In the first part of the thesis we explore the sensitivity to sterile neutrinos by using a novel kaon tagging technology: ENUBET, the proposed experiment could decisively test indications from the experiments Neutrino-4 and IceCube. In the second part of the thesis we discuss the current status of sterile neutrino searches at nuclear reactors, we present a study with the optimization of a green field, two baseline reactor experiment with respect to the sensitivity for electron anti-neutrino disappearance in search of a light sterile neutrino at both research and commercial reactors. We find that a total of 5 tons of detectors deployed at a commercial reactor with a closest approach of 25 m can probe the mixing angle sin²2θ down to ∼ 5 × 10⁻³ around ∆m² ∼ 1 eV² . The same detector mass deployed at a research reactor can be sensitive up to ∆m² ∼ 20 − 30 eV² assuming a closest approach of 3 m and excellent energy resolution, such as that projected for TAO. We also find that lithium doping of the reactor could be effective in increasing the sensitivity for higher ∆m² values. / Master of Science / A sterile neutrino is a particle that is not included in the actual content of matter at the fundamental level. Our goal in this thesis was to search for an imprint of this particle at neutrino experiments. We performed numerical simulations using the experimental specification given in the literature to predict what this signal should look like. The importance of searching for this particle arises from indications at neutrino nuclear experiments, if this particle exists, that would imply new physics beyond our actual understanding of the matter content in the universe. The first search was performed at an experimental facility called ENUBET and the second search was performed at nuclear reactors. Testing this elusive particle means we need to determine two parameters from a model. The results of the aforementioned parameter space searches are presented in this thesis. The statistical significance in our findings is not entirely conclusive to either confirm or refute the sterile neutrino. The benefits of studying neutrinos at nuclear reactors is that they are produced in generating electrical power as well as monitoring nuclear weapons.

This thesis will present a search for sterile neutrinos using data taken with the MINOS experiment between 2005 and 2012. MINOS is a two-detector on-axis experiment based at Fermilab. The NuMI neutrino beam encounters the MINOS Near Detector 1km downstream of the neutrino-production target before travelling a further 734km through the Earth's crust, to reach the Far Detector located at the Soudan Underground Laboratory in Northern Minnesota. By searching for oscillations driven by a large mass splitting, MINOS is sensitive to the existence of sterile neutrinos through looking for any energy-dependent perturbations using a charged-current sample, as well as looking at any relative deficit in neutral current events between the Far and Near Detectors. This thesis will discuss the novel analysis that enabled a search for sterile neutrinos covering five orders of magnitude in the mass splitting and setting a limit in previously unexplored regions of the sterile neutrino parameter space, where a 3+1-flavour phenomenological model was used to extract parameter limits. The results presented in this thesis are sensitive to the sterile neutrino parameter space suggested by the LSND and MiniBooNE experiments.

Neutrinooszillation, ein Phänomen, das den Neutrino-Flavour nach ihrer Ausbreitung durch den Weltraum verändern kann, ist ein Beweis für nicht-verschwindende Neutrinomassen und ein Hinweis auf eine neue Physik außerhalb des Standardmodells. Diese Arbeit präsentiert die erste Messung zu atmosphärischen Neutrinooszillationen, die sechs Jahre zwischen Mai 2011 und Mai 2017 des IceCube DeepCore Experiment umfasst. Sie erweitert die bisher verfügbare Ereignisauswahl um eine neue Ereignissignatur und einen großeren Energiebereich. Diese Arbeit beschreibt die Methoden, die für die Simulationen der Wechselwirkungen der Neutrinos, die Ereignisauswahl, die Rekonstruktion und die statistische Behandlung von Messdaten und systematischen Messunsicherheiten benutzt werden. Die beste Abschätzung für die Neutrino-Mischungsparameter ist $\Delta m^2_{32} = 