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In-medium width of the η' mesonNiblaeus, Carl January 2013 (has links)
In this master’s thesis the width of the <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Ceta'" /> meson is studied as a function of temperature. We consider a background medium consisting of a pion gas and assume a vanishing net baryon chemical potential. The width is obtained in the framework of large <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?N_%7Bc%7D" /> chiral perturbation theory and we consider terms up to next-to-leading order in the effective Lagrangian. We use a low-density approximation to calculate the width increase due to scattering with pions from the heat bath. The results suggest that the in-medium width for the <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Ceta'" /> may become of considerable size: at a temperature <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?T%5Capprox%2075" /> MeV we find a width increase of <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5CDelta%5CGamma%5Capprox%203-4" /> MeV, comparable to the inverse lifetime of the fireball created in a heavy-ion collision. The <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Ceta'" /> is of particular interested since it is intimately connected to the <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?U(1)_%7BA%7D" /> anomaly of Quantum Chromodynamics (QCD). It has been speculated that <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?U(1)_%7BA%7D" /> may become a symmetry of QCD at high temperatures and studies of the <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Ceta'" /> could indicate if that is the case. Depending on for how large temperatures the results from large <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?N_%7Bc%7D" /> can be trusted, they indicate that the <img src="http://www.diva-portal.org/cgi-bin/mimetex.cgi?%5Ceta'" /> may be possible to study experimentally in heavy-ion collisions due to its sizable in-medium width.
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Cutting rules for Feynman diagrams at finite temperature.Chowdhury, Usman 13 January 2010 (has links)
The imaginary part of the retarded self energy is of particular interest as it contains a lot of physical information about particle interactions. In higher order loop diagrams the calculation become extremely tedious and if we have to do the same at finite temperature, it includes an extra dimension to the difficulty. In such a condition we require to switch between bases and select the best basis for a particular diagram. We have shown in our calculation that in higher order loop diagrams, at #12;finite temperature, the R/A basis is most convenient on summing over the internal vertices and very efficient on calculating some particular diagrams while the result is most easily interpretable in the Keldysh basis for most other complex diagrams.
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Violação da invariância de Lorentz no regime de temperatura finita / Lorentz invariance violation in the finite temperature regimeLeite, Júlio Rafael da Silva 27 July 2012 (has links)
In this master s thesis, we have studied the possibility of Lorentz invariance violation by considering some terms of the standard-model extesion (SME), specifically, those terms which refer to the quantum electrodynamics extension. We have performed quantum corrections in the fermionic sector of the usual quantum electrodynamics with added terms which violate the Lorentz and CPT symmetries in two different configurations. Firstly, adding operators governed by the coefficient gκλμ and, in a later time, adding operators governed by the coefficient bμ. In the theory with the coefficient gκλμ, we have performed quantum corrections in order to induce, in the bosonic sector of the theory, the conventional Chern-Simons and the higher-derivative terms, and analyze the behavior of both terms in the finite temperature regime, by using the method of derivative expansion and the Matsubara formalism. On the other hand, in the model with the coefficient bμ, we have performed quantum corrections in order to induce, in the bosonic sector, the higher-derivative Chern-Simons term, by using the method of derivative expansion and, subsequently, analyze the behavior of this term in the finite temperatura regime, where we have used the Matsubara formalism again. Thus, for the first case, we have observed that the conventional Chern-Simons term is nonzero only at finite temperature, whereas the higher-derivative term is finite at zero temperature, however, this term goes to zero when the temperature increases infinitely. In the second part of our study, we have shown that the higher-derivative Chern-Simons term is induced at zero temperature, nevertheless, this term goes to zero when the temperature increases too much. / Conselho Nacional de Desenvolvimento Científico e Tecnológico / Nesta dissertação, estudamos a possibilidade de violação da invariância de Lorentz levando em conta alguns termos do modelo padrão estendido, mais especificamente, da parte deste modelo que trata da eletrodinâmica quântica estendida. Realizamos correções quânticas no setor fermiônico da eletrodinâmica quântica usual adicionada de termos que violam