The T2K Experiment is a long-baseline accelerator neutrino oscillation experiment, whose primary aim is to look for ν_e appearance in a ν_μ beam. A predominantly ν_μ beam is produced at J-PARC in Tokai on the east coast of Japan, and neutrino interactions are measured both by a near detector complex, 280 m from the neutrino production target,and by a large water Cerenkov detector, Super-Kamiokande, 295 km away. This thesis is concerned with oscillations ν_μ → ν_e, within both a standard three neutrino model and a model in which there is one additional sterile neutrino. By looking at ν_e appearance over the T2K baseline, this thesis looks for oscil- lations involving one additional sterile neutrino. A region of the short baseline neutrino oscillation parameter space favoured by other experiments is excluded at 3σ. The ND280 is also used to search for ν_e appearance over a short baseline. A ν_e selection is developed, and limits on the short baseline oscillation parameter space are set. Sensitivity predictions are also made for future T2K running. The T2K ND280 is interesting for this work since the detector technology is different to that of other experiments that have seen indications of short baseline electron-neutrino appearance. In the standard three-flavour neutrino oscillation picture, a combined analy- sis of the electron-neutrino appearance results of T2K and another long-baseline accelerator neutrino experiment, MINOS, is presented. Combining the two re- sults with the Feldman-Cousins method results in sin² 2θ₁₃ = 0 being excluded at 2.7σ, assuming the normal neutrino mass hierarchy.

Thesis (M. Sc.)--Carleton University, 1995. / Also available in electronic format on the Internet.

Neutrinos are the ideal cosmic messengers, and can be used to explore the most powerful accelerators in the Universe, in particular the mechanisms for producing and accelerating cosmic rays to incredible energies. By studying clustering of neutrino candidate events in the IceCube detector we can discover sites of hadronic acceleration. We present results on searches for point-like sources of astrophysical neutrinos located in the Southern hemisphere, at energies between 100 GeV and a few TeV. The data were collected during the first year of the completed 86-string detector, corresponding to a detector livetime of 329 days. The event selection focuses on identifying events starting inside the instrumented volume, utilizing several advanced veto techniques, successfully reducing the large background of atmospheric muons. An unbinned maximum likelihood method is used to search for clustering of neutrino-like events. We perform a search in the full Southern hemisphere and a dedicated search using a catalog of 96 pre-defined known gamma-ray emitting sources seen in ground-based telescopes. No evidence of neutrino emission from point-like sources is found. The hottest spot is located at R.A. 305.2° and Dec. -8.5°, with a post-trial p-value of 88.1%. The most significant source in the a priori list is QSO 2022-077 with a post-trial p-value of 14.8%. In the absence of evidence for a signal, we calculate upper limits on the flux of muon-neutrinos for a range of spectra. For an unbroken E-2 neutrino spectrum, the observed limits are between 2.8 and 9.4×10-10 TeV cm-2 s-1, while for an E-2 neutrino spectrum with an exponential cut-off at 10 TeV, the observed limits are between 0.6 and 3.6×10-9 TeV cm-2 s-1. / IceCube

L'expérience Double Chooz a pour but de mesurer l'angle de mélange leptonique θ₁₃ avec précision. Pour ce faire, l'expérience met à profit deux détecteurs identiques -- remplis de liquide scintillant dopé au gadolinium -- afin d'étudier les $antinue$ produits par les deux réacteurs nucléaires de 4.25GWth de la centrale de Chooz. Le détecteur lointain -- situé à une distance moyenne de $SI{1050}{m}$ des coeurs -- prend des données depuis Avril 2011. Le détecteur proche -- à une distance moyenne de 400m des coeurs -- observe les réacteurs depuis Décembre 2014. Le paramètre de mélange θ₁₃ conduit à la disparition d'$antinue$, lorsque ceux-ci voyagent des coeurs jusqu'aux sites de détection; la dépendance en énergie de ce déficit permet d'extraire la valeur de sin² 2θ₁₃, par ajustement. Les systématiques de détection, ainsi que l'incertitude sur la prédiction du flux $antinue$, sont formidablement réduites par le biais de corrélations entre les détecteurs et la configuration iso-flux du site. Par conséquent, la précision relative à la mesure de θ₁₃ est dominée par les incertitudes sur les bruits de fond et sur la normalisation relative des taux d'$antinue$. Le bruit de fond principal provient de la désintégration des émetteurs βn -- produits par spallation des muons -- dans le détecteur lui-même. Les spectres de ces isotopes cosmogéniques ont été simulés et complétés par un traitement d'erreur rigoureux. Ces prédictions sont en bon accord avec les données, elles-mêmes extraites à l'aide d'un veto actif, dont la performance a été étudiée pour les deux sites. Le taux d'évènements cosmogéniques restant parmi les candidats $antinue$ a également été estimé. En outre, la normalisation relative du taux d'$antinue$, liée aux nombres de protons dans les cibles de chaque détecteur, a été évaluée. Tous ces travaux se sont inscrits au sein des premières analyses Double Chooz à l'aide de deux détecteurs, produisant sin² (2θ₁₃) = 0.111±0.018. / The Double Chooz experiment aims at accurately measuring the value of the leptonic mixing angle θ₁₃. To this intent, the experiment makes the most of two identical detectors -- filled with gadolinium-loaded liquid scintillator -- observing $antinue$'s released by the two 4.25GWth nuclear reactors of the French Chooz power plant. The so-called "far detector" -- located at an average distance of 1050m from the two nuclear cores -- has been taking data since April 2011. The "near detector" -- at an average distance of 400}m from the cores -- has monitored the reactor since December 2014. The θ₁₃ mixing parameter leads to an energy dependent disappearance of $antinue$'s as they propagate from the nuclear cores to the detection sites, which allows for a fit of the sin² 2θ₁₃ value. By reason of correlations between the detectors and an iso-flux site layout, the detection systematics and the $antinue$ flux uncertainty on the θ₁₃ measurement are dramatically suppressed. In consequence, the precision of the θ₁₃ measurement is dominated by the uncertainty on the backgrounds and the relative normalisation of the $antinue$-rates. The main background originates from the decay of βn-emitters -- generated by $mu$-spallation -- within the detector itself. The energy spectra of these cosmogenic isotopes have been simulated and complemented by a diligent error treatment. These predictions have been successfully compared to the corresponding data spectra, extracted by means of an active veto, whose performance has been studied at both sites. The rate of cosmogenic background remaining within the $antinue$ candidates has also been assessed. Addtionally, the normalisation of the $antinue$ rates, bound to the number of target protons within each detector, has been evaluated. All these works were part of the first Double Chooz multi-detector results, yielding sin² (2θ₁₃) = 0.111±0.018.