2.54^{+0.11}_{-0.12}\cdot 10^{-3}$~eV$^2$ und $\sin^2 \theta_{23} = 0.51\pm0.05$ (68\% C.L.) und gehört zurzeit zu den präzisesten Messungen atmosphärischer Neutrinos. Darüber hinaus wird in dieser Arbeit das Standard-Drei-Flavour-Modell überprüft, indem ein steriles Neutrino mit einer Masse in der Größenordnung von 1 eV eingeführt wird. Die Suche nach Effekten steriler Neutrinos auf atmosphärischen Neutrinooszillationen wird auf drei Jahren Daten, genommen zwischen Mai 2011 und Mai 2014, durchgeführt. Die Ergebnisse stimmen mit dem Standard-Modell der Drei-Neutrino-Oszillation überein, was zu den Obergrenzen für sterilen Neutrino-Mischungsparameter $|U_{\mu4}|^2<0.11$ und $|U_{\tau4}|^2<0.15$ (90\% C.L.) für $\Delta m^2_{41}=1$~eV$^2$ führt. Dieser Ergebnis ist derzeit die stringenste Obergrenze für $|U_{\tau4}|^2$. / Neutrino oscillations, a phenomenon that can change the flavour of neutrinos after their propagation through space, are a proof of non-zero neutrino masses and are an indication of new physics beyond the Standard Model. This work presents the first measurement of the atmospheric neutrino oscillations using six years of IceCube DeepCore data taken between May 2011 and May 2017. It extends the previously available event selection to include new event signatures and to use an extended energy range. This work discusses the techniques used for simulation of neutrino interactions, event selection, reconstruction, and the statistical treatment of data and systematic uncertainties. The best estimates for the neutrino mixing parameters are $\Delta m^2_{32} = 2.54^{+0.11}_{-0.12}\cdot 10^{-3}$~eV$^2$ and $\sin^2 \theta_{23} = 0.51\pm0.05$ (68\% C.L.), which are currently among the most precise measurements obtained with atmospheric neutrinos. In addition, this work tests the standard three-flavour paradigm by introducing one sterile neutrino with a mass on the order of 1~eV. The search for sterile neutrino effects in atmospheric neutrino oscillations is performed with three years of data taken between May 2011 and May 2014. The results are consistent with the standard three-neutrino oscillation picture, leading to limits on the allowed sterile neutrino mixing of $|U_{\mu4}|^2<0.11$ and $|U_{\tau4}|^2<0.15$ (90\% C.L.) for $\Delta m^2_{41}=1$~eV$^2$. Currently, the limit for $|U_{\tau4}|^2$ is the most stringent in the World.

Neutrinooszillationen sind das einzige Phänomen jenseits des Standardmodells, das experimentell mit hoher statistischer Signifikanz bestätigt wurde. Diese Arbeit präsentiert eine Messung der atmosphärischen Neutrinooszillationen unter Verwendung von acht Jahren an Daten, die zwischen 2011 und 2019 vom IceCube DeepCore-Detektor aufgenommen wurden. Die Ereignisauswahl wurde im Vergleich zu früheren DeepCore-Messungen verbessert, wobei ein besonderes Augenmerk auf ihre Robustheit gegenüber systematischen Unsicherheiten in den Detektoreigenschaften gelegt wurde. Die Oszillationsparameter werden über eine Maximum-Likelihood-Fit an gebinnte Daten in der gemessenen Energie und Zenitwinkel geschätzt, wobei die Erwartungswerte aus gewichteten simulierten Ereignissen abgeleitet werdem. Diese Arbeit diskutiert den Simulations- und Datenauswahlprozess sowie die statistischen Methoden, die verwendet werden, um einen genauen Erwartungswert unter variablen Detektoreigenschaften und anderen systematischen Unsicherheiten zu liefern. Die Messung wird zunächst unter Verwendung des Standardmodells der Drei-Flavor-Oszillation durchgeführt, wobei das atmosphärische Massensplitting und der Mischwinkel auf $\Delta m^2_{32} = 2.42_{-0.75}^{+0.77} \times10^{-3};\mathrm{eV}^2$ und $\sin^2\theta_{23} = 0.507_{-0.053}^{+0.050}$ geschätzt werden. Das Drei-Flavor-Modell wird dann um einen zusätzlichen Masseneigenzustand erweitert, der einem sterilen Neutrino mit Massensplitting $\Delta m^2_{41} = 1;\mathrm{eV}^2$ entspricht und mit den aktiven $\nu_\mu$- und $\nu_\tau$-Flavorzuständen mischen kann. Es wird kein signifikantes Signal eines sterilen Neutrinos beobachtet, und die Mischungsamplituden zwischen den sterilen und aktiven Zuständen werden auf $|U_{\mu 4}|^2 < 0.0534$ und $|U_{\tau 4}|^2 < 0.0574$ bei 90\% C.L. begrenzt. Diese Grenzwerte sind um den Faktor zwei bis drei strenger als das vorherige DeepCore-Ergebnis, und die Einschränkung von $|U_{\tau 4}|^2$ ist