as simetrias de Lorentz e CPT em duas configurações diferentes. Primeiramente, adicionando operadores governados pelo coeficiente gκλμ e, em um momento posterior, adicionando operadores governados pelo coeficiente bμ. Para a teoria com o coeficiente gκλμ, realizamos correções quânticas com a intenção de induzir, no setor bosônico da teoria, os termos de Chern-Simons convencional e o de derivada superior, e analisar o comportamento de ambos os termos no regime de temperatura finita, fazendo o uso do método da expansão derivativa e do formalismo de Matsubara. Já para o modelo com o coeficiente bμ, realizamos correções quânticas a fim de induzir, no setor bosônico, o termo de Chern-Simons de derivada superior, usando o método da expansão derivativa e, posteriormente, analisar o comportamento deste termo no regime de temperatura finita, ao utilizarmos novamente o formalismo de Matsubara. Assim, para o primeiro caso, notamos que o termo de Chern-Simons convencional é diferente de zero apenas à temperatura finita, enquanto que, o termo de derivada superior é finito em temperatura zero, porém, tal termo vai a zero quando a temperatura cresce infinitamente. Na segunda parte do nosso estudo, mostramos que o termo de Chern-Simons de derivada superior é induzido no regime de temperatura zero, contudo, tal termo vai a zero quando a temperatura cresce muito.
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Geração de termos com violação de simetria de Lorentz à temperatura finita / Generation of terms with violation of Lorentz symmetry at finite temperatureFreire, Wellington Romero Serafim 03 July 2015 (has links)
In this thesis, we investigated the generation of the higher derivative Chern-Simons term, as well as the ether-like term, both at finite temperature. We also examined the question of the large gauge invariance, with respect to the Chern-Simons term and the hight derivative one. We observed that the coefficient of the Chern-Simons term vanishes, when we taken into account the limit of high temperature. This happens to occur also for the other high derivative terms. With respects to the Chern-Simons term, we observed that the action of the Chern-Simons term and the high derivative one are invariant under the large gauge transformation. For this, we calculated the exactly effective action, however, for a specific gauge choice. Finally, with respects to the ether-like term, we observed that the calculation at finite temperature is ambiguous, as well as it is observed for the calculation at zero temperature. / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Nesta tese, investigamos a geração do termo de Chern-Simons de derivada superior, assim como o termo tipo éter, ambos à temperatura finita. Também estudamos a questão da invariância de gauge ampla, com o termo de Chern-Simons e Chern-Simons de derivada superior. Constatamos que o coeficiente do termo de Chern-Simons de derivada superior anula-se, quando tomamos o limite de altas temperaturas. Esse resultado parece se repetir para os demais termos de derivada superior. Dentro ainda do contexto de Chern-Simons, observamos que a ação de Chern-Simons e a ação de Chern-Simons de derivada superior são invariantes sob transformação de gauge ampla. Para isso, conseguimos calcular a ação efetiva exata, contudo, para uma escolha de gauge específica. Finalmente, com relação ao termo tipo éter, constatamos que o cálculo à temperatura finita e ambíguo, assim como observado no cálculo à temperatura zero.
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Rede de defeitos em estrelas de sólitons em temperatura finita.SILVA, Waldson Marcelo dos Santos. 08 November 2018 (has links)
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Previous issue date: 2010-12-22 / Capes / Neste trabalho tratamos de soluções de paredes de domínios em modelos de campos
escalares reais acoplados que envolve a simetria Z2× Z3 em um modelo não supersi-
métrico. Exploramos a ideia de uma rede de paredes de domínios aparecer na
superfície de uma estrela de sóliton. Considerando isto para um ajuste fino satisfatório entre os parâmetros do modelo, podemos encontrar modo zero fermiônico na rede de paredes de domínios. Neste cenário a estrela de sóliton torna-se instável e decai em partículas livres antes do aparecimento de uma massa limite. Porém se os férmions não se ligarem à rede de paredes de domínios, a rede torna-se neutra, impondo um valor limite na carga da estrela de sóliton, aumentando ligeiramente a sua massa crítica. Usamos os efeitos de temperatura finita para controlar esses dois cenários. / In this work we study domain wall solutions in models of coupled real scalar fields involving Z2 × Z3 symmetry in a non supersymmetric model. We explore the idea of a network of domain walls appearing in a surface of a soliton star. Using a fine-tuning among the parameters of the model, we find a fermionic zero mode in the network of
domain walls. In this scenario the soliton star becomes unstable and decays into free
particles before the appearance of a mass limit. But if the fermions do not bind to the network of domain walls, the network becomes neutral, imposing a limit on the charge of the soliton star slightly increasing its critical mass. We use effects of finite temperature to control these scenarios.
