La mesure récente de l'angle de mélange theta-13, à laquelle l'expérience Double Chooz contribue, a ouvert la voie aux futures expériences de la physique des neutrinos. Dans ce manuscrit, la caractérisation de certains bruits de l'expérience sont décrits. Les muons cosmiques qui s'arrêtent et se désintègrent dans le détecteur sont mal reconstruits, résultant en distorsion de la distribution temporelle des signaux laquelle peut être utilisée pour identifier ce type de fond. Les neutrons rapides créés par spallation par les muons cosmiques produisent de nombreux protons de recul qui peuvent entraîner un décalage dans la distribution temporelle des signaux et ainsi être identifiés. Ces distributions temporelles ont aussi été utilisées pour identifier la formation de l'état d'orthopositronium en observant et en mesurant un délai entre l'ionisation du positron et l'annihilation de celui-ci, pouvant permettre une séparation positron-électron. / The measurement of the theta-13 mixing angle, to which the Double Chooz experiment contributed, paves the way to future findings in neutrino physics. In this manuscript, we describe the characterization of some Double Chooz backgrounds. Cosmic muons that stop and decay in the detector are characterized by anisotropic emission of the scintillation light, causing the vertex to be poorly reconstructed. The resulting pulse shape distortion can be used to tag and remove such background. Fast spallation neutrons producing multiple recoil protons may produce a similar distortion in the pulse shape and can also be tagged. Pulse shapes are also used to identify the formation of ortho-positronium. The tagging of such electron-positron bound state is made possible by the induced distortion in the pulse shape due to the delay in the positron annihilation, and can be used for an electron-positron separation.

[EN] Neutrino astronomy is a booming field in astroparticle physics. Due to the particular characteristics of neutrinos, these particles offer great advantages as probes for the study of the far and high-energy Universe. It is extensively accepted by the scientific community that a multi-messenger approach with the combination of information provided by neutrinos, photons and charged particles (cosmic rays) is possible to obtain a more complete image of the fundamental astrophysics processes taking place in our Universe. Since neutrinos are neutral and very weak interacting particles they can reach the Earth from astrophysical sources without deflection by magnetic fields and almost without energy losses and absorption, contrarily to the rest of messengers. The other side of the coin of neutrino properties is that detection of neutrinos is very challenging and big highly instrumented detection volumes are needed. Natural media (deep sea, lakes or ice in the Antarctica) host this kind of experiments using the water (or ice) as target material where the neutrino interaction is produced. ANTARES is the first undersea neutrino telescope, located at 2475 m depth in the Mediterranean Sea. ANTARES is optimized for optical detection of the Cerenkov light induced by relativistic muons produced by high energy neutrino interactions near the detector. The charge, position and arrival time of the photons to the optical modules which compose the detector allows the muon track reconstruction, and thus, knowing the neutrino coming direction. Some information of the event energy is also derived. ANTARES is also hosting the AMADEUS experiment which is investigating the feasibility of the acoustic detection of Ultra-High Energy (UHE) neutrinos. The framework of this thesis is the ANTARES experiment. As commonly done in the thesis developed in this experiment (and in this field), the work has been divided in two different areas. On the one hand, a part more devoted to technological aspects related to the detector and, on the other hand, a part dedicated to ANTARES data analysis. The first part of the thesis is focused in the development of a calibrator able to reproduce the acoustic signal generated in the UHE neutrino interaction with a water nucleus which, roughly speaking, generates a highly directive bipolar acoustic pulse. Having a good calibrator is crucial to test and tune the telescope response for this kind of signals. The second part of the thesis, the data analysis part, is centred in the analysis of the ANTARES data in order to constrain possible Dark Matter models. This work is focused on the detection of products resulting of the Dark Matter annihilation trapped in the centre of the Sun. Specifically, the Secluded Dark Matter (SDM) model has been tested by the detection of di-muons (co-linear muon pair) and/or neutrinos coming from Sun direction. Broadly speaking, this model is based on the idea of the existence of a mediator resulting of the Dark Matter annihilation which, subsequently, would decay into standard model particles as