Since the construction of the IceCube Neutrino Observatory was completed in 2010, many amazing discoveries have been made in the field of neutrino physics. Recently a neutrino event has been linked to an blazar-type active galactic nucleus source, bringing us one step closer to understanding the production of high-energy extragalactic neutrinos and ushering in a new era of multimessenger astronomy. This was found by linking the neutrino event to one of the Fermi Collaboration’s gamma ray sources which had a blazar counterpart. The quest to link other neutrino events to AGN (active galactic nuclei) sources through collaboration with the Fermi Large Area Telescope has turned up some interesting candidates. The fact that some of these potential sources are not blazars is curious and, although unconfirmed as neutrino sources, these objects merit further investigation due to their unusual nature.

The IceCube Neutrino Observatory has sensitivity to MeV electron antineutrinos from core-collapse supernovae through an excess of the detection rate over the background. Wavelength-shifting sensors have the potential to greatly increase photon collection making it a promising candidate for improving the measurement of the supernova neutrino light curve in IceCube-Gen2. For high-energy neutrino reconstruction, the competing effect of increased photon collection and the broader time distribution necessitate detailed simulations to determine the impact. In this thesis, we investigate the sensitivity gain caused by wavelength shifters in a future IceCube-Gen2 detector regarding the detection of faint modulations of the supernova neutrino lightcurve. Furthermore, we lay the groundwork for a future high-energy reconstruction through the implementation and integration of wavelength-shifting sensors into the IceCube simulation framework.

<p>A search for a diffuse flux of cosmic neutrinos with energies in excess of 10¹⁴ eV was performed using two years of AMANDA-II data, collected in 2003 and 2004. A 20% evenly distributed sub-sample of experimental data was used to verify the detector description and the analysis cuts. A very good agreement between this 20% sample and the background simulations was observed. The analysis was optimised for discovery, to a relatively low price in limit setting power. The background estimate for the livetime of the examined 80% sample is 0.035 ± 68% events with an additional 41% systematical uncertainty.</p><p>The total neutrino flux needed for a 5σ discovery to be made with 50% probability was estimated to 3.4 ∙ 10^-7 <i>E</i>^-2 GeV s^-1 sr^-1 cm^-2 equally distributed over the three flavours, taking statistical and systematic uncertainties in the background expectation and the signal efficiency into account. No experimental events survived the final discriminator cut. Hence, no ultra-high energy neutrino candidates were found in the examined sample. A 90% upper limit is placed on the total ultra-high energy neutrino flux at 2.8 ∙ 10^-7 <i>E</i>^-2 GeV s^-1 sr^-1cm^-2, taking both systematical and statistical uncertainties into account. The energy range in which 90% of the simulated <i>E</i>^-2signal is contained is 2.94 ∙ 10¹⁴ eV to 1.54 ∙ 10¹⁸eV (central interval), assuming an equal distribution over the neutrino flavours at the Earth. The final acceptance is distributed as 48% electron neutrinos, 27% muon neutrinos, and 25% tau neutrinos.</p><p>A set of models for the production of neutrinos in active galactic nuclei that predict spectra deviating from <i>E</i>^-2 was excluded.</p>

A search for a diffuse flux of cosmic neutrinos with energies in excess of 1014 eV was performed using two years of AMANDA-II data, collected in 2003 and 2004. A 20% evenly distributed sub-sample of experimental data was used to verify the detector description and the analysis cuts. A very good agreement between this 20% sample and the background simulations was observed. The analysis was optimised for discovery, to a relatively low price in limit setting power. The background estimate for the livetime of the examined 80% sample is 0.035 ± 68% events with an additional 41% systematical uncertainty. The total neutrino flux needed for a 5σ discovery to be made with 50% probability was estimated to 3.4 ∙ 10-7 E-2 GeV s-1 sr-1 cm-2 equally distributed over the three flavours, taking statistical and systematic uncertainties in the background expectation and the signal efficiency into account. No experimental events survived the final discriminator cut. Hence, no ultra-high energy neutrino candidates were found in the examined sample. A 90% upper limit is placed on the total ultra-high energy neutrino flux at 2.8 ∙ 10-7 E-2 GeV s-1 sr-1 cm-2, taking both systematical and statistical uncertainties into account. The energy range in which 90% of the simulated E-2 signal is contained is 2.94 ∙ 1014 eV to 1.54 ∙ 1018 eV (central interval), assuming an equal distribution over the neutrino flavours at the Earth. The final acceptance is distributed as 48% electron neutrinos, 27% muon neutrinos, and 25% tau neutrinos. A set of models for the production of neutrinos in active galactic nuclei that predict spectra deviating from E-2 was excluded.