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Correntes induzidas e condensado fermiônico no espaço-tempo de uma corda cósmicaBragança, Eduardo André de Figueiredo 20 February 2017 (has links)
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Previous issue date: 2017-02-20 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In this thesis, we investigate the e ect of the topology and the magnetic eld on the
vacuum
uctuations associated to bosonic and fermionic charged elds. Firstly, we analize
the vacuum induced current densities for a bosonic eld in a (D+1)-dimensional cosmic
string spacetime. For this analysis, we consider the presence of a magnetic eld along of
the azimuthal and axial directions and that the z-axis is compacti ed to a circle of lenght
L. This analysis is performed using the positive frequencies Wightman function, that is
necessary to calculate the vacuum expectation values of the induced current densities. Our
second investigation is related with fermionic elds. We investigate the expectation values of
the fermionic condensate, h i, charge and current densities for a massive fermionic eld in
thermal equilibrium with T temperature, with a nonzero chemical potential, . We consider
the background of a (2+1)-dimensional conical spacetime and the presence of a magnetic
eld in the cone apex. In this analysis, we consider two separeted cases. Firstly, we consider
the case where j j 6 m and after that the case where j j > m. / Nesta tese, investigamos o efeito da topologia e de um campo magnético nas flutuações do vácuo associadas a campos bosônicos e fermiônicos carregados. Primeiramente, analisamos as densidades de correntes induzidas no vácuo para um campo bosônico em um espaço-tempo de uma corda cósmica com (D+1) dimensões. Para esta analise, consideramos a presença de um fluxo magnético ao longo das direções axial e azimutal e que o eixo z é compactificado em um circulo de comprimento L. Esta analise é realizada fazendo use da função de Wightman de frequências positivas, que é necessária para calcular os valores esperados no vácuo das densidades de correntes induzidas. Nossa segunda investigação é relacionada a campos fermiônicos. Investigamos os valores esperados do condensado fermiônico, (N), carga e densidades de corrente para u campo fermiônico massivo em equilíbrio térmico a uma temperatura T, e com um potencial químico não-nulo, µ. Consideramos um espaço-tempo cônico em (2+1) dimensões na presença de um campo magnético no vértice do cone. Para esta analise, dois casos foram considerados separadamente. Primeiramente, consideramos o caso em que |µ| < m, e ern seguida, o caso ern que 1p,1 > m.
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Alguns resultados exatos a Temperatura Finita da Eletrodinâmica CPT-par do Modelo Padrão Estendido / Some exact results of the Finite Temperature Electrodynamics CPT-pair of the Standard Model ExtendedSilva, Madson Rubem Oliveira 30 April 2010 (has links)
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Previous issue date: 2010-04-30 / Maxwell s electrodynamics is a field theory which contains in its structure three fundamental
physical symmetries: The Lorentz symmetry, the CPT-symmetry and the local gauge symmetry. The
Lorentz covariance and the CPT-symmetry are fundamental in the construction of any field theory
describing elementary (or not elementary) particles. Both together with the local gauge symmetry are
the cornerstones in the construction of the Standard Model and of others modern field theories. However,
it is cogitate that as much the Lorentz covariance as the CPT-symmetry can be spontaneously
broken at Planck energy scale (or in the very early Universe when energies are close to the Planck
scale) due to quantum gravity effects. The possible residual effects of such spontaneous symmetry
breaking are studied within the structure of the Standard Model Extension (SME). The U(1)-local
gauge symmetry sector of the SME describes the effects produced in Maxwell s electrodynamics due
to the Lorentz-covariance violation and the spontaneous symmetry breaking of the CPT-invariance.
Here, we study the finite temperature properties of the CPT-even electrodynamics of SME, represented
by the term (kF )ανρφFανFρφ. First, we construct a well-defined and gauge invariant partition
function in the functional integration formalism for an arbitrary tensor (kF ). Then, we specialize for
the leading-order-nonbirefringent coefficients of the tensor (kF ) and we study in separate the parityeven
and the parity-odd sectors. Consequently, for both sectors, the partition function is exactly
caculated by showing that it is a power of Maxwell s partition function. Such power is an explicit
function of the respective parameters ruling the Lorentz-covariance violation. This way, Planck s radiation
law retains its frequency dependence and the Stefan-Boltzmann law is maintained, except for
a change in Stefan-Boltzmann s constant that is multiplied by a global factor containing all the LIV
contributions. Nevertheless, in general, it is observed that the LIV coefficients induce an anisotropy
in the angular distribution of the black body energy density. / A eletrodinâmica de Maxwell é uma teoria de campo que contém em sua estrutura três tipos de
simetrias fundamentais na física: A simetria de Lorentz, a simetria CPT e a simetria de calibre local. A
covariância de Lorentz e a simetria CPT são fundamentais na construção de qualquer teoria de campo
que descreva partículas elementares e não elementares. Ambas simetrias juntamente com a simetria de
calibre local são os pilares na construção do Modelo Padrão e de outras modernas teorias de campo. No
entanto, cogita-se que ambas, a covariância de Lorentz e a simetria CPT, poderiam sofrer uma quebra
espontânea de simetria na escala de energia de Planck (ou no Universo primordial quando as energias
eram da ordem de magnitude) devido aos efeitos produzidos pelo gravidade quântica. Os possíveis
efeitos residuais dessa quebra espontânea, tanto da covariância de Lorentz como da simetria CPT,
são estudados dentro da estrutura do Modelo Padrão Estendido (MPE). Assim, o setor de simetria
de calibre local U(1) do MPE descreve os efeitos sofridos pela eletrodinâmica de Maxwell devido à
violação da covariância de Lorentz e da quebra espontânea da invariância CPT.
O intuito da Dissertação é estudarmos as propriedades à temperatura finita da eletrodinãmica
CPT-par do MPE representada pelo termo (kF )ανρφFανFρφ. O primeiro passo é construir uma
função de partição, bem definida e invariante de gauge, para uma configuração arbitrária do tensor
(kF )ανρφ. Como estamos interessados em conhecer efeitos não perturbativos ou exatos da quebra
espontânea da simetria de Lorentz, concentramos nossa atenção nas componentes do tensor (kF )
cujas contribuições, em primeira ordem não nula, para as relações de dispersão da eletrodinâmica de
Maxwell ainda as mantém não birrefringentes. Para uma maior clareza ou um melhor entendimento,
estudamos separadamente esses coeficientes não birrefringentes pertencentes aos setores de paridadepar
e de paridade-ímpar do tensor (kF ) . Consequentemente, para ambos os setores, mostramos que
a função de partição é calculada exatamente e resulta ser uma potência da função de partição de
Maxwell. Tal potência é uma função explícita somente dos respectivos parâmetros que controlam a
violação da simetria de Lorentz (VSL). Esse resultado demonstra que as propriedades termodinâmicas,
do setor não birrefringente da eletrodinâmica CPT-par do MPE, como densidade de energia, pressão,
entropia, etc, sejam as mesmas da eletrodinâmica de Maxwell multiplicadas por uma função que
depende somente nos respectivos coeficientes não birrefringentes. Desse modo, a lei de radiação de
Planck mantém a mesma dependência funcional na freqüência e a lei de Stefan-Boltzmann conservasse
proporcional a T4. Entretanto, a constante de Stefan-Boltzmann usual sofre uma mudança, pois
resulta multiplicada justamente por um fator global que contém as contribuições da VSL. No entanto,
observa-se que, em geral, os coeficientes do VSL induzem uma anisotropia na distribuição angular da
densidade de energia emitida pelo corpo negro.
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Efeitos da quebra espontânea da simetria-CPT e da invariância de Lorentz no fenômeno da condensação de Bose-Einstein / Effects of spontaneous symmetry breaking-CPT and Lorentz invariance of the phenomenon the Bose-EinsteinSilva, Kleber Anderson Teixeira da 29 April 2011 (has links)
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Previous issue date: 2011-04-29 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The Bose-Einstein condensate (BEC), also known as the fifth state of the matter, was predicted
theoretically by Albert Einstein in 1925 and verified experimentally 70 years later in 1995. This
dissertation addresses the effects of spontaneous broken of the CPT-symmetry and of the invariance
Lorentz (also called simply the violation of Lorentz symmetry) in the Bose-Einstein condensation
of an ideal bosonic gas in the limits nonrelativistic and will ultrarelativ´ıstico. The work is based
on a model of fields theory described by a massive complex scalar field in the framework of spontaneous
breaking of the CPT-symmetry and of the Lorentz invariance. First we study the CBE
in the nonrelativistic limit starting from the non relativistic version of our model. Thus, the existence
of the CBE imposes strong restrictions on some parameters governing the breach of Lorentz
invariance (VIL). We observed that only the critical temperature is modified by the VIL. Also,
we use experimental data to obtain limits upper for the coefficients that control the VIL. Because
of our model describe consistently the CBE in the non relativistic limit We can use it to study
the CBE ultrarelativıstica of an ideal bosonic gas loaded. Thus, we show that the construction of
a partition function well defined to describe the relativistic ideal gas, imposes strong restrictions
on two parameters that control the VIL. The analysis of the CBE in the ultrarelativistico limit
shows that both the critical temperature as the chemical potential are affected by the spontaneous
breaking of the invariance of Lorentz. / A condensação de Bose-Einstein (CBE), também conhecida como o quinto estado da matéria, foi
prevista teoricamente por Albert Einstein em 1925 e verificado experimentalmente 70 anos depois,
em 1995. Esta dissertação aborda os efeitos da quebra espontânea da simetria-CPT e da invariância
de Lorentz (também chamada simplesmente de violação da simetria de Lorentz) na condensação de
Bose-Einstein de um gás ideal bosônico nos limites não relativístico e ultrarelativístico. O trabalho
é baseado em um modelo de teoria de campos descrito por um campo escalar complexo massivo
no marco da quebra espontânea da simetria-CPT e da invariância de Lorentz. Primeiro estudamos
a CBE no limite não relativístico partindo da versão não relativística do nosso modelo. Desse
modo, a existência da CBE impõe restrições severas sobre alguns parâmetros que regem a violação
da invariância de Lorentz (VIL). Observamos que somente a temperatura crítica é modificada
pela VIL. Também, usamos dados experimentais para obter limites superiores para os coeficientes
que controlam a VIL. Pelo fato do nosso modelo descrever de modo consistente a CBE no limite
não relativíıstico podemos usá-lo para estudar a CBE ultrarelativística de um gás ideal bosônico
carregado. Assim, mostramos que a construção de uma função de partição bem definida, para
descrever o gás ideal relativístico, impõe restrições severas sobre dois parâmetros que controlam a
VIL. A análise da CBE no limite ultrarelativístico mostra que tanto a temperatura crítica como o
potencial químico são afetados pela quebra espontânea da invariância de Lorentz.
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Etude par dynamique moléculaire des propriétés structurales, dynamiques et thermodynamiques d'agrégats moléculaires / Molecular dynamics study of structural, dynamical and thermodynamical properties of molecular clustersKorchagina, Kseniia 28 October 2016 (has links)
Les agrégats de molécules d'eau constituent une classe d'espèces chimiques importante du fait de leur rôle central dans de nombreux processus physico-chimiques et biologiques terrestres, en particulier, les processus atmosphériques. Leurs propriétés physiques et chimiques sont particulièrement sensibles aux effets de taille et aux effets de température, ce qui les rend particulièrement difficile à caractériser expérimentalement. Ainsi, mes travaux de thèse ont porté sur l'étude théorique des propriétés structurales, dynamiques et thermodynamiques ainsi que sur la réactivité de divers agrégats de molécules d'eau avec pour objectif de mettre en place des outils de modélisation adaptés, permettant une description plus fine de ces systèmes. Pour cela, nous avons utilisé des approches de dynamique moléculaire de type "parallel-tempering" qui ont été couplées avec des calculs d'énergie et de gradient réalisés par la méthode Self-Consistent-Charge Density-Functional based Tight-Binding (SCC-DFTB). Trois grands volets ont été abordés au cours de ces travaux. Dans la première partie, l'analyse détaillée des structures des agrégats d'eau (H2O)nSO24- et (H2O)nH2SO4 avec n=1-20 est présentée. Nous avons mis en évidence l'influence de la nature de l'impureté soufrée sur le réseau de liaisons hydrogène de ces agrégats. La deuxième partie de cette thèse porte sur l'étude de la transition de phase "solide-liquide" dans divers agrégats de molécules d'eau. En plus des agrégats soufrés évoqués ci-dessus, nous avons également étudié des agrégats d'eau protonés contenant de 19 à 23 molécules d'eau. Pour mieux comprendre le mécanisme de transition de phase, nous avons considéré divers changements structuraux associés à la transition tels que l'évolution des distributions d'angles intermoléculaires et l'évolution du nombre de cycles de molécules dans l'agrégat. Nous avons également caractérisé la transition de phase au travers d'indicateurs dynamiques tels que la fréquence de transfert du proton en excès. La dernière partie de cette thèse est consacrée à l'étude de l'influence de petits agrégats d'eau (allant de 1 à 10 molécules d'eau) sur la réaction de recombinaison entre l'atome H et la molécule CO. Cette réaction est la première étape de formation des molécules organiques oxygénées simples dans le milieu interstellaire. Elle présente donc un intérêt particulier. Grâce à l'analyse de dynamiques collisionnelles entre H et CO ainsi qu'au calcul de sections efficaces de réaction, nous avons montré que la présence des molécules d'eau joue un rôle important dans le processus de formation du radical HCO. / Water clusters constitute an important class of chemical species due to their central role in many physico-chemical and biological processes, in particular, atmospheric processes. Their physical and chemical properties are particularly sensitive to size and finite-temperature effects, which makes them particularly difficult to characterize experimentally. This thesis focused on the theoretical investigation of the structural, dynamical and thermodynamical properties as well as on the reactivity of various water clusters with the aim to implement appropriate modeling tools to enable a more detailed description of these systems. To do so, we used the paralleltempering molecular dynamics approach that was coupled with calculations of energies and gradients carried out by the Self-Consistent-Charge Density-Functional based Tight-Binding (SCC-DFTB) method.Three main areas were addressed during the work. In the first part, a detailed analysis of the structure of water clusters (H2O)nSO24- and (H2O)nH2SO4 with n=1-20 is performed. This study highlights the influence of the nature of the sulfur impurity on the hydrogen bond network of these species.The second part of this thesis focuses on the study of the "solid-liquid" phase transition in various water clusters. In addition to the sulfur-containing water clusters mentioned above, we also investigated protonated water clusters containing from 19 to 23 water molecules. To better understand the phase transition mechanism, we considered various structural changes associated with the transition, such as the evolution of the distributions of intermolecular angles and the evolution of the number of molecular rings in the cluster. We also characterized the phase transition through dynamical indicators such as the crossover frequency of the excess proton. The last part of this thesis is devoted to the study of the influence of small water clusters (from 1 to 10 water molecules) on the recombination reaction between the H atom and the CO molecule. This reaction is the first step in the formation of simple oxygenated organic molecules in the interstellar medium. It is therefore of particular interest. Due to the analysis of collisional dynamics between H and CO and the calculation of effective reaction cross sections we showed that the presence of water molecules plays an important role in the HCO radical formation.
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Perturbative perspectives on the Phase diagram of Quantum ChromoDynamics / Points de vue perturbatifs sur le diagramme de phases de la chromodynamique quantiqueMaelger, Jan 10 October 2019 (has links)
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.
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