muons or neutrinos. These models have been proposed in order to explain some experimental "anomalies" observed, such as the electron-positron ratio spectrum detected in satellites, measured recently with high accuracy by AMS-II. The study of this thesis constitutes the first search of experimental evidences of this kind of models in neutrino telescopes. / [ES] La astronomía de neutrinos es un campo en auge dentro de la Física de Astropartículas. Los neutrinos ofrecen grandes ventajas como sondas para estudiar el Universo lejano y de alta energía. Es extensamente aceptado que mediante la combinación de la información que proporcionan los neutrinos junto a la obtenida mediante fotones de alta energía (rayos gamma) y partículas cargadas (rayos cósmicos) se podría obtener una imagen más completa de los procesos astrofísicos fundamentales que tienen lugar a lo largo de nuestro Universo.La razón fundamental por la que los neutrinos son tan altamente valorados como mensajeros es la baja interacción con el medio que los rodea. Al ser partículas sin carga interactúan muy débilmente con la materia, por ello pueden escaparse de la fuente donde se han producido y, al contrario de lo que ocurre con el resto de mensajeros, pueden llegar a la Tierra sin ser desviados por los campo magnéticos y sin prácticamente pérdida de energía. Esta misma razón que los hace tan valorados es a su vez la que los hace tan difíciles de detectar. Se impone la necesidad de construir detectores de grandes volúmenes, del orden del km3, altamente instrumentados. Se utilizan medios naturales (en el fondo del mar, en lagos o en enterrados en el hielo de la Antártida) aprovechando el agua (o hielo) como material diana donde se espera que interaccione el neutrino. ANTARES es el primer telescopio submarino de neutrinos construido en el fondo del mar Mediterráneo. Está optimizado para la detección óptica de la luz Cherenkov inducida por los muones relativistas producidos en la interacción de neutrinos de alta energía en los alrededores del detector. La información de la carga, posición y tiempo de llegada de los fotones a los fotomultiplicadores que componen el detector permite tanto la reconstrucción de la trayectoria del neutrino como el conocimiento de su energía. Además, ANTARES acoge el experimento AMADEUS mediante el cual se está investigando y testeando la detección acústica de neutrinos de muy alta energía que, al interaccionar en el agua, producen un pulso termo-acústico que se pretende registrar con una red de hidrófonos. El trabajo desarrollado en esta tesis se engloba bajo el marco del experimento ANTARES. Como es común en las tesis desarrolladas en este experimento, el trabajo se ha dividido en dos áreas diferenciadas: por un lado, una parte de enfoque más tecnológico y, por otro lado, una parte analítica de datos tomados por el telescopio. La primera parte de la tesis está centrada en el desarrollo de un calibrador capaz de reproducir la señal acústica que se emite en la interacción de un neutrino de alta energía con un núcleo de agua que, generalizando, es un pulso bipolar altamente directivo. El disponer de un buen calibrador es clave a la hora de testear la detección acústica en el telescopio y poder sintonizar y "entrenar" los los receptores para este tipo de señales. La segunda parte de la tesis se ha centrado en el análisis de datos registrados por ANTARES con el fin de contrastar posibles modelos astrofísicos para la búsqueda de materia oscura. Este trabajo ha focalizado en la detección de los productos de la aniquilación de materia oscura atrapada en el centro del Sol. Se ha testeado el modelo de Secluded Dark Matter (SDM) a través de la detección de di-muones (pareja de muones co-lineales) y neutrinos en la dirección del Sol. A grandes rasgos, este modelo se basa en la idea de la existencia de un mediador resultado de la aniquilación de materia oscura que posteriormente decaería en partículas del modelo estándar como muones o neutrinos. Estos modelos han sido propuestos con el fin de explicar ciertas 'anomalías' experimentales observadas, tales como el espectro del flujo de positrones detectado en satélites, medido recientemente con gran precisión por AMS-II. realizado en esta tesis constituye la primera búsqueda de evidencias / [CAT] L'astronomia de neutrins és un camp en auge dins la Física d'Astropartícules. Els neutrins ofereixen grans avantatges com a sondes per estudiar l'Univers llunyà i d'alta energia. Es extensament acceptat que mitjançant la combinació de la informació proporcionada pels neutrins junt a la obtinguda mitjançant fotons d'alta energia (rajos gamma) i partícules carregades (rajos còsmics) es podria obtindre una imatge més completa dels processos astrofísics fonamentals que es donen al llarg del nostre Univers. La raó fonamental per la qual els neutrins són altament valorats com a missatgers és la baixa interacció amb el medi que els envolta. Al ser