with the MEM at NLO accuracy from the generated events. The extracted estimators are in agreement with the input values from the event generation. Repeating the mass determinations from the same events, without NLO corrections in the predictions, results in biased estimators. These shifts may not be accounted for by estimated theoretical uncertainties rendering the estimation of the theoretical uncertainties unreliable in the leading-order analysis. The results emphasise the importance of the inclusion of NLO corrections into the MEM.

La Chromodynamique Quantique (QCD) sur réseau permet d'étudier de façon ab-initio et non-perturbative les processus d'interaction forte. Ce formalisme, qui permet une régularisation covariante de la théorie de l'interaction forte, fournit aussi un cadre naturel pour le calcul et la simulation numérique de la Chromodynamique Quantique. Dans cette thèse, après un tour d'horizon des principales propriétés de la QCD et une présentation détaillée de notre discrétisation de cette théorie sur un réseau, nous étudions de façon approfondie deux problèmes de physique hadronique : le phénomène de diffusion résonante et la structure du nucléon. Les calculs sont réalisés avec les configurations de jauge de la Collaboration Budapest-Marseille-Wuppertal, générées avec une action de Wilson améliorée avec 2+1 saveurs de quarks dynamiques. Elles couvrent une large gamme de pas de réseau, de volumes et de masses des quarks différents, permettant ainsi une étude fine de la sensibilité de nos résultats à ces paramètres, et fournissant un bon contrôle sur l'extrapolation au continu. Notre étude de la diffusion de particules sur le réseau est menée grâce à une méthode proposée par M. Lüscher. Nous avons choisi le cas particulier de la diffusion pion-pion dans le canal résonant du méson rho, et analysé nos données avec une méthode variationnelle aux valeurs propres généralisées. Nous présentons les déphasages pion-pion ainsi que les paramètres de la résonance obtenus de façon détaillée, tout en garantissant un bon contrôle de nos erreurs systématiques. Nos résultats apportent une avancée importante dans le panorama des études de diffusion sur le réseau car ce sont les premiers réalisés à la masse physique du pion, pour laquelle la désintégration du rho en deux pions peut effectivement avoir lieu. Les valeurs obtenues pour les paramètres de la résonance du méson rho sont accord avec l'expérience, et confirment la faible dépendance du couplage entre le rho et les deux pions à la masse du pion. L'exploration de la structure du nucléon se fait à travers un calcul complet des facteurs de forme électrofaibles isovectoriels, avec une étude approfondie du rayon de charge électrique et de la charge axiale. Notre analyse présente aussi des données à la masse physique du pion, ce qui s'avère crucial pour maîtriser les extrapolations au point physique, étant données les variations violentes prédites par la perturbation chirale de ces quantités. Notre calcul utilise une projection sur les états du nucléon à la source et au puits, et une méthode de fit combinant les fonctions de corrélation à deux et trois points afin de réduire et de contrôler au maximum les contaminations pouvant venir des états excités. Bien que davantage de données seraient nécessaires pour déterminer très précisément le rayon et la charge axiale au point physique avec une évaluation pertinente des erreurs systématiques, les valeurs que nous obtenons sont en bon accord avec l'expérience, et suggèrent que les effets dus aux états excités sont faibles et sous contrôle. Notre analyse souligne aussi que l'utilisation de configurations de jauge avec des masses de pion proches de la valeur physique et avec des grands volumes semble indispensable à une étude précise de la structure du nucléon sur réseau. / The formalism of Quantum Chromodynamics on the lattice (or Lattice QCD) allows to perform ab-initio non-perturbative studies of strong-interaction driven processes, as it provides both a covariant regularisation of the theory of QCD and a natural framework for numerical computations. In this work, after a review of the main features of QCD and a step-by-step presentation of our discretization of QCD on a lattice, we undertake detailed studies of two problems of hadronic physics: the phenomenon of resonant