One-Loop Integrals for Purely Virtual Particles

Quantum field theories with purely virtual particles, or fakeons, require suitable modifications in one-loop integrals. We provide the expressions for the modified scalar integrals in the case of the bubble, triangle and box diagrams. The new functions are defined by means of their difference with the `t Hooft-Veltman scalar integrals. The modifications do not affect the derivation of the Passarino-Veltman reduction and one-loop integrals with nontrivial numerators can be decomposed in the same fashion. Therefore, the new functions can be directly used to study the phenomenology of any models with standard particles and fakeons. We compare our results with standard amplitudes and show that the largest differences are often localized in relatively small energy ranges and are characterized by additional nonanalyticities. Finally, we give explicit examples in the context of a toy model, where cross sections and decay widths of standard particles are modified by the presence of fakeons.Comment: 34 pages, 9 figures. Sec. 2 rearranged, new example with plot in sec. 3, other minor corrections. Published version, pr

Slepton Non-Universality in the Flavor-Effective MSSM

Supersymmetric theories supplemented by an underlying flavor-symmetry $\mathcal{G}_f$ provide a rich playground for model building aimed at explaining the flavor structure of the Standard Model. In the case where supersymmetry breaking is mediated by gravity, the soft-breaking Lagrangian typically exhibits large tree-level flavor violating effects, even if it stems from an ultraviolet flavor-conserving origin. Building on previous work, we continue our phenomenological analysis of these models with a particular emphasis on leptonic flavor observables. We consider three representative models which aim to explain the flavor structure of the lepton sector, with symmetry groups $\mathcal{G}_f = \Delta(27)$, $A_4,$ and $S_3$.Comment: References added, minor typos corrected. 28 pages, 8 figure

The Origin of Flavor in Physics Beyond the Standard Model

En esta disertación, investigamos la interacción entre la supersimetría y los modelos basados ​​en diferentes grupos de simetría de sabor: A4, S3, A5 y Δ (27). Aunque su capacidad de predicción depende de ellos, el SM no da pistas sobre el valor de ninguno de sus parámetros de sabor. Pueden considerarse insumos externos necesarios del modelo. Esto se conoce con el nombre de puzzle de sabor. Entre las ideas propuestas para abordar el rompecabezas del sabor, el uso de simetrías de sabor, desde el mecanismo más simple de Froggatt Nielsen (FN) hasta simetrías discretas no abelianas, sigue siendo la herramienta más popular para los constructores de modelos. Esta avenida se ha explorado especialmente en el sector de los leptones. Una plétora de posibles opciones para la simetría del sabor y su ruptura son consistentes con los datos de sabor existentes. Un problema bien conocido del SM es que no podemos recuperar completamente los parámetros de sabor fundamentales del SM Lagrangian, las matrices Yukawa. Este problema es especialmente crítico en el sector de neutrinos, donde el mecanismo de Seesaw enreda los acoplamientos de neutrinos Yukawa y las masas de neutrinos Majorana. Por lo tanto, es posible que nunca seamos capaces de detectar qué simetría de sabor se encuentra detrás del Origen del sabor. Física más allá del modelo estándar, que predice nuevas interacciones de sabores, como, por ejemplo, la supersimetría, es probablemente la única oportunidad para resolver el puzzle de sabor. En esta tesis, tomamos el punto de vista de que el problema del sabor SUSY no puede separarse del problema del sabor SM. De hecho, es natural pensar que el mismo mecanismo que genera las estructuras de sabor en los acoplamientos Yukawa también es responsable de la estructura en los términos de ruptura suave de SUSY. Mostramos que la combinación de límites sobre los procesos que violan el sabor de los leptones, especialmente μ → eγ, puede restringir significativamente el espacio de parámetros de la supersimetría mucho más allá de las búsquedas directas y, al mismo tiempo, proporcionar predicciones comprobables independientes que serán (des) probadas por el próximo búsquedas de infracciones de sabor. Para el más prometedor de estos modelos, también investigamos la posibilidad de una Leptogénesis viable, que vincula la resolución de la asimetría bariónica del universo (BAU) con el rompecabezas del sabor. Un modelo completo debería tener en cuenta la BAU observada, que proporciona restricciones adicionales sobre sus parámetros. En particular, como veremos en el análisis, la coincidencia con el BAU observado nos permite restringir los parámetros de otro modo desconocidos del sector de neutrinos RH. Los últimos años han sido testigos del surgimiento de varios indicios de fenómenos no estándar a partir de observables de precisión que involucran sabores de leptones. Discutimos las restricciones impuestas por el momento magnético anómalo (g-2) μ, (g-2) ey μ → eγ en la estructura leptónica de Yukawa. Para un modelo genérico, una contribución considerable del momento magnético anómalo, como lo requieren las discrepancias observadas, está intrínsecamente relacionada con los procesos LFV y con la generación de masa. Sin embargo, cómo suprimir estas contribuciones es un tema que a menudo no se aborda explícitamente en la literatura. Proponemos un mecanismo original en el que una corrección de flavones radiativos a las masas de los leptones aporta una contribución considerable al momento magnético anómalo.In this dissertation, we investigate the interplay between Supersymmetry and models based on different flavor symmetry groups: A4, S3, A5, and Δ(27). Although its predicting ability relies on them, the SM gives no clue about the value of any of its flavor parameters. They can be regarded as necessary external inputs of the model. This goes under the name of the flavor puzzle. Among the proposed ideas to tackle the flavor puzzle, the use of flavor symmetries, from the simplest Froggatt Nielsen (FN) mechanism down to non-Abelian discrete symmetries, remains the most popular tool for model builders. This avenue has been especially explored in the lepton sector. A plethora of possible choices for the flavor symmetry and its breaking are consistent with the existing flavor data. A well-known problem of the SM is that we can not fully recover the fundamental flavor parameters of the SM Lagrangian, the Yukawa matrices. This problem is especially critical in the neutrino sector, where the Seesaw mechanism entangles the neutrino Yukawa couplings and the right-handed neutrino Majorana masses. Hence, we may never be able to detect which flavor symmetry lies behind the Origin of Flavor. Physics Beyond the Standard Model, which predicts new flavor interactions, like, for example, Supersymmetry, is probably the only opportunity to sort out the flavor puzzle. In this thesis, we take the point of view that the SUSY flavor problem can not be detached from the SM flavor problem. Indeed, it is natural to think that the same mechanism generating the flavor structures in the Yukawa couplings is also responsible for the structure in the SUSY soft-breaking terms. We show that the combination of bounds over lepton flavor violating processes, especially μ→eγ, can significantly restrict the parameter space of Supersymmetry well beyond direct searches and, at the same time, provide detached testable predictions to be (dis) proven by the upcoming flavor violation searches. For the most promising of these models, we also investigate the possibility of viable Leptogenesis, which links the resolution of the Baryon Asymmetry of the Universe (BAU) with the flavor puzzle. A complete model ought to account for the observed BAU, which provides additional constraints on its parameters. In particular, as we shall see in the analysis, matching to the observed BAU allows us to constrain the otherwise unknown parameters of the RH neutrino sector. Recent years have been witnessing the arising of several hints for non-standard phenomena from precision observables involving lepton flavors. We discuss the constraints imposed by the anomalous magnetic moment (g-2)μ, (g-2)e, and μ→eγ on the leptonic Yukawa structure. For a generic model, a sizeable contribution of the anomalous magnetic moment, as required by the observed discrepancies, is intrinsically related to LFV processes and to the mass generation. However, how to suppress these contributions is an issue that often is not explicitly addressed in the literature. We propose an original mechanism in which a radiative flavon correction to the lepton masses gives a sizable contribution to the anomalous magnetic moment