partícules sense càrrega interactuen molt dèbilment amb la matèria, per això poden escapar-se de la font on s'han produït i, al contrari del que ocorre amb la resta de missatgers, poden arribar a La Terra sense desviar-se pels camps electromagnètics i sense pràcticament pèrdua d'energia. Aquesta mateixa raó que els fan tan valorats és al mateix temps la que els fa tan difícil de detectar. S'imposa la necessitat de construir detectors amb grans volums de detecció, de l'ordre del km3, altament instrumentats. S'utilitzen medis naturals (al fons de la mar, en llacs, al gel de l'Antàrtida) aprofitant l'aigua (o el gel) com a material diana on interaccionen el neutrins. ANTARES és el primer telescopi submarí de neutrins construït al fons de la mar Mediterrània. Està optimitzat per a la detecció òptica de la llum de Cherenkov induïda pels muons relativistes produïts en la interacció de neutrins d'alta energia als voltants del detector. La informació de la carrega, posició i temps d'arribada dels fotons als fotomultiplicadors que composen el detector permet tant la reconstrucció de la trajectòria del neutrí, amb gran resolució angular, com el coneixement de la seua energia. A més, ANTARES acull l'experiment AMADEUS mitjançant el qual s'està investigant i testejant la detecció acústica de neutrins de molt alta energia, que, al interaccionar a l'aigua produeixen un pols termo-acústic que es pretén registrar amb una xarxa d'hidròfons. El treball dut a terme en esta tesi s'engloba baix el marc de l'experiment ANTARES. Com es comú en les tesis desenvolupades en aquest experiment, el treball s'ha dividit en dues àrees diferenciades: per una banda una part d'enfocament mes tecnològic i, d'altra banda, una part analítica de les dades preses pel telescopi. La primera part de la tesi està centrada en el desenvolupament d'un calibrador capaç de reproduir la senyal acústica que es genera en la interacció d'un neutrí d'alta energia amb un nucli de l'aigua que, generalitzant, és un pols bipolar altament directiu. Disposar d'un bon calibrador es clau a l'hora de testejar la detecció acústica al telescopi i poder sintonitzar i "entrenar" els receptors a aquest tipus de senyals. La segona part de la tesi, amb caràcter d'anàlisi de dades, s'ha centrat en l'anàlisi de les dades registrades per ANTARES amb el fi de contrastar possibles models astrofísics per a la recerca de matèria fosca. Aquest treball es centra en la detecció dels productes d'aniquilació de matèria fosca atrapada al centre del Sol. En concret, s'ha testejat el model de Secluded Dark Matter (SDM) a través de la detecció de di-muons (parell de muons co-lineals) i neutrins en la direcció del Sol. A grans trets, aquest model es basa en la idea de l'existència d'un mediador resultat de l'aniquilació de matèria fosca que posteriorment decauria en partícules del model estàndard com muons o neutrins. Aquests models han sigut proposats amb la fi d'explicar certes "anomalies" experimentals observades, tals com l'espectre del flux de positrons detectat en satèl¿lits, mesurat recentment amb gran precisió per AMS-II. L'estudi realitzat en esta tesi constitueix la primera recerca d'evidències experimentals d'aquest tipus de models en telescopis de neutrins. / Adrián Martínez, S. (2015). Design and Development of an Acoustic Calibrator for Deep-Sea Neutrino Telescopes and First Search for Secluded Dark Matter with ANTARES [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/48877 / TESIS

L’expérience SuperNEMO cherche à observer la double désintégration beta sans émission de neutrinos, uniquement possible si le neutrino est une particule de Majorana. Le premier module, aussi appelé démonstrateur, est en cours de construction au Laboratoire Souterrain de Modane. Sa capacité à détecter les particules individuelles en plus d’en mesurer l’énergie en fait un détecteur unique. Le démonstrateur peut contenir 7 kg de ⁸²Se sous forme de fines feuilles source. Ces feuilles source sont entourées par une chambre à fils, permettant ainsi la reconstruction en 3 dimensions des traces de particules chargées. Un calorimètre segmenté, composé de scintillateurs plastiques couplés à des photomultiplicateurs, assure quant à lui la mesure de l’énergie de chaque particule. De plus, la chambre à fils peut être soumise à un champ magnétique afin d’identifier la charge des particules. SuperNEMO est donc capable d’effectuer la reconstruction complète de la cinématique d’un événement ainsi que d’identifier la nature des particules impliquées dans ce dernier : électrons, positrons, particules α ou encore particules γ. En pratique, la reconstruction des particules repose sur divers algorithmes implémentés dans un logiciel de simulation et de reconstruction développé par et pour la collaboration SuperNEMO. La reconstruction des particules γ est particulièrement délicate puisque ces particules ne laissent pas de traces dans la chambre à fils et sont