scattering and the structure of the nucleon. The lattice calculations are performed with the Budapest-Marseille-Wuppertal Collaboration's 2+1-flavour gauge configurations, which give access to a wide range of lattice spacings, volumes and quarks masses, thereby allowing to study the sensibility of our results on these parameters, and to perform a complete continuum extrapolation. These configurations include dynamical quarks, and use a clover-improved Wilson QCD action. To investigate the scattering of particles on the lattice, we set up a Lüscher analysis for the emblematic case of pion-pion scattering in the channel of the rho meson resonance. We analyse our data with a variational generalized eigenvalue method, and give an in-depth calculation of the scattering phase-shifts and the corresponding resonance parameters, with a full control of the systematic errors. Our results provide an important step for lattice studies of scattering states, as they are the first to be performed at the physical pion mass, where one can see the actual decay of the rho into two pions. The obtained rho meson parameters are in good agreement with the experimental values, and consistent with a weak pion mass dependence of the coupling between the rho and two pions. As for our probe of the structure of the nucleon, we present a complete extraction of the electroweak isovector form factors, with a comprehensive study of the electric charge squared radius and of the axial charge. Our analysis also feature data at the physical pion mass, which turns out to be crucial in order to perform safe extrapolations to the physical point, as the chiral perturbation theory predicts violent variations of these quantities near the massless-quarks point. Our calculation includes source and sink projections onto the nucleon state, as well as a combined fit method between the two-point and three-point correlation functions to control the contamination of our data by excited states. Although one would need more data to perform a high-accuracy determination of the nucleon radius and axial charge at the physical point with a relevant estimation of the systematic errors, the results we obtain are in good agreement with the experiment and suggest that the excited-state effects are under control. Our analysis also highlights that gauge configurations ensembles near the physical pion mass and with large volumes must be used in order to extract accurate information about the nucleon structure from lattice calculations.

O principal tema desenvolvido nesta tese é o estudo de efeitos nucleares na produção de diléptons por meio do processo Drell–Yan para energias compatíveis com as de RHIC e de LHC. Dois modelos são usados: o formalismo de dipolos de cor e o modelo de pártons com momentum transversal intrínseco. No primeiro capítulo, uma breve introdução histórica e a motivação para o estudo são apresentadas. No Cap. 2, o espalhamento profundamente inelástico (EPI) no modelo de pártons é revisado e são discutidas as distribuições partônicas de prótons (CTEQ) e de nucleons (EKS, EPS08 e EPS09). O EPI no formalismo de dipolos também é discutido. O Cap. 3 é dedicado ao embasamento teórico do formalismo de dipolos e ao estudo das seção de choque de dipolos. As principais equações que governam a evolução de dipolos são expostas, seguidas pelas parametrizações fenomenológicas GBW, DHJ, BUWe ABGS. Um novo modelo é proposto: a parametrização AGBS com flutuações. Quando ajustada aos dados de HERA, a nova parametrização não difere da antiga AGBS, indicando que flutuações não são necessárias para descrever os dados de EPI nas presentes energias. No Cap. 4, é apresentado o modelo de pártons no processo Drell–Yan. O formalismo é discutido em ordem dominante, em ordem seguinte à dominante (OSD) e em OSD com momentum transversal intrínseco, já que apenas na última possibilidade o momentum transversal do dilépton pode ser gerado de maneira consistente com os experimentos. Posteriormente, o formalismo de dipolos aplicado ao mesmo processo é debatido, que em ordem dominante possui distribuição em momentum transversal consistente com os experimentos. Os resultados são cálculos para o fator de modificação nuclear (RpA) para rapidezes negativas como função de rapidez e momentum transversal. A aplicação para rapidezes negativas do