[pt] Embora sua detecção seja difícil, neutrinos são considerados como boa ferramenta na astrofísica para obtermos informações das Supernovas, Explosão de Raios Gama e do interior de objetos celestes como o Sol e Núcleos Ativos de Galáxias, porque não é possível explorá-los por meios ópticos. Por interagir muito fracamente com a matéria usual, neutrinos são capazes de percorrer grandes distâncias e atravessar astros e galáxias sem perder energia e, ao serem detectados na Terra, eles trazem informações sobre a fonte emissora, mecanismos de produção destes neutrinos energéticos e, nos permitem estudar as propriedades ainda não completamente conhecidas dos neutrinos, como massas e ângulos de misturas. Um excelente exemplo é o telescópio de neutrinos IceCube que, recentemente, registrou eventos de neutrinos de altas energias, na faixa de TeV e PeV, cujas origens são extragalácticas. Tal pioneirismo abre uma nova janela para estudarmos o Universo. Nessa dissertação, através do fenômeno de oscilação de neutrinos, estudaremos como ocorre a mudança dos sabores leptônicos dos fluxos dos neutrinos de altas energias de origem extragaláctica ao longo de seu caminho, desde a sua criação até a Terra. Analisaremos também o comportamento da razão de sabor dos fluxos de neutrinos em função da fase de violação CP e do ângulo de mistura de sabor leptônico entre segunda e terceira gerações. / [en] Depite that it is difficult to detect neutrinos, they are an interesting tool in astrophysics to obtain information about astronomical objects and events such as the Sun, Supernovae, Active Galactic Nuclei and Gamma Ray Bursts because it is impossible to explore them by optic ways. Due to their very weak interaction with ordinary matter, neutrinos are able to propagate very large distances and pass through stars and galaxies with no energy loss. By detecting neutrinos at Earth, we can get information about neutrino sources as well as the production mechanism and we are also able to study neutrino properties which are not well understood yet, such as mass hierarchy and mixing angles. A great exemple is a neutrino telescope called IceCube which recently recorded, for the first time, high energy neutrino events in the range of TeV and PeV whose source is extragalactic. These events have opened a new window to study the universe. In this dissertation we use neutrino oscillation to study how is the change in the leptonic flavors of high energy neutrino fluxes from their extragalactic sources to the Earth. We also analyse the flavor ratio behavior of neutrino fluxes as a function of CP-violating phase and the mixing angle between second and third generation.

