This thesis presents a study of neutral-current π⁰ production from <i>ν_µ</i> interactions. The study uses data taken at the Tokai-to-Kamiokande off-axis experiment's near detector (ND280). Monte Carlo studies were used to develop selection criteria to reconstruct event signatures produced from such interactions. This physics analysis focuses on the tracker region of the ND280 subdetector. Other π⁰ analyses using different detector regions are either underway or completed. The physics analysis, after being developed from Monte Carlo studies, was then run on real data. This analysis goes partway towards measuring the neutral-current π⁰-producing neutrino interaction cross-section, by measuring the rate of such interactions in the T2K experiment's near detector, ND280.

The Minimal Scale Invariant extension of the Standard Model (MSISM) is a model of low-energy particle physics which is identical to the Standard Model except for the inclusion of an additional complex singlet scalar and tree-level scale invariance. Scale invariance is a classical symmetry which is explicitly broken by quantum corrections whose interplay with the quartic couplings can be used to trigger electroweak symmetry breaking. The scale invariant Standard Model suffers from a number of problems, however the inclusion of a complex singlet scalar results in a perturbative and phenomenologically viable theory. We present a thorough and systematic investigation of the MSISM for a number of representative scenarios along two of its three classified types of flat direction. In these scenarios we determine the permitted quartic coupling parameter space, using both theoretical and experimental constraints, and apply these limits to make predictions of the scalar mass spectrum and the energy scale at which scale invariance is broken. We calculate the one-loop effective potential and