Controlled flavor violation in the MSSM from a unified Δ(27)\Delta(27) flavor symmetry

We study the phenomenology of a unified supersymmetric theory with a flavor symmetry $\Delta(27)$. The model accommodates quark and lepton masses, mixing angles and CP phases. In this model, the Dirac and Majorana mass matrices have a unified texture zero structure in the $(1,1)$ entry that leads to the Gatto-Sartori-Tonin relation between the Cabibbo angle and ratios of the masses in the quark sectors, and to a natural departure from zero of the $\theta_{13}^\ell$ angle in the lepton sector. We derive the flavor structures of the trilinears and soft mass matrices, and show their general non-universality. This causes large flavor violating effects. As a consequence, the parameter space for this model is constrained, allowing it to be (dis)proven by flavor violation searches in the next decade. Although the results are model specific, we compare them to previous studies to show similar flavour effects (and associated constraints) are expected in general in supersymmetric flavor models, and may be used to distinguish them.Comment: 21 pages, 6 figure

Leptogenesis in Δ(27)\Delta(27) with a Universal Texture Zero

We investigate the possibility of viable leptogenesis in an appealing $\Delta(27)$ model with a universal texture zero in the (1,1) entry. The model accommodates the mass spectrum, mixing and CP phases for both quarks and leptons and allows for grand unification. Flavoured Boltzmann equations for the lepton asymmetries are solved numerically, taking into account both $N_1$ and $N_2$ right-handed neutrino decays. The $N_1$-dominated scenario is successful and the most natural option for the model, with $M_1 \in [10^9, 10^{12}]$ GeV, and $M_1/M_2 \in [0.002, 0.1]$, which constrains the parameter space of the underlying model and yields lower bounds on the respective Yukawa couplings. Viable leptogenesis is also possible in the $N_2$-dominated scenario, with the asymmetry in the electron flavour protected from $N_1$ washout by the texture zero. However, this occurs in a region of parameter space which has a stronger mass hierarchy $M_1/M_2 < 0.002$, and $M_2$ relatively close to $M_3$, which is not a natural expectation of the $\Delta(27)$ model.Comment: v2: 20 pages, 2 figures. Version accepted in JHE