Cosmic rays reach Earth from beyond the Milky Way and with energies up to 10^20 eV.The responsible accelerators have to date not been discovered. However, multi-messenger astronomy can shed light on the question, based on the principle that protons and nuclei accelerated in dense and energetic environments would also produce gamma rays and neutrinos. Such environments may be found in "blazars", which are therefore cosmic ray accelerator candidates. Their gamma-ray emission has been observed to increase, sometimes by orders of magnitude, during "flares" as observed in light curves taken by the Large Angle Telescope on the Fermi satellite. When the latter was launched in 2008, the IceCube Neutrino Observatory had also started taking data, detecting the Cherenkov light from high-energy neutrino interactions in the glacier ice under the geographic South Pole. These two experiments have enabled multi-messenger searches for neutrinos in time correlation with the gamma-ray emission from blazars. This work builds on this principle and extends it by "stacking" the signal from multiple blazar flares. Thus, their individually undetectable neutrino emission could still be discoverable. One first analysis focused on the blazar TXS 0506+056, whose flare in 2017 coincided with arrival of the neutrino IceCube-170922A. Extending into a lower energy range than the alert, the search found no additional excess neutrinos associated with the flare. A second analysis used 179 bright and variable blazars. They were divided in two specific blazar classes and weighted relatively to each other, with two weighting schemes motivated physically using the observed gamma-ray luminosity and a third, generic weighting to cover unconsidered scenarios. No significant neutrino excess was found in the unblinded likelihood fits for any of the source catalogues and weighting schemes. Their combined trial-corrected p-value was p=(79.1 +/- 0.3)%. The limits derived from this analysis are also discussed and its relation with other searches considered. Since that was the first "blazar flare stacking", this work also proposes further improvements to the analysis which will help advance the search for cosmic ray accelerators. / Les rayons cosmiques proviennent d'au-delà de la Voie lactée et atteignent la Terre avec des énergies pouvant aller jusqu'à 10^20 eV. Les objets qui accélèrent ces rayons cosmiques n'ont toujours pas été découverts. Toutefois, l'astronomie multimessager peut apporter un élément de réponse à cette question, en supposant que les protons et les noyaux accélérés dans des environnements denses et énergétiques pourraient également produire des rayons gamma et des neutrinos. Les "blazars" sont de possibles candidats pour les accélérateurs de rayons cosmiques. Une augmentation de leurs émissions de rayons gamma, parfois de plusieurs ordres de grandeur, a été observée lors de phénomènes qu'on appelle "éruption", comme le montrent les courbes de lumière prises par le télescope spatial Fermi-LAT. Lorsque ce dernier a été lancé en 2008, l'observatoire de neutrinos IceCube avait également commencé à prendre des données, détectant la lumière Tcherenkov provenant d'interactions de neutrinos à haute énergie dans la glace qui se trouve sous le Pôle Sud géographique. Ces deux expériences ont permis de mener à bien des recherches multi-messagers de neutrinos en corrélation temporelle avec l'émission de rayons gamma des blazars. Ce principe est le point de départ de cette thèse, qui va plus loin en employant la méthode du "stacking", qui consiste à combiner les signaux provenant de plusieurs éruptions de blazars. Ainsi, leurs émissions individuelles de neutrinos, habituellement indétectables, pourraient être découvertes après combinaison. Une première