die stärkste der Welt. / Neutrino oscillations are the only phenomenon beyond the Standard Model that has been confirmed experimentally to a very high statistical significance. This work presents a measurement of atmospheric neutrino oscillations using eight years of data taken by the IceCube DeepCore detector between 2011 and 2019. The event selection has been improved over that used in previous DeepCore measurements with a particular emphasis on its robustness with respect to systematic uncertainties in the detector properties. The oscillation parameters are estimated via a maximum likelihood fit to binned data in the observed energy and zenith angle, where the expectation is derived from weighted simulated events. This work discusses the simulation and data selection process, as well as the statistical methods employed to give an accurate expectation value under variable detector properties and other systematic uncertainties. The measurement is first performed first under the standard three-flavor oscillation model, where the atmospheric mass splitting and mixing angle are estimated to be $\Delta m^2_{32} = 2.42_{-0.75}^{+0.77} \times10^{-3}\;\mathrm{eV}^2$ and $\sin^2\theta_{23} = 0.507_{-0.053}^{+0.050}$, respectively. The three-flavor model is then extended by an additional mass eigenstate corresponding to a sterile neutrino with mass splitting $\Delta m^2_{41} = 1\;\mathrm{eV}^2$ that can mix with the active $\nu_\mu$ and $\nu_\tau$ flavor states. No significant signal of a sterile neutrino is observed and the mixing amplitudes between the sterile and active states are constrained to $|U_{\mu 4}|^2 < 0.0534$ and $|U_{\tau 4}|^2 < 0.0574$ at 90\% C.L. These limits are more stringent than the previous DeepCore result by a factor between two and three and the constraint on $|U_{\tau 4}|^2$ is the strongest in the world.

[pt] Nesta dissertação, estudamos do ponto de vista fenomenológico, os efeitos de neutrinos estéreis para os observáveis de massas de neutrinos baseado nos dados do experimento LSND e nos resultados divulgados recentemente pela colaboração MiniBooNE. Consideramos observáveis de massa as seguintes quantidades: o parâmetro de massa cinemática cuja medida é realizada em experimentos com o decaimento beta do tritium tendo seu valor atual fornecido pelos experimentos Mainz e Troitsk; a massa efetiva de Majorana, que é uma quantidade que pode ser obtida em experimentos com o duplo decaimento beta sem neutrinos; finalmente, a soma de massas dos neutrinos, a qual é vinculada por dados cosmológicos. Nossa análise é realizada considerando os possíveis ordenamentos de massas para o caso em que temos dois neutrinos estéreis além dos três neutrinos ativos usuais, cuja adição é necessária para explicar os resultados de LSND e MiniBooNE ao mesmo tempo. Neste cenário, temos oito possíveis ordenamentos de massas, os quais dividimos em três grupos. No primeiro grupo, temos que os dois neutrinos estéreis são mais pesados que os três neutrinos ativos. No segundo grupo, os dois neutrinos estéreis são mais leves que os três neutrinos ativos. Cada um destes dois grupos tem possibilidades que dependem do ordenamento de massas dos neutrinos ativos que pode ser normal ou invertido. No terceiro e último grupo temos que um neutrino estéril é mais leve e o outro mais pesado que os três neutrinos ativos. Neste grupo, existem quatro possibilidades de ordenamento associada ao posicionamento dos neutrinos estéreis e ao ordenamento dos neutrinos do setor ativo. Investigamos os observáveis de massas em cada um destes cenários. / [en] In this dissertation we study, from the phenomenological point of view, the effects of sterile neutrinos for the observables related to neutrino masses based on the data of the LSND experiment and on the results releasedrecently by the MiniBooNE collaboration. We consider the following mass related obsevables: the kinematic mass parameter which is obtained in tritium beta decay experiments whose current value is provided by Mainzand Troitsk experiments; the Majorana effective mass, it is a quantity that can be obtained in neutrinoless double beta decay experiments. In additionto these quantities, we also consider the sum of neutrinos masses, which isconstrained by cosmological data. Our analysis is performed by considering the possible mass