Muon and electron g−2g-2 and lepton masses in flavor models

The stringent experimental bound on $\mu \rightarrow e \gamma$ is compatible with a simultaneous and sizable new physics contribution to the electron and muon anomalous magnetic moments $(g-2)_\ell$ ($\ell=e,\,\mu$), only if we assume a non-trivial flavor structure of the dipole operator coefficients. We propose a mechanism in which the realization of the $(g-2)_\ell$ correction is manifestly related to the mass generation through a flavor symmetry. A radiative flavon correction to the fermion mass gives a contribution to the anomalous magnetic moment. In this framework, we introduce a chiral enhancement from a non-trivial $\mathcal{O}(1)$ quartic coupling of the scalar potential. We show that the muon and electron anomalies can be simultaneously explained in a vast region of the parameter space with predicted vector-like mediators of masses as large as $M_\chi\in [0.6,2.5]$~TeV.Comment: 18 pages, 3 figures, 2 table

Implications of the Muon g-2 result on the flavour structure of the lepton mass matrix

The confirmation of the discrepancy with the Standard Model predictions in the anomalous magnetic moment by the Muon g-2 experiment at Fermilab points to a low scale of new physics. Flavour symmetries broken at low energies can account for this discrepancy but these models are much more restricted, as they would also generate off-diagonal entries in the dipole moment matrix. Therefore, if we assume that the observed discrepancy in the muon $g-2$ is explained by the contributions of a low-energy flavor symmetry, lepton flavour violating processes can constrain the structure of the lepton mass matrices and therefore the flavour symmetries themselves predicting these structures. We apply these ideas to several discrete flavour symmetries popular in the leptonic sector, such as $\Delta (27)$, $A_4$, and $A_5 \ltimes {\rm CP}$.Comment: 21 pages; v3: comments added, typos corrected, version accepted for publication in EPJ

Anomaly-free ALP from non-Abelian flavor symmetry

Motivated by the Xenon1T excess in electron-recoil measurements, we investigate the prospects of probing axion-like particles (ALP) in lepton flavor violation experiments. In particular, we identify such ALP as a pseudo-Goldstone from the spontaneous breaking of the flavor symmetries that explain the mixing structure of the Standard Model leptons. We present the case of the flavor symmetries being a non-Abelian U(2) and the ALP originating from its U(1) subgroup, which is anomaly-free with the Standard Model group. We build two explicit realistic examples that reproduce leptonic masses and mixings and show that the ALP which is consistent with Xenon1T anomaly could be probed by the proposed LFV experiments.Comment: 18 pages, 1 figur

LFV and g-2 in non-universal SUSY models with light higgsinos

We consider a supersymmetric type-I seesaw framework with non-universal scalar masses at the GUT scale to explain the long-standing discrepancy of the anomalous magnetic moment of the muon. We find that it is difficult to accommodate the muon g-2 while keeping charged-lepton flavor violating processes under control for the conventional SO(10)-based relation between the up sector and neutrino sector. However, such tension can be relaxed by adding a Georgi-Jarlskog factor for the Yukawa matrices, which requires a non-trivial GUT-based model. In this model, we find that both observables are compatible for small mixings, CKM-like, in the neutrino Dirac Yukawa matrix.Comment: 27 pages, 9 figure

Phenomenology of a Fake Inert Doublet Model

We introduce a new way of modeling the physics beyond the Standard Model by considering fake, strictly off-shell degrees of freedom: the fakeons. To demonstrate the approach and exemplify its reach, we re-analyze the phenomenology of the Inert Doublet Model under the assumption that the second doublet is a fakeon. Remarkably, the fake doublet avoids the most stringent $Z$-pole constraints regardless of the chosen mass scale, thereby allowing for the presence of new effects well below the electroweak scale. Furthermore, the absence of on-shell propagation prevents fakeons from inducing missing energy signatures in collider experiments. The distinguishing features of the model appear at the loop level, where fakeons modify the Higgs boson $h\to\gamma\gamma$ decay width and the Higgs trilinear coupling. The running of Standard Model parameters proceeds as in the usual Inert Doublet Model case. Therefore, the fake doublet can also ensure the stability of the Standard Model vacuum. Our work shows that fakeons are a valid alternative to the usual tools of particle physics model building, with the potential to shape a new paradigm, where the significance of the existing experimental constraints towards new physics must necessarily be reconsidered.Comment: 12 pages, 7 figures, version accepted by JHE