NOvA is a two detector long-baseline neutrino oscillation experiment using Fermilab's 700 kW NuMI neutrino beam. It studies the disappearance of muon (anti)neutrinos and the appearance of electron (anti)neutrinos in the beam over a distance of 810 km be- tween the detectors. This thesis presents the latest 2020 update of the NOvA neutrino oscillation analysis within the standard model of three neutrinos mixing. With about +50% new data in NuMI neutrino mode (+22% of the total available data) and nu- merous analysis upgrades compared to the previously reported results, the experiment has made over 4σ-significant observation of electron antineutrino appearance in muon antineutrino beam and constrained the oscillation parameters |∆m2 32|, sin2 θ23 and δCP. The text closely depicts the analysis and all its novelties and changes. A detailed inspec- tion is dedicated to the systematic uncertainties and their estimation and validation. Although statistical uncertainties currently dominate in these measurements, under- standing the major sources of systematic uncertainties and their correlations is vital for both the interpretation and precision of the results and for further improvements of the analysis. 1

Une revue générale est faite de la situation expérimentale concernant la mesure des masses, du mélange et des oscillations de neutrinos, ainsi que de la phénoménologie sous-jacente. Mes contributions à travers le programme expérimental du CERN, CHARM-II, CHORUS et OPERA, font l'objet d'un développement particulier. Le dernier chapitre tente de mettre en perspective le programme expérimental à moyen et long terme. The experimental situation concerning the measurements of neutrinos masses, mixing and oscillation is reviewed, as well as of the underlying phenomenology. A particular attention is given to the CERN experimental program to which I took or take part, CHARM-II, CHORUS and OPERA. In the last chapter, I try to put into perspective the medium and long term experimental programme.

Neutrino telescopes open a new observational window on the universe. Neutrino interactions in these detectors can give rise to a combination of electromagnetic cascades, hadronic cascades and long range muons. Cerenkov radiation from these products is detected by the neutrino telescope. In this thesis the observational signatures associated with various neutrino-nucleon interaction products are investigated. Cerenkov radiation is emitted at a distinctive angle, about 40o in ice. The maximum number of optical photons that can be produced per unit charged tracklength is calculated to be 562 photons cm−1. The simulation programs Pythia and GEANT are used to study neutrino interactions using ice as the medium. The production of tau from the tau neutrino interaction is considered and it is found that the Cerenkov angle from tau is not distinctive at low energies, due to its lifetime tau decays before travelling an observable distance. The energy required for a tau neutrino to produce a sharp tau Cerenkov signal is on the order of 1 PeV. In a high energy electron neutrino interaction the resulting hadronic cascade contains high energy pions and kaons. These particles decay, often producing muons that are also high energy and therefore long range. Due to the muons travelling faster than the local speed of light in ice, their signal may be received by the detector earlier than the signal resulting from the event that created the muon. This can complicate the reconstruction of electron neutrino events.

Large-scale neutrino telescopes will be powerful tools to observe multitude of mysterious phenomena happening in the Universe. The dark matter puzzle is listed as one of them. In this study, indirect detection of dark matter via neutrino signals is presented. The upward muon, the contained muon and the hadronic shower fluxes are calculated, assuming annihilation/decay of the dark matter in the core of the astrophysical objects and in the Galactic center. Direct neutrino production and secondary neutrino production from the decay of Standard Model particles produced in the annihilation/decay of dark matter are studied. The results are contrasted to the ones previously obtained in the literature, illustrating the importance of properly treating muon propagation and energy loss for the upward muon flux. The dependence of the dark matter signals on the density profile, the dark matter mass and the detector threshold are discussed. Different dark matter models (gravitino, Kaluza-Klein and leptophilic) which can account for recent observations of some indirect searches are analyzed regarding their detection in the kilometer size neutrino detectors in the near future. Muon and shower rates and the minimum observation times in order to reach 2σ detection significance are evaluated, with the result suggesting that the optimum cone half angles chosen about the Galactic center are about 10° (50°) for the muon (shower) events. A detailed analysis shows that for the annihilating dark matter models such as the leptophilic and Kaluza-Klein models, upward and contained muon as well as showers yield promising signals for dark matter detection in just a few years of observation, whereas for decaying dark matter models, the same observation times can only be reached with showers. The analytical results for the final fluxes are also obtained as well as parametric forms for the muon and shower fluxes for the dark matter models considered in this study.

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