orderings for the cases where we have two sterile neutrinosbeyond the three standard active neutrinos, whose addition is necessary to explain the results of LSND andMiniBooNE simultaneously. In this scenariot here are eight possible mass orderings, which are divided into three groups.In the first group we have two sterile neutrinos which are heavier thanthe three active neutrinos. In the second group the two sterile neutrinosare lighter than the three active neutrinos. Each of these two groups canbe further divided into 2 subgroups depending on the mass ordering ofthe active neutrinos that can be normal or inverted. In the third and lastgroup we have one sterile neutrino lighter and the other heavier than thethree active neutrinos. In this group there are four possibilities of ordering depending on the positioning of the sterile neutrinos with respect to theactive ones and on the mass ordering of the active states. We investigate systematically the masses observable in each of these scenarios.

Le travail présenté dans cette thèse porte sur la recherche de neutrino stérile à l'aide de désintégrations β dans les expériences SOX et TRISTAN. Le neutrino stérile est une particule hypothétique, solidement établi théoriquement, qui ne prendrait part à aucune interaction fondamentale, gravité mise à part. Étant entendu que le neutrino stérile se mélange avec les neutrinos actifs connus, l'existence de ces premiers peut être étudiée directement en laboratoire. L'expérience SOX a été conçue pour explorer l'existence d'un neutrino stérile d'une masse autour de l'électronvolt (eV). Un neutrino stérile avec une telle masse permettrait d'expliquer plusieurs anomalies observées à courte distance de sources (quelques mètres) lors de mesures d'oscillations de neutrinos de basses énergies (quelques MeV). SOX avait pour projet d'utiliser le détecteur de neutrinos solaire déjà existant Borexino, et d'observer un signal d'oscillation vers le stérile à l'intérieur même du volume actif du détecteur. La source radioactive de 5.5 PBq et positionnée à 8.5 m du centre du détecteur, émettrait des antineutrinos électroniques via la désintégration β du ¹⁴⁴Ce et du ¹⁴⁴Pr. Une des clés de l'observation de cette oscillation, est la connaissance précise de l'activité de la source. Une telle activité peut être déterminée en mesurant la chaleur dégagée par la source. C'est la raison pour laquelle l'INFN Genova et la TUM ont développé conjointement un calorimètre dédié. La chaleur dégagée par la radioactivité est alors captée par un échangeur puis transmise à un circuit d'eau étroitement contrôlé. Le calorimètre a été assemblé, optimisé puis étalonné avec succès. La perte de chaleur du circuit fut déterminée lors des mesures d'étalonnage grâce à un chauffage électrique. Des variations des conditions expérimentales et une isolation thermique sophistiquée ont permis d'opérer avec des pertes de chaleur négligeables. Il a ainsi été démontré que la puissance thermique de la source pouvait être estimée, en 5 jours seulement, avec une précision supérieure à 0,2%. Malheureusement, le programme SOX a dû être annulé. Le projet TRISTAN, quant à lui, tend à démontrer l'existence d'un neutrino stérile avec une masse de l'ordre du kilo-électronvolt (keV). Si le neutrino stérile à l'eV tente d'apporter une réponse aux différentes anomalies observées lors de mesures d'oscillation, le neutrino stérile au keV, en tant que potentiel candidat matière noire. Le projet TRISTAN cherche à mesurer l'empreinte de ce nouvel état de masse sur le spectre du tritium dans le cadre de l'expérience KATRIN. Cette dernière vise à déterminer la masse effective du neutrino (actif) en mesurant l'extrémité du spectre de tritium avec une excellente résolution et un faible taux de comptage. Une fois la mesure achevée, le détecteur de KATRIN sera modifié afin d'effectuer une mesure différentielle et intégrale de l'ensemble du spectre en tritium: c'est le projet TRISTAN. Le détecteur actuel sera remplacé par un nouveau détecteur de silicium à dérive (SDD) de 3500 pixels permettant une résolution de 3% à 6 keV et pouvant supporter un taux de comptage montant jusqu'à 10⁸ coups par seconde, activité maximum attendue. Un prototype a été testé avec succès et une première mesure de tritium a été réalisé au spectromètre de masse neutrino Troitsk afin