formalismo de dipolos é uma contribuição original, bem como a comparação deste formalismo com o modelo de pártons com momentum transversal intrínseco. Efeitos de grande (efeito EMC e anti-sombreamento) e de pequeno x (sombreamento) são observados. Mostra-se que o momentum transversal intrínseco é particularmente importante, alterando o fator de modificação nuclear em torno de 10%. Quando as diferentes parametrizações da seção de choque de dipolos são comparadas, a produção de diléptons não apresenta variações significativas, indicando que ela não é sensível aos detalhes das parametrizações, como a possível violação de escalamento geométrico presente na parametrização DHJ. Adicionalmente, os resultados do modelo de pártons são estendidos para rapidezes positivas e comparados com resultados do condensado de vidros de cor. Para energias de RHIC, os formalismos concordam, enquanto que para LHC, a menos que a parametrização nuclear tenha um sombreamento muito forte (caso da EPS08), os formalismos discordam, devido aos comportamentos distintos do sombreamento de glúons e do sombreamento de quarks. Como perspectiva, é discutido o modelo unidimensional, que é uma simplificação drástica da evolução de dipolos sem deixar de incluir os efeitos de flutuações. Em particular, as possibilidades de escalamento geométrico e escalamento difusivo nas seções de choque são discutidas. / The main topic of this thesis is the study of nuclear effects in Drell–Yan dilepton production at RHIC and LHC energies. Two approaches are employed: the color dipole approach and the parton model with intrinsic transverse momentum. In the first chapter, a brief introduction and the motivation to the study are presented. Chapt. 2 reviews the deep inelastic scattering (DIS) in the parton model and the parton distribution functions of protons (CTEQ) and of nucleons (EKS, EPS08 e EPS09) are examinated. The DIS in the dipole frame is also discussed. Chapt. 3 reviews the theoretical foundations of the color dipole approach and the dipole cross section. The main equations that drive the dipole evolution are shown, followed by the phenomenological parameterizations GBW, DHJ, BUW, and ABGS. A new model is proposed: the AGBS parameterization with fluctuations. When fitted to HERA data, the new parameterization does not differ from the old AGBS, indicating that fluctuations are not needed to reproduce DIS data at current energies. Chapt. 4 discusses the parton model in the Drell–Yan process. This approach is considered at leading order, at next-to-leading order (NLO), and at NLO with intrinsic transverse momentum, as only in the last case the dilepton transverse momentum distribution can be obtained in agreement with experiments. Afterwards, the color dipole approach applied to the process is examined, giving at leading order a transverse momentum distribution compatible with experiments. The results are calculations to the nuclear modification factor (RpA) at backward rapidities as function of rapidity and transverse momentum. The use of backward rapidities of the color dipole approach is an original contribution, as well as the comparison of this approach with the parton model with intrinsic transverse momentum. Effects of large (EMC effect and antishadowing) and of small x (shadowing) are seen. It is shown that the intrinsic transverse momentum is particularly important, changing the nuclear modification factor of about 10%. When different parameterizations of the dipole cross section are compared, the dilepton production does not show significant discrepancies, suggesting the it is not sensitive to the parametrization details, such as the possibility of geometric scaling breaking present in DHJ parameterization. Furthermore, the parton model results are extended to forward rapidities and compared with results of the color glass condensate. At RHIC energies, the approaches agree, while at LHC energies, unless the nuclear parameterization shows a very strong nuclear shadowing (EPS08 case), the approaches disagree, due to the different behaviors of gluon shadowing and quark shadowing. The unidimensional model, a toy model of the dipole evolution that includes the fluctuation effects is also investigated. Specificaly, the possibilities of geometric scaling and difusive scaling in cross sections are discussed.