analyse s'est concentrée sur le blazar TXS 0506+056, dont l'éruption en 2017 a coïncidée avec l'arrivée de l'évènement IceCube-170922A. En considérant une gamme d'énergie inférieure à celle de l'alerte 170922-A, pas d’autres neutrino excédentaire n’a été associé à l'éruption. Une deuxième analyse est basée sur 179 blazars lumineux et variables. Ces blazars ont été répartis en deux classes spécifiques, et chacun d'entre eux a reçu un poids relatif. Trois schémas de pondération ont été considérés :les deux premiers étant motivés par des observations, le troisième étant plus générique. Aucun excès significatif de neutrinos n'a été observé après avoir effectué des ajustements par maximum de vraisemblance sur les données non masquées, pour les différents catalogues de sources et schémas de pondération. Leur valeur-p combinée est de p=(79.1 +/- 0.3)%. Les limites dérivées de cette analyse sont discutées ainsi que leur rapport avec les résultats d'autres recherches. Puisqu'il s'agit du premier stacking d'éruptions de blazars, nous suggérons également des améliorations à apporter à l'analyse afin de permettre la poursuivre de la recherche d'accélérateurs de rayons cosmiques. / Kosmische straling afkomstig van buiten de Melkweg bereikt de Aarde met energieën tot wel 10^20 eV. De astrofysische bronnen waarin deze deeltjes worden versneld zijn tot op heden nog niet ontdekt. De multi-boodschapperastronomie kan een nieuw licht werpen op de oorsprong van kosmische straling, aangezien protonen en atoomkernen die worden versneld in een dichte en energetische omgeving ook gammastralen en neutrino’s produceren. „Blazars” zijn mogelijke kandidaat-versnellers. Observaties van blazars, gemaakt met de ruimtetelescoop Fermi-LAT ,tonen aan dat hun gammastraling tijdens zogenaamde „flakkers” toeneemt. Rond de tijd dat deze werd gelanceerd, begon het IceCube Neutrino Observatorium ook gegevens te verzamelen. Deze laatste detecteert hoog-energetische neutrino’s aan de hand van het Cherenkovlicht dat geproduceerd wordt tijdens hun interacties met de ijskap bij de geografische zuidpool. Deze twee experimenten hebben het mogelijk gemaakt om een multibooschapperzoektocht te verrichten naar neutrino’s van blazars die een tijdscorrelatie hebben met diens flakkers van gammastraling. Dit is het uitgangspunt van dit proefschrift, waarbij er ook een zogenaamde „stapelmethode” wordt toegepast. Op deze manier kan de neutrino-emissie van indivuele blazarflakkers, die afzonderlijk te zwak is om te detecteren, gecombineerd worden en mogelijks toch worden ontdekt. Een eerste analyse legt de focus op de blazar TXS 0506+056, waarvan een flakker in 2017 samenviel met de aankomst van het neutrino IceCube 170922-A. In een relatief lager energiebereik wordt er geen surplus aan neutrino’s gevonden gecorreleerd met de flakker. In een tweede analyse maken we gebruik van de stapelmethode om neutrino’s te zoeken afkomstig van 179 heldere en variabale blazars. Deze worden onderverdeeld in twee specifieke klassen en krijgen elks een zeker gewicht in de stapelanalyse. Hiervoor worden twee wegingsschema’s gebruikt die gemotiveerd zijn door de geobserveerde gammastraling, alsook een derde generieke weging. Ook hierwordt er geen significant neutrinosignaal geobserveerd. De gecombineerde p waarde is p=(79.1 +/- 0.3)%. Hieruit worden limieten afgeleid, en worden de verbanden met andere zoekacties besproken. Aangezien dit werk de eerste analyse omvat naar neutrino’s afkomstig van blazarflakkers gebruik makende van een stapelmethode, worden er in dit werk ook verdere verbeteringen van de analyse voorgesteld. Deze zullen als een startpunt dienen voor toekomstige zoektochten naar de nog onbekende bronnen van kosmische straling. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