d'étudier les erreurs systématiques et de développer des méthodes d'analyses pertinentes. Un premier ajustement cohérent du spectre tritium différentiel acquis lors de cette installation, a démontré la faisabilité du projet. TRISTAN lui-même est toujours en cours de développement mais les caractérisations du détecteur et les études de systématiques sont plus qu'encourageantes pour la poursuite du projet. La première investigation de neutrino stérile avec le détecteur de TRISTAN sur le site de KATRIN est prévue après la mesure de masse, en cours à Karlsruhe, aux alentours de 2024. / The work presented in this thesis is about the sterile neutrino search with the two experiments SOX and TRISTAN based on the β-decay. Sterile neutrinos are theoretically well motivated particles that do not participate in any fundamental interaction except for the gravitation. With the help of these particles one could elegantly explain the origin of the neutrino mass, dark matter and the matter-antimatter asymmetry in the universe. As sterile neutrinos can mix with the known active neutrinos, they could be discovered in laboratory searches. The SOX experiment was designed to search for a sterile neutrino with a mass in the eV-range. This particular mass range is motivated by several anomalous observations at short-baseline neutrino experiments that could be explained by an additional oscillation with a length in the order of meters that arises from an eV-scale sterile neutrino. For SOX it was planned to use the existing Borexino solar neutrino detector to search for an oscillation signal within the detector volume. The neutrinos are emitted from a 5.5 PBq electron-antineutrino source made of the β-decaying isotopes ¹⁴⁴Ce and ¹⁴⁴Pr, located at 8.5 m distance from the detector center. For the analysis of the signal it is crucial to know the source activity. This parameter is determined by measuring the decay heat of the source with a thermal calorimeter that was developed by TUM and INFN Genova. The decay heat is measured through the temperature increase of a well-defined water flow in a heat exchanger that surrounds the source. The calorimeter was assembled, optimized and characterized. Heat losses were determined through calibration measurements with an electrical heat source. Adjustable measurement conditions and an elaborate thermal insulation allowed an operation with negligible heat losses. It was proven that the power of a decaying source can be measured with <0.2% uncertainty in a single measurement that lasts ~5 days. Unfortunately the SOX experiment was canceled after a technological problem rendered the source production with the required activity and purity impossible. The TRISTAN project is an attempt to discover sterile neutrinos with masses in the order of keV. In contrast to eV-scale sterile neutrinos that are motivated by several anomalies observed in terrestrial experiments, the existence of sterile neutrinos with masses in the keV range could resolve cosmological and astrophysical issues, as they are dark matter candidates. The TRISTAN project is an extension of the KATRIN experiment to search for the signature of keV-scale sterile neutrinos in the tritium β-spectrum. KATRIN itself is attempting to determine the effective neutrino mass by measuring the end point of the tritium spectrum at low counting rates. The KATRIN setup will be modified after the neutrino mass measurements are finished to conduct a differential and integral measurement of the entire tritium spectrum. This project is called TRISTAN. The current detector will be replaced by a novel 3500-pixel silicon drift detector system that has an outstanding energy resolution of a few hundred eV and can handle rates up to 10⁸ counts per second as they occur when the entire spectrum is scanned. Prototype detectors were successfully tested and first tritium data was taken at the Troitsk ν-mass spectrometer to study systematic effects and develop analysis methods. A successful fit of the differential tritium spectrum proved the feasibility of this approach. TRISTAN itself is still at an early stage, but the detector development and systematic studies are well on track and delivered so far encouraging results. The sterile neutrino search is scheduled after the KATRIN neutrino mass program is finished in ~2024.