Die nicht perturbative Formulierung der Quantenchromodynamik (QCD) auf dem vierdimensionalen euklidischen Gitter in Zusammenhang mit der sogenannten Finite-Size-Scaling Methode ermoeglicht die nicht-perturbative Renormierung der QCD-Parameter. Um praezise Vorhersagen aus der Gitter-QCD zu erhalten, ist es noetig, die dynamischen Fermion-Freiheitsgrade in den Gitter-QCD-Simulationen zu beruecksichtigen. Wir betrachten QCD mit zwei und vier O(a)-verbesserten Wilson-Quark-Flavours, wobei deren Masse degeneriert ist. In dieser Dissertation verbessern wir die vorhandenen Bestimmungen des fundamentalen Parameters der Zwei- und Vier-Flavor-QCD. In der Vier-Flavor-Theorie berechnen wir den praezisen Wert des Lambda-Parameters in Einheiten der Skale Lmax, welche im hadronischen Bereich definiert ist. Zudem geben wir auch die praezise Bestimmung der laufenden Schoedinger-Funktional-Kopplung in Vier-Flavor-Theorie an sowie deren Vergleich zu perturbativen Resultaten. Die Monte-Carlo Simulationen der Gitter-QCD in der Schroedinger-Funktional-Formulierung wurden mittels der plattformunabhaengigen Software Schroedinger-Funktional-Mass-Preconditioned- Hybrid-Monte-Carlo (SF-MP-HMC) durchgefuehrt, die als Teil dieses Projektes entwickelt wurde. Schliesslich berechnen wir die Masse des Strange-Quarks und den Lambda-Parameter in Zwei-Flavor-Theorie, wobei die voll-kontrollierte Kontinuums- und chirale Extrapolation zum physikalischen Punkt durchgefuehrt wurden. Um dies zu erreichen, entwickeln wir eine universale Software fuer Simulationen der zwei Wilson-Fermionen-Flavor mit periodischen Randbedingungen, namens Mass-Preconditioned-Hybrid-Monte-Carlo (MP-HMC). Die MP-HMC wird verwendet um Simulationen mit kleinen Gitterabstaenden und in der Naehe der physikalischen Pionmasse ausfuehrlich zu untersuchen. / The non-perturbative formulation of Quantumchromodynamics (QCD) on a four dimensional space-time Euclidean lattice together with the finite size techniques enable us to perform the renormalization of the QCD parameters non-perturbatively. In order to obtain precise predictions from lattice QCD, one needs to include the dynamical fermions into lattice QCD simulations. We consider QCD with two and four mass degenerate flavors of O(a) improved Wilson quarks. In this thesis, we improve the existing determinations of the fundamental parameters of two and four flavor QCD. In four flavor theory, we compute the precise value of the Lambda parameter in the units of the scale Lmax defined in the hadronic regime. We also give the precise determination of the Schroedinger functional running coupling in four flavour theory and compare it to the perturbative results. The Monte Carlo simulations of lattice QCD within the Schroedinger Functional framework were performed with a platform independent program package Schroedinger Funktional Mass Preconditioned Hybrid Monte Carlo (SF-MP-HMC), developed as a part of this project. Finally, we compute the strange quark mass and the Lambda parameter in two flavour theory, performing a well-controlled continuum limit and chiral extrapolation. To achieve this, we developed a universal program package for simulating two flavours of Wilson fermions, Mass Preconditioned Hybrid Monte Carlo (MP-HMC), which we used to run large scale simulations on small lattice spacings and on pion masses close to the physical value.

Orientador: Gastão Inácio Krein / Resumo: Existem poucas dúvidas de que a QCD seja a teoria correta das interações fortes. As dificuldades em resolver a teoria em baixas energias no regime fortemente acoplado e não perturbativo tem deixado sem respostas muitas questões importantes, tais como a natureza do confinamento e o mecanismo de hadronização. Diversos métodos têm sido usados para estudar suas propriedades e consequências a baixas energias. Esses métodos incluem a QCD na rede, as equações de Dyson- Schwinger, a teoria de perturbação chiral e os modelos de quarks. Recentemente, a dualidade gauge/gravidade tem fornecido uma nova maneira de acessar o regime fortemente acoplado de uma teoria de calibre via uma teoria de gravidade dual, em especial da QCD através de modelos holográficos. Tais modelos são usualmente denominados modelos holográficos para a QCD, ou apenas modelos AdS/QCD. Nesta tese investigamos importantes problemas de interesse atual em física hadrônica envolvendo as quebras das simetrias chiral e de sabor usando modelos holográficos para a QCD. Estes problemas são: (1) o desaparecimento das constantes de decaimento leptônicas dos estados excitados do pion no limite quiral; (2) os efeitos da quebra de simetria de sabor