seulement détectées par le calorimètre, parfois même plusieurs fois. Différentes approches ont été explorées durant cette thèse. Ce travail a abouti à la création d’un nouvel algorithme permettant à la fois d’optimiser l’efficacité de reconstruction des particules γ mais aussi d’améliorer la reconstruction de leurs énergies. D'autres programmes assurant l’identification des particules et l’opération des mesures topologiques pertinentes à chaque événement ont aussi été développés. La valeur du champ magnétique a été optimisée pour la recherche de la désintégration 0νββ à l’aide de simulations Monte-Carlo. Les performances des blindages magnétiques ainsi que leur influence sur le champ magnétique ont été évaluées via des mesures effectuées grâce à des bobines magnétiques à échelle réduite. Le démonstrateur SuperNEMO est capable de mesurer ses propres contaminations en bruits de fond grâce à des canaux d’analyse dédiés. À l’issue d’une première prise de données de 2,5 ans, les activités visées pour les principaux bruits de fond devraient être connues précisément. En outre, la demi-vie du processus 2νββ pour le ⁸²Se devrait être mesurée avec une incertitude totale de 0,3 %.À la différence d’autres expériences double beta se basant uniquement sur la somme en énergie des deux électrons, SuperNEMO a accès à la totalité de la cinématique d’un événement et donc à de plus nombreuses informations topologiques. Une analyse multivariée reposant sur des arbres de décision boostés permet ainsi une amélioration d’au moins 10 % de la sensibilité pour la recherche de la désintégration 0νββ. Après 2,5 ans, et si aucun excès d'événements 0νββ n'est observé, le démonstrateur pourra établir une limite inférieure sur la demi-vie du processus 0νββ : T > 5.85 10²⁴ ans, équivalant à une limite supérieure sur la masse effective du neutrino mββ < 0.2 − 0.55 eV. En extrapolant ce résultat à une exposition de 500 kg.an, ces mêmes limites deviendraient T > 10²⁶ ans et mββ < 40 − 110 meV. / SuperNEMO is an experiment looking for the neutrinoless double beta decay in an effort to unveil the Majorana nature of the neutrino. The first module, called the demonstrator, is under construction and commissioning in the Laboratoire Souterrain de Modane. Its unique design combines tracking and calorimetry techniques. The demonstrator can study 7 kg of ⁸²Se, shaped in thin source foils. These source foils are surrounded by a wire chamber, thus allowing a 3-dimensional reconstruction of the charged particles tracks. The individual particles energies are then measured by a segmented calorimeter, composed of plastic scintillators coupled with photomultipliers. A magnetic field can be applied to the tracking volume in order to identify the charge of the particles. SuperNEMO is thus able to perform a full reconstruction of the events kinematics and to identify the nature of the particles involved: electrons, positrons, α particles or γ particles. In practice, the particle and event reconstruction relies on a variety of algorithms, implemented in the dedicated SuperNEMO simulation and reconstruction software. The γ reconstruction is particularly challenging since γ particles do not leave tracks in the wire chamber and are only detected by the calorimeter, sometimes multiple times. Several γ reconstruction approaches were explored during this thesis. This work lead to the creation of a new algorithm optimizing the γ reconstruction efficiency and improving the γ energy reconstruction. Other programs allowing the particle identification and performing the topological measurements relevant to an event were also developed. The value of the magnetic field was optimized for the 0νββ decay search, based on Monte-Carlo simulations. The magnetic shieldings performances and their impact on the shape of the magnetic field were estimated with measurements performed on small scale magnetic coils. The SuperNEMO demonstrator is able to measure its own background contamination thanks to dedicated analysis channels. At the end of the first 2.5 years data taking phase, the main backgrounds target activities should be measured accurately. The ⁸²Se 2νββ half-life should be known with a 0.3 % total uncertainty. Unlike other double beta decay experiments relying solely on the two electrons energy sum, SuperNEMO has access to the full events kinematics and thus to more topological information. A multivariate analysis based on Boosted Decision Trees was shown to guarantee at least a 10 % increase of the sensitivity of the 0νββ decay search. After 2.5 years, and if no excess of 0νββ events is observed, the SuperNEMO demonstrator should be able to set a limit on the 0νββ half-life of T > 5.85 10²⁴ y, translating into a limit on the effective Majorana neutrino mass mββ < 0.2 − 0.55 eV. Extrapolating this result to the full-scale SuperNEMO experiment, i.e. 500 kg.y, the sensitivity would be raised to T > 10²⁶ y or mββ < 40 − 110 meV.