The neutrino is, by its nature, an elusive particle that requires massive detectors with small backgrounds to capture a handful of events. Nevertheless, neutrino experiments stand at the heart of the current mysteries of particle physics and astrophysics. These include the origin and size of neutrino mass, the existence of additional types of neutrinos, CP violation and the matter--antimatter asymmetry, the amount of metals in the Sun's core, and the existence of non-nuclear energy sources in the Sun. This dissertation concerns the the use of a novel detector technology, the Raghavan Optical Lattice (ROL), in the Low-Energy Neutrino Spectrometer (LENS) and Neutrino Lattice (NuLat) experiments. LENS will measure the solar neutrino luminosity and the Sun's core metallicity using a ROL with indium-loaded liquid scintillator. NuLat will probe the existence of light sterile neutrinos with masses of $ \sim 1\,\mathrm{eV} $ using a ROL made from $ ^{6}\mathrm{Li} $-loaded plastic scintillator. For LENS we present an overview of the experiment and the present the ROL construction results from the LENS R\andD program. In particular we will present results from the micro- and mini-LENS prototypes. For both LENS and NuLat we present the development of an event reconstruction algorithm for ROLs and we apply these to the expected signals for these experiments. For NuLat we present an overview of the experiment including its theory of operation and its sensitivity to sterile neutrino oscillations. Finally, we present work toward the full-sized NuLat detector through bench-top tests and construction of the NuLat demonstrator. / Ph. D.

In dieser Arbeit verwenden wir Flavourinvarianten, um systematisch Lösungen für Probleme des Standardmodells (SM) der Teilchenphysik mit Hilfe verschiedener effektiver Feldtheorien (EFTs) zu untersuchen. In Teil I untersuchen wir die CP-Verletzung im SM und in der SM EFT erweitert mit leichten, sterilen Neutrinos. Wir konstruieren die erzeugende Menge von Flavourinvarianten im νSM, mit der jede Observable als Polynom der Invarianten, sowie die Bedingungen für die CP-Verletzung auf flavourinvariante Weise ausgedrückt werden können. Anschließend weiten wir die Ergebnisse auf die EFT-Wechselwirkungen für verschiedene Szenarien der Neutrinomassen aus. Hier ändert sich die Form der EFT-Flavourinvarianten und ihre Unterdrückung mit der Skala der neuen Physik drastisch mit der untersuchten Art der Neutrinomassen. In Teil II untersuchen wir verschiedene Aspekte der Symmetriebrechung in EFTs von axionartigen Teilchen (ALPs). Wegen ihrer pseudo-Nambu–Goldstone-Natur ist eine wesentliche Eigenschaft der ALPs ihre Shiftsymmetrie (ShS). Wir formulieren flavourinvariante Ordnungsparameter der ShS, die das Powercounting der EFT führender Ordnung bei einer leicht gebrochenen ShS korrekt implementieren lassen. Mit der Hilbertreihe zählen wir die Anzahl der Operatoren, die in der ALP EFT mit und ohne ShS oberhalb und unterhalb der elektroschwachen Skala auftreten, womit wir Operatorbasen konstruieren, die Beziehungen der ShS auf höhere Ordnung verallgemeinern und die CP-verletzenden Flavourinvarianten führender Ordnung konstruieren. Die Axionlösung des starken CP-Problems kann durch neue CP-Verletzung im Ultravioletten durch kleine Instantonen gestört werden. Mit einer SMEFT-Parametrisierung der neuen CP-Verletzung zeigen wir, dass neu konstruierte CP-verletzende SMEFT-Flavourinvarianten explizit in den Instantonberechnungen auftauchen und zur Systematisierung der Berechnungen verwendet werden können, wodurch wir bessere Limits für kleine Instanton- und Flavourszenarien ableiten. / In this thesis, we use flavour invariants to systematically study solutions to problems of the Standard Model (SM) of particle physics with different effective field theories (EFTs). In Part I, we study Charge–Parity (CP) violation in the SM and SM EFT extended with light sterile neutrinos. We construct the generating set of flavour invariants in the νSM allowing us to express any observable as a polynomial of those invariants. In addition, the invariants enable us to express the conditions for CP violation in a flavour-invariant way. We extend the results to the EFT interactions with different scenarios for the neutrino masses. Here, the form of the EFT flavour invariants and their suppression with the scale of new physics changes drastically depending on the nature of the neutrino masses. In Part II, we study different aspects of symmetry breaking in the EFTs of axionlike particles (ALPs). An essential property of ALPs is their shift symmetry (ShS) due to their pseudo-Nambu–Goldstone nature. We formulate flavour-invariant order parameters of ShS, which allow us to properly impose the power counting of the leading order EFT in the presence of a softly broken ShS. Using the Hilbert series, we count the number of operators appearing in the ALP EFT with and without a ShS above and below the electroweak scale. We use this information to construct operator bases, generalise the relations imposing ShS to higher orders and construct the leading order CP-odd flavour invariants. The axion solution to the strong CP problem can be spoiled by new CP violation in the ultraviolet in the presence of small instantons. Parameterising the new CP violation in the SMEFT, we show that newly constructed CP-odd SMEFT flavour invariants, featuring the strong CP angle, explicitly appear in the instanton computations and vice-versa that they can be used to systematise the computations. Using these results, we derive bounds on different small instanton and SMEFT flavour scenarios.