no acoplamentos do méson rho aos mésons charmosos D and D^{*} e seus fatores de forma eletromagnéticos; (3) os efeitos de um campo magnético e da temperatura sobre o condensado quiral, sinalizando uma catálise magnética inversa. / Abstract: There is little doubt that QCD is the correct theory for the strong interactions. The difficulties in solving the theory at low energies in the strongly interacting, non-perturbative regime have left unanswered many important questions, such as the nature of confinement and the mechanism of hadronization. Several approaches have been used to study its properties and consequences at low energies. These include lattice QCD, Dyson-Schwinger equations, chiral perturbation theory and quark models. More recently, the gauge/gravity duality has provided a new way to access the strongly coupled regime of a gauge theory via a dual gravity theory, in special of QCD through holographic models. Such models are usually named as holographic QCD models, or just AdS/QCD models. In this thesis, we investigate three problems of contemporary interest in hadronic physics involving the chiral and flavor symmetries holographic QCD models. These problems are: (1) the vanishing of the leptonic decay constants of the excited states of the pion in the chiral limit; (2) the effects of the flavor symmetry breaking on the strong couplings of the rho meson to the charmed D and D^{*} mesons and the their electromagnetic form factors; (3) the effects of a magnetic field and temperature on the chiral condensate, signalizing inverse magnetic catalysis. / Doutor

O principal tema desenvolvido nesta tese é o estudo de efeitos nucleares na produção de diléptons por meio do processo Drell–Yan para energias compatíveis com as de RHIC e de LHC. Dois modelos são usados: o formalismo de dipolos de cor e o modelo de pártons com momentum transversal intrínseco. No primeiro capítulo, uma breve introdução histórica e a motivação para o estudo são apresentadas. No Cap. 2, o espalhamento profundamente inelástico (EPI) no modelo de pártons é revisado e são discutidas as distribuições partônicas de prótons (CTEQ) e de nucleons (EKS, EPS08 e EPS09). O EPI no formalismo de dipolos também é discutido. O Cap. 3 é dedicado ao embasamento teórico do formalismo de dipolos e ao estudo das seção de choque de dipolos. As principais equações que governam a evolução de dipolos são expostas, seguidas pelas parametrizações fenomenológicas GBW, DHJ, BUWe ABGS. Um novo modelo é proposto: a parametrização AGBS com flutuações. Quando ajustada aos dados de HERA, a nova parametrização não difere da antiga AGBS, indicando que flutuações não são necessárias para descrever os dados de EPI nas presentes energias. No Cap. 4, é apresentado o modelo de pártons no processo Drell–Yan. O formalismo é discutido em ordem dominante, em ordem seguinte à dominante (OSD) e em OSD com momentum transversal intrínseco, já que apenas na última possibilidade o momentum transversal do dilépton pode ser gerado de maneira consistente com os experimentos. Posteriormente, o formalismo de dipolos aplicado ao mesmo processo é debatido, que em ordem dominante possui distribuição em momentum transversal consistente com os experimentos. Os resultados são cálculos para o fator de modificação nuclear (RpA) para rapidezes negativas como função de rapidez e momentum transversal. A aplicação para rapidezes negativas do formalismo de dipolos é uma contribuição original, bem como a comparação deste formalismo com o modelo de pártons com momentum transversal intrínseco. Efeitos de grande (efeito EMC e anti-sombreamento) e de pequeno x (sombreamento) são observados. Mostra-se que o momentum transversal intrínseco é particularmente importante, alterando o fator de modificação nuclear em torno de 10%. Quando as diferentes parametrizações da seção de choque de dipolos são comparadas, a produção de diléptons não apresenta variações significativas, indicando que ela não é sensível aos detalhes das parametrizações, como a possível violação de escalamento geométrico presente na parametrização DHJ. Adicionalmente, os resultados do modelo de pártons são estendidos para rapidezes positivas e comparados com resultados do condensado de vidros de cor. Para energias de RHIC, os formalismos concordam, enquanto que para LHC, a menos que a parametrização nuclear tenha um sombreamento muito forte (caso da EPS08), os formalismos discordam, devido aos comportamentos distintos do sombreamento de glúons e do sombreamento de quarks. Como perspectiva, é discutido o modelo unidimensional, que é uma simplificação drástica da evolução de dipolos sem deixar de incluir os efeitos de flutuações. Em particular, as possibilidades de escalamento geométrico e escalamento difusivo nas seções de choque são discutidas. / The main topic of this thesis is the study of nuclear effects in Drell–Yan dilepton production at RHIC and LHC energies. Two approaches are employed: the color dipole approach and the parton model with intrinsic transverse momentum. In the first chapter, a brief introduction and the motivation to the study are presented. Chapt. 