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.

Im Jahr 2013 ist es dem IceCube-Experiment gelungen, einen Fluss von Neutrinos extraterrestrischen Ursprungs nachzuweisen, und damit das Neutrino als weiteres kosmisches Botenteilchen zu etablieren. Die Frage nach dem Ursprung der Neutrinos, die einen komplementären Blick auf die Quellen bieten, kann möglicherweise die alte Frage nach dem Ursprung der kosmischen Strahlung lösen. Zeitunabhängige Suchen nach Neutrinoquellen konnten bisher keine einzelnen Kandidaten isolieren. Zeitlich veränderliche Quellen kommen daher als Ursprung in Betracht. IceCube ermöglicht es, kontinuierlich den gesamten Himmel nach aufflackernden Neutrinoquellen abzusuchen und die astronomische Gemeinschaft schnellstmöglich zu benachrichtigen. In dieser Arbeit wird die Echtzeitidentifikation und -rekonstruktion von Myonneutrinokandidaten mit IceCube verbessert. Die erreichte Sensitivität ist mit etablierten nicht-Echtzeit Analysen vergleichbar. Kontinuierlich vom Experiment am Südpol übermittelte Informationen werden sofort auf bemerkenswerte Ereignisse hin analysiert. Bekannte astrophysikalische Quellen von Gammastrahlung werden auf Neutrinoemission hin beobachtet. Eine verallgemeinerte Methode erlaubt die Suche nach Signalen überall, unbeeinflusst von vorher bekannten Quellen. Weiterhin werden die hochenergetischsten Neutrinokandidaten, die wahrscheinlich astrophysikalischen Ursprungs sind, sofort identifiziert und global bekannt gemacht. Abschließend werden die Suchalgorithmen am Beispiel zweier Blazare demonstiert, 1ES 1959+650 und TXS 0506+56. In letzterem Fall wurden erstmals Anzeichen für eine Quelle hochenergetischer, kosmischer Neutrinos gefunden. Die im Rahmen dieser Arbeit entwickelte Infrastruktur erlaubt es, die astronomische Gemeinschaft auf signifikante Neutrinoereignisse, oder sich entwickelnde Neutrinocluster hinzuweisen. Auch die zügige Suche nach Neutrinos in Reaktion auf interessante astrophysikalische Ereignisse, wie z.B. Gravitationswellen, ist möglich. / The discovery of an flux of neutrinos of astrophysical origin with the IceCube experiment in 2013 has broadened our understanding of cosmic messengers and opened a new window on the universe. By addressing the newly pertinent question about their sources, neutrinos can provide a complementary view on cosmic accelerators and may help solving the long-standing puzzle of the origin of the cosmic rays. As traditional time-integrated searches for sources of neutrinos have not been able to isolate individual candidates, variable and transient sources shift into focus. IceCube's design allows to continuously search the entire sky for neutrino flares, and alert the community with the lowest possible latency in the case of a detection. This thesis improves the identification and reconstruction of muon neutrino candidates with IceCube in real-time, achieving a sensitivity comparable to dedicated offline analyses. The stream of neutrino candidates is analyzed for interesting events in order to alert partner experiments and inspire follow-up observations. First, known gamma-ray emitters are monitored for time-variable neutrino emission. Second, a generalization of this method monitors the entire sky for neutrino flares, regardless of pre-defined source lists. Third, the most-energetic neutrino candidates with the highest chance to be of astrophysical origin are selected for alerts. Eventually, the search methods are applied to the blazars 1ES 1959+650 and TXS 0506+056. In the latter case, evidence for source of high-energetic, astrophysical neutrinos was found for the first time. The infrastructure built in this work allows to notify the community whenever significant neutrino events are recorded, or significant flares develop on time-scales from days to weeks. It also allows to quickly perform neutrino follow-up searches in response to interesting astrophysical events, such as the observation of gravitational waves.

Neutrino astronomy is expanding into the ultra-high energy (&gt;1017eV) frontier with the use of in-ice detection of Askaryan radio emission from neutrino-induced particle showers. There are already pilot arrays for validating the technology and the next few years will see the planning and construction of IceCube-Gen2, an upgrade to the current neutrino telescope IceCube. This thesis aims to facilitate that planning by providing faster simulations using deep learning surrogate models. Faster simulations could enable proper optimisation of the antenna stations providing better sensitivity and reconstruction of neutrino properties. The surrogates are made for two parts of the end-to-end simulations: the signal generation and the signal propagation. These two steps are the most time-consuming parts of the simulations. The signal propagation is modelled with a standard fully connected neural network whereas for the signal generation a conditional Wasserstein generative adversarial network is used. There are multiple reasons for using these types of models. For both problems the neural networks provide the speed necessary as well as being differentiable -both important factors for optimisation. Generative adversarial networks are used in the signal generation because of the inherent stochasticity in the particle shower development that leads to the Askaryan radio signal. A more standard neural network is used for the signal propagation as it is a regression task. Promising results are obtained for both tasks. The signal propagation surrogate model can predict the parameters of interest at the desired accuracy, except for the travel time which needs further optimisation to reduce the uncertainty from 0.5 ns to 0.1 ns. The signal generation surrogate model predicts the Askaryan emission well for the limited parameter space of hadronic showers and within 5° of the Cherenkov cone. The two models provide a first step and a proof of concept. It is believed that the models can reach the required accuracies with more work.

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