[pt] Nesse trabalho realizamos, em princípio, um estudo fenomenológico de alguns experimentos prévios com neutrinos, que utilizaram reatores nucleares como fonte. Tais experimentos buscaram evidências de distorção espectral que corroborassem o mecanismo de conversão de sabor entre neutrinos. Nessa etapa reunimos suficiente informação sobre a metodologia empregada para a detecção e análise do espectro de neutrinos de reatores, bem como os limites impostos por esses experimentos sobre os parâmetros de oscilação. A partir desse estudo, investigamos a possibilidade de se explorar uma região do espaço de parâmetros, caracterizada por um pequeno ângulo de mistura e elevado autoestado de massa, ainda não excluída pelos resultados experimentais atuais. Como uma escala de massa tão elevada não pode estar associada a qualquer um dos três sabores ativos (eletrônico, muônico ou tauônico), em nossa análise, utilizamos uma extensão do modelo de três neutrinos ativos, incorporando um quarto autoestado estéril. Por fim, discutimos uma configuração experimental, baseada em um reator nuclear, que apresente a sensibilidade necessária para atingir esse conjunto de parâmetros. / [en] In this dissertation we have done, at first, a phenomenological study of some previous neutrino experiments, that had searched evidences of spectral distortion that corroborated the mechanism of flavor conversion, using nuclear reactors as source. We collected enough information about the methodology used for detection and analisys of neutrino spectrum of reactors, as well as the constraints on oscillation parameters provided by that experiments. Starting from this study, we investigated the possibility to explore a region of the space of parameters, characterized by a small mixing angle and high mass eigenstate, that not yet have been excluded by experimental results, but incompatible with the extracted current mass scale of the experiments with solar, atmospheric, accelerator (except for the LSND experiment) and reactor neutrinos. For this inquiry, we assume the simplest extension of the standart oscillation model, incorporating a new eigenstate responsible for the higher scale mass. Finally, we discuss a experimental configuration, based on nuclear reactor, which presents the sensitivity required to reach this set of parameters.

In this thesis I test the modified Newtonian dynamics as an alternative to the cold dark matter hypothesis. In the Milky Way, I show that the dynamics of the dwarf galaxies are well described by the paradigm and I confirm its distant low surface brightness globular clusters provide a strong test, for which I make predictions. Through analysis of a sample of 26 X-ray bright galaxy groups and clusters I demonstrate that the three active neutrinos and their anti-particles are insufficient to reconcile modified Newtonian dynamics with the observed temperatures of the X-ray emitting gas, nor with weak-lensing measurements, in particular for the bullet cluster. To this end, I propose an 11eV sterile neutrino to serendipitously resolve the residual mass problem in X-ray bright groups and clusters, as well as matching the angular power spectrum of the Cosmic Microwave Background. With this in mind, I show that the large collision velocity of the bullet cluster and the high number of colliding clusters is more naturally reproduced in MOND than in standard dynamics.