2 reviews the deep inelastic scattering (DIS) in the parton model and the parton distribution functions of protons (CTEQ) and of nucleons (EKS, EPS08 e EPS09) are examinated. The DIS in the dipole frame is also discussed. Chapt. 3 reviews the theoretical foundations of the color dipole approach and the dipole cross section. The main equations that drive the dipole evolution are shown, followed by the phenomenological parameterizations GBW, DHJ, BUW, and ABGS. A new model is proposed: the AGBS parameterization with fluctuations. When fitted to HERA data, the new parameterization does not differ from the old AGBS, indicating that fluctuations are not needed to reproduce DIS data at current energies. Chapt. 4 discusses the parton model in the Drell–Yan process. This approach is considered at leading order, at next-to-leading order (NLO), and at NLO with intrinsic transverse momentum, as only in the last case the dilepton transverse momentum distribution can be obtained in agreement with experiments. Afterwards, the color dipole approach applied to the process is examined, giving at leading order a transverse momentum distribution compatible with experiments. The results are calculations to the nuclear modification factor (RpA) at backward rapidities as function of rapidity and transverse momentum. The use of backward rapidities of the color dipole approach is an original contribution, as well as the comparison of this approach with the parton model with intrinsic transverse momentum. Effects of large (EMC effect and antishadowing) and of small x (shadowing) are seen. It is shown that the intrinsic transverse momentum is particularly important, changing the nuclear modification factor of about 10%. When different parameterizations of the dipole cross section are compared, the dilepton production does not show significant discrepancies, suggesting the it is not sensitive to the parametrization details, such as the possibility of geometric scaling breaking present in DHJ parameterization. Furthermore, the parton model results are extended to forward rapidities and compared with results of the color glass condensate. At RHIC energies, the approaches agree, while at LHC energies, unless the nuclear parameterization shows a very strong nuclear shadowing (EPS08 case), the approaches disagree, due to the different behaviors of gluon shadowing and quark shadowing. The unidimensional model, a toy model of the dipole evolution that includes the fluctuation effects is also investigated. Specificaly, the possibilities of geometric scaling and difusive scaling in cross sections are discussed.

O principal tema desenvolvido nesta tese é o estudo de efeitos nucleares na produção de diléptons por meio do processo Drell–Yan para energias compatíveis com as de RHIC e de LHC. Dois modelos são usados: o formalismo de dipolos de cor e o modelo de pártons com momentum transversal intrínseco. No primeiro capítulo, uma breve introdução histórica e a motivação para o estudo são apresentadas. No Cap. 2, o espalhamento profundamente inelástico (EPI) no modelo de pártons é revisado e são discutidas as distribuições partônicas de prótons (CTEQ) e de nucleons (EKS, EPS08 e EPS09). O EPI no formalismo de dipolos também é discutido. O Cap. 3 é dedicado ao embasamento teórico do formalismo de dipolos e ao estudo das seção de choque de dipolos. As principais equações que governam a evolução de dipolos são expostas, seguidas pelas parametrizações fenomenológicas GBW, DHJ, BUWe ABGS. Um novo modelo é proposto: a parametrização AGBS com flutuações. Quando ajustada aos dados de HERA, a nova parametrização não difere da antiga AGBS, indicando que flutuações não são necessárias para descrever os dados de EPI nas presentes energias. No Cap. 4, é apresentado o modelo de pártons no processo Drell–Yan. O formalismo é discutido em ordem dominante, em ordem seguinte à dominante (OSD) e em OSD com momentum transversal intrínseco, já que apenas na última possibilidade o momentum transversal do dilépton pode ser gerado de maneira consistente com os experimentos. Posteriormente, o formalismo de dipolos aplicado ao mesmo