This thesis is devoted to the study of neutrino physics in general and the study of neutrino mixing and oscillations in particular. In the standard model of particle physics, neutrinos are massless, and as a result, they do not mix or oscillate. However, many experimental results now seem to give evidence for neutrino oscillations, and thus, the standard model has to be extended in order to incorporate neutrino masses and mixing among different neutrino flavors. When neutrinos propagate through matter, the neutrino mixing, and thus, also the neutrino oscillations, may be significantly altered. While the matter effects may be easily studied in a framework with only two neutrino flavors and constant matter density, we know that there exists (at least) three neutrino flavors and that the matter density of the Universe is far from constant. This thesis includes studies of three-flavor effects and a solution to the two-flavor neutrino oscillation problem in matter with an arbitrary density profile. Furthermore, there have historically been attempts to describe the neutrino flavor transitions by other effects than neutrino oscillations. Even if these effects now seem to be disfavored as the leading mechanism, they may still give small corrections to the neutrino oscillation formulas. These effects may lead to erroneous determination of the fundamental neutrino oscillation parameters and are also studied in this thesis in form of damping factors. / QC 20101124

Das IceCube Neutrino Teleskop hat einen diffusen Fluss hochenergetischen astrophysikalischen Neutrinos entdeckten. Allerdings sind die Quellen die für die Mehrzahl der nachgewiesenen Neutrinos verantwortlich sind, noch unbekannt. Diese Arbeit untersucht die Möglichkeit, dass der beobachtete Neutrino-Fluss im Zentrum von aktiven galaktischen Kernen (AGN) erzeugt wird. Eine Stacking-Analyse wird durchgeführt, um die Korrelation zwischen verschiedenen Subpopulationen von AGN und hochenergetischen Neutrinos unter Verwendung von IceCube-Daten aus acht Jahren zu testen. AGN werden anhand ihrer Radioemission, Infrarot-Farbeigenschaften und ihres Röntgenflusses. Die Leuchtkraft der Akkretionsscheibe wird verwendet, um den Beitrag ausgewählter Galaxien zum Neutrinosignal zu gewichten. Die leuchtende AGN-Population trägt zu ~52% des von IceCube gemessenen diffusen Flusses bei 100 TeV mit einem Best-Fit-Spektralindex von 2 bei mit 2.83 sigma post-trial Signifikanz. Für die AGN-Probe mit geringer Leuchtkraft wird eine Signifikanz nach dem Versuch von nur 0.66 sigma gefunden, daher werden Obergrenzen festgelegt. Unter Annahme des Spektralindex für den astrophysikalischen Fluss von 2 und einer gleichverteilten gleiche Zusammensetzung Neutrinoflavour-Zusammensetzung auf der Erde, wird eine obere Flussgrenze berechnet, die den maximalen Beitrag der Kerne von AGN mit geringer Leuchtkraft zum diffusen TeV-PeV-Neutrino-Fluss auf ~51% bei 100 TeV beschränkt. Für diese Arbeit wurde auch eine neue Rekonstruktionsmethode entwickelt. In IceCube werden hochenergetische Myon-neutrinos durch die sekundären Myonen identifiziert, die durch Wechselwirkungen über geladene Ströme mit dem Eis erzeugt werden. In dem hier vorgestellten Rekonstruktionsschema wird die erwartete Ankunftszeitverteilung durch ein vorbestimmtes stochastisches Myon-Energieverlustprofil parametrisiert. Diese realistischere Parametrisierung führt zu einer Verbesserung der Myon-Winkelauflösung in IceCube um etwa 20%. / The IceCube neutrino telescope has measured a diffuse flux of high-energy astrophysical neutrinos. However, the sources responsible for the emission of the majority of the detected neutrinos are still unknown. The goal of this thesis is to explore the possibility that the neutrino flux observed by IceCube is produced in the cores of Active Galactic Nuclei (AGN). A stacking analysis is conducted to test for a correlation between various sub-populations of AGN and high-energy neutrinos using eight years of IceCube data. AGN are selected based on their radio emission, infrared color properties, and X-ray flux using the NVSS, AllWISE, ROSAT and XMMSL2 catalogs. The accretion disk luminosity estimated by the observed soft X-ray flux is used as a proxy for the contribution of selected galaxies to the neutrino signal. Two of the three AGN samples tested in this analysis show over-fluctuations, with the highest significance being of 2.83 sigma after trial correction. The luminous AGN population is found to contribute to ~52% of the diffuse flux measured by IceCube at 100 TeV with a best-fit spectral index of 2. For the low-luminosity AGN sample a post-trial significance of only 0.66 sigma is found, therefore upper limits are set. Assuming the spectral index for the astrophysical flux to be 2 and an equal composition of neutrino flavours arriving at Earth, an upper flux limit is calculated which constrains the maximal contribution of the cores of low-luminosity AGN to the diffuse TeV-PeV neutrino flux to be ~51% at 100 TeV. A new reconstruction method has also been developed for this thesis. In IceCube high-energy muon neutrinos are identified through the secondary muons produced via charge current interactions with the ice. In the reconstruction scheme presented in this thesis, the expected arrival time distribution is parameterized by a predetermined stochastic muon energy loss pattern, leading to an improvement of about 20% to the muon angular resolution of IceCube.

Blazars are a type of Active Galaxy that emit strong astrophysical jets. The association of a HE gamma-ray flare from the blazar TXS 0506+056 to the IceCube-170922A neutrino event in 2017, opened the possibility to a link between these two events. In this thesis, we will look at the HE gamma-ray behavior of TXS 0506+056 using data obtained from the Fermi-LAT by taking into account the other set of neutrino events associated with this source from 2014-2015. We will investigate whether both neutrino events present with comparable HE gamma-ray behavior by analyzing the lightcurves and the spectra for a quiet state, the 2014-2015 period, and the flare centered around the neutrino event from 2017. The results of the analysis performed in this thesis show no strong indication of a change in the gamma-ray behaviour in these potential neutrino detections.

We consider a simple model for invisible neutrino decay as a sub-leading effect in the standard three-flavor neutrino oscillation framework, and use the Cayley–Hamilton formalism to obtain a full set of neutrino oscillation probabilities in matter. These are given as analytical series expansions in the small parameters α ∼ O(λ^2) and s_13 ∼ O(λ), where λ ≡ 0.2 is a “book-keeping parameter” denoting the order of the expansion. We produce explicit formulas for P_eµ, P_eτ , P_µµ, P_µτ , and P_ττ to order O(λ^3), and for P_ee to order O(λ^2), all having first corrections of order O(λ^4). Moreover, we also present vacuum limits of our expressions, as well as discuss the effect of decay on unitarity. We show that all rows in the unitarity table have corrections of order O(λ^2), with the second and third rows having additional corrections of order O(1). In the limit of no decay, unitarity is restored, and we furthermore recover known results for all probabilities. / Vi betraktar en enkel modell för osynligt neutrinosönderfall som en icke-ledande effekt inom det vedertagna ramverket för oscillationer med tre neutrinosmaker och använder Cayley–Hamilton-formalismen för att erhålla en fullständig uppsättning av sannolikheter för neutrinooscillationer i materia. Dessa ges som analytiska serieutvecklingar i de små parametrarna α ∼ O(λ^2) och s_13 ∼ O(λ), där λ ≡ 0.2 är en gemensam “bokföringsparameter” som anger serieutvecklingens ordning. Vi tar fram explicita uttryck för P_eµ, P_eτ , P_µµ, P_µτ och P_ττ till ordning O(λ^3) och för P_ee till ordning O(λ^2), med första korrigeringar för alla sannolikheterna till ordning O(λ^4). Därutöver presenterar vi gränsvärden för våra uttryck i vakuum samt diskuterar sönderfallets inverkan på unitaritet. Vi finner att alla rader i unitaritetstabellen har korrigeringar till ordning O(λ^2) och därtill korrigeringar till ordning O(1) för den andra och tredje raden. I avsaknad av sönderfall återfås unitaritet och vi reproducerar dessutom kända resultatför alla sannolikheter.

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] Conforme entramos na era de precisão da astronomia multimensageira, novas janelas se abrem para compreendermos melhor o Universo, desde a escala quântica até a escala cósmica. Em particular, o estudo de fenômenos astrofísicos de altas energias tem nos permitido acessar os ambientes mais extremos conhecidos pela humanidade, bem como obter avanços sem precedentes no domínio da física de partículas. Esta tese resume as descobertas importantes da astrofísica multi-mensageira ao longo dos anos, e, em seguida, foca a sua atenção em três tópicos relevantes que estão atualmente sendo investigados neste campo. Primeiramente, abordamos o problema da propagação de raios gama no espaço. Interações durante este processo levam à formação de cascatas eletromagnéticas que se desenvolvem ao longo de distâncias cosmológicas. Apresentaremos um código semi-analítico chamado “γ-Cascade”, que calcula os fluxos na Terra resultantes de tais cascatas. Também exploramos a possibilidade de se produzir neutrinos em cascatas ocorrendo a energias ultra-altas. Em segundo lugar, estabeleceremos uma relação multimensageira nova e original entre os fluxos medidos de neutrinos astrofísicos entre TeV–PeV e raios cósmicos ultra-energéticos. Para isso, utilizaremos nossas observações precisas de raios gama em energias abaixo de TeV, demonstrando o poder de uma análise multimensageira. Finalmente, estudaremos a evolução da composição de sabor de neutrinos produzidos em supernovas. Nosso novo método permite previsões genéricas sobre os possíveis sabores de neutrinos medidos na Terra. São levados em consideração os efeitos de matéria dentro dos ambientes densos de supernovas, enquanto permanecemos completamente agnósticos em relação ao resultado das conversões auto-induzidas de sabor em seus núcleos. / [en] As we enter the precision era of multi-messenger astronomy, new windows are opened for us to better understand the Universe, from quantum to cosmic scales. In particular, the study of high-energy astrophysical phenomena has allowed us to probe the most extreme environments known to mankind, as well as obtain unprecedented breakthroughs within the realm of particle physics. This thesis summarizes the important findings of multi-messenger astrophysics over the years, before focusing its attention to three relevant topics currently being investigated in the field. Firstly, we tackle the problem of γ-ray propagation in space. High center-of-momenta interactions during this process leads to the formation of electromagnetic cascades that develop over cosmological distances. We describe a semi-analytical code called “γ-Cascade, which calculates the fluxes at the Earth resulting from such cascades. We also explore the possibility of producing neutrinos in ultra-high-energy cascades. Secondly, we establish a new, original multi-messenger connection between the measured fluxes of TeV–PeV astrophysical neutrinos and ultra-high-energy cosmic rays. This is done by taking advantage of our precise γ-ray observations at sub-TeV energies, demonstrating the power of multi-messenger analyses. Finally, we study the evolution of the flavor composition of supernova neutrinos in a model-independent way. Our novel method allows for predictions of the neutrino flavor content measured at the Earth from supernovae, accounting for matter effects within its dense environment, while remaining completely agnostic about the outcome of self-induced flavor conversions in its core.

Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: See fulltext for abstract. / AFRIKAANSE OPSOMMING: Sien volteks vir opsomming.