Ref HAL: tel-04131500_v1
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Cette thèse couvre les recherches conduites à l'université de Montpellier, durant les trois années de contrat financé par l'école doctorale I2S, dans le contexte de la théorie quantique des champs dans un milieu chaud et dense. Cette théorie est un cadre de travail permettant de décrire les champs quantiques à températures et densités finies. Un intérêt spécial est donné au groupe de renormalisation et ses propriétés afin de définir un meilleur schéma de resommation des divergences infrarouges, affligeant la théorie quantique des champs dans un milieu thermal. Nous commençons par définir la méthode dite ``renormalization group optimized perturbation theory'' (RGOPT), consistant à optimiser la série perturbative en accord avec le groupe de renormalisation, un cadre de travail permettant de resommer les divergences infrarouges, puis nous discutons son application dans différents modèles. Premièrement, l'application au modèle scalaire, plus simple que la chromodynamique quantique (QCD), au troisième ordre perturbatifs où nous constatons une forte amélioration de la convergence de la série perturbative ainsi qu'une diminution drastique de la dépendence d'échelle de renormalisation résiduelle. Puis, nous explorons le cas de la QCD à haute densités et température nulle, où nous déterminons une resommation originale à tous les ordres des logarithmes dominants et sous-dominants. La discussion est ensuite étendue pour incorporer l'application de la méthode de resommation RG dans le secteur des quarks, et une première approche à l'extension de ce formalisme au secteur des gluons massifs, décrit par le formalisme Hard Thermal Loop (HTL), est exploré. Finalement, nous discutons une première application de cette méthode de resommation pour la détermination d'une équation d'état pour les étoiles à neutrons.

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Ref HAL: hal-03722260_v1
Ref Arxiv: 2206.13820
Ref INSPIRE: 2102704
DOI: 10.1103/PhysRevD.106.083513
WoS: 000875169200002
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2 Citations
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Combining Supernovae, Baryon Acoustic Oscillations and Redshift-Space Distortions data from the next generation of (Stage-IV) cosmological surveys, we aim to reconstruct the expansion history up to large redshifts using forward-modeling of $f_{\mathrm DE}(z) = \rho_\mathrm{DE}(z)/\rho_\mathrm{DE,0}$ with Gaussian processes (GP). In order to reconstruct cosmological quantities at high redshifts where few or no data are available, we adopt a new approach to GP which enforces the following minimal assumptions: a) Our cosmology corresponds to a flat Friedman-Lemaître-Robertson-Walker (FLRW) universe; b) An Einstein de Sitter (EdS) universe is obtained on large redshifts. This allows us to reconstruct the perturbations growth history from the reconstructed background expansion history. Assuming various DE models, we show the ability of our reconstruction method to differentiate them from $\Lambda$CDM at $\gtrsim2\sigma$.

Ref HAL: hal-03635867_v1
Ref Arxiv: 2204.02981
DOI: 10.1007/JHEP07(2022)090
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We use string field theory to fix the normalization of the D-instanton corrections to the superpotential involving the moduli fields of type II string theory compactified on an orientifold of a Calabi-Yau threefold in the absence of fluxes. We focus on $O(1)$ instantons whose only zero modes are the four bosonic modes associated with translation of the instanton in non-compact directions and a pair of fermionic zero modes associated with the two supercharges broken by the instanton. We work with a generic superconformal field theory and express our answer in terms of the spectrum of open strings on the instanton. We analyse the contribution of multi-instantons of this kind to the superpotential and argue that it vanishes when background fluxes are absent.

Ref HAL: hal-03635855_v1
Ref Arxiv: 2204.02207
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We investigate the modularity constraints on the generating series $h_r(\tau)$ of BPS indices counting D4-D2-D0 bound states with fixed D4-brane charge $r$ in type IIA string theory compactified on complete Intersection Calabi-Yau threefolds with $b_2 = 1$. For unit D4-brane, $h_1$ transforms as a (vector-valued) modular form under the action of $SL(2,Z)$ and thus is completely determined by its polar terms. We propose an Ansatz for these terms in terms of rank 1 Donaldson-Thomas invariants, which incorporates contributions from a single D6-anti-D6 pair. Using an explicit overcomplete basis of the relevant space of weakly holomorphic modular forms (valid for any $r$), we find that for 10 of the 13 allowed threefolds, the Ansatz leads to a solution for $h_1$ with integer Fourier coefficients, thereby predicting an infinite series of DT invariants.For $r > 1$, $h_r$ is mock modular and determined by its polar part together with its shadow. Restricting to $r = 2$, we use the generating series of Hurwitz class numbers to construct a series $h^{\rm an}_2$ with exactly the same modular anomaly as $h_2$, so that the difference $h_{2}-h^{\rm an}_2$ is an ordinary modular form fixed by its polar terms. For lack of a satisfactory Ansatz, we leave the determination of these polar terms as an open problem.

Ref HAL: hal-03454983_v1
Ref Arxiv: 2111.07319
Ref INSPIRE: 1968869
DOI: 10.1088/1475-7516/2022/01/052
Ref. & Cit.: NASA ADS
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Bouncing non-singular isotropic cosmological solutions are investigated in a simple model of scalar-tensor gravity. New families of such solutions are found and their properties are presented and analyzed using an effective potential as the main tool. Bouncing solutions are shown to exist for a Higgs-like self-interaction potential which is bounded from below, in contrast to previous solutions that appeared in the literature based on potentials which were unbounded from below. In the simplest version of a scalar field with the quartic potential and conformal coupling to gravity, bouncing spatially flat solutions either have the Hubble function diverging in the past before the bounce, but with a well-behaved future, or are globally regular but unstable with respect to anisotropic or inhomogeneous perturbations at some finite values of the scalar field and curvature. Regular solutions can only exist in the part of the parameter space where the maximum of the effective potential is larger than the first zero of the potential, and gravity becomes repulsive at the bounce.

Ref HAL: hal-03410948_v1
Ref Arxiv: 2110.09126
DOI: 10.1103/PhysRevD.105.115041
WoS: WOS:000822568800005
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New physics in the lepton sector may account for neutrino masses, affect electroweak precision observables, induce charged-lepton flavour violation, and shift dipole moments. The low-energy predictions of different models are most conveniently compared within the formalism of effective field theory. To illustrate the benefits of this approach, we derive the Wilson coefficients for a set of representative models: the fermionic seesaw mechanisms (type I and III), the Zee model, and a minimal leptoquark model. In each case, the Weinberg and the dipole operators have qualitatively different origins. In parallel, we present the model-independent constraints on the Wilson coefficients coming from various lepton observables. We then show that it becomes straightforward to understand the allowed parameter space for each model, and to discriminate between them. The Zee and leptoquark models are suitable to address the muon g − 2 anomaly.We also confront the models with the anomalies in the W-boson mass and semileptonic B-meson decays.