processo é debatido, que em ordem dominante possui distribuição em momentum transversal consistente com os experimentos. Os resultados são cálculos para o fator de modificação nuclear (RpA) para rapidezes negativas como função de rapidez e momentum transversal. A aplicação para rapidezes negativas do formalismo de dipolos é uma contribuição original, bem como a comparação deste formalismo com o modelo de pártons com momentum transversal intrínseco. Efeitos de grande (efeito EMC e anti-sombreamento) e de pequeno x (sombreamento) são observados. Mostra-se que o momentum transversal intrínseco é particularmente importante, alterando o fator de modificação nuclear em torno de 10%. Quando as diferentes parametrizações da seção de choque de dipolos são comparadas, a produção de diléptons não apresenta variações significativas, indicando que ela não é sensível aos detalhes das parametrizações, como a possível violação de escalamento geométrico presente na parametrização DHJ. Adicionalmente, os resultados do modelo de pártons são estendidos para rapidezes positivas e comparados com resultados do condensado de vidros de cor. Para energias de RHIC, os formalismos concordam, enquanto que para LHC, a menos que a parametrização nuclear tenha um sombreamento muito forte (caso da EPS08), os formalismos discordam, devido aos comportamentos distintos do sombreamento de glúons e do sombreamento de quarks. Como perspectiva, é discutido o modelo unidimensional, que é uma simplificação drástica da evolução de dipolos sem deixar de incluir os efeitos de flutuações. Em particular, as possibilidades de escalamento geométrico e escalamento difusivo nas seções de choque são discutidas. / The main topic of this thesis is the study of nuclear effects in Drell–Yan dilepton production at RHIC and LHC energies. Two approaches are employed: the color dipole approach and the parton model with intrinsic transverse momentum. In the first chapter, a brief introduction and the motivation to the study are presented. Chapt. 2 reviews the deep inelastic scattering (DIS) in the parton model and the parton distribution functions of protons (CTEQ) and of nucleons (EKS, EPS08 e EPS09) are examinated. The DIS in the dipole frame is also discussed. Chapt. 3 reviews the theoretical foundations of the color dipole approach and the dipole cross section. The main equations that drive the dipole evolution are shown, followed by the phenomenological parameterizations GBW, DHJ, BUW, and ABGS. A new model is proposed: the AGBS parameterization with fluctuations. When fitted to HERA data, the new parameterization does not differ from the old AGBS, indicating that fluctuations are not needed to reproduce DIS data at current energies. Chapt. 4 discusses the parton model in the Drell–Yan process. This approach is considered at leading order, at next-to-leading order (NLO), and at NLO with intrinsic transverse momentum, as only in the last case the dilepton transverse momentum distribution can be obtained in agreement with experiments. Afterwards, the color dipole approach applied to the process is examined, giving at leading order a transverse momentum distribution compatible with experiments. The results are calculations to the nuclear modification factor (RpA) at backward rapidities as function of rapidity and transverse momentum. The use of backward rapidities of the color dipole approach is an original contribution, as well as the comparison of this approach with the parton model with intrinsic transverse momentum. Effects of large (EMC effect and antishadowing) and of small x (shadowing) are seen. It is shown that the intrinsic transverse momentum is particularly important, changing the nuclear modification factor of about 10%. When different parameterizations of the dipole cross section are compared, the dilepton production does not show significant discrepancies, suggesting the it is not sensitive to the parametrization details, such as the possibility of geometric scaling breaking present in DHJ parameterization. Furthermore, the parton model results are extended to forward rapidities and compared with results of the color glass condensate. At RHIC energies, the approaches agree, while at LHC energies, unless the nuclear parameterization shows a very strong nuclear shadowing (EPS08 case), the approaches disagree, due to the different behaviors of gluon shadowing and quark shadowing. The unidimensional model, a toy model of the dipole evolution that includes the fluctuation effects is also investigated. Specificaly, the possibilities of geometric scaling and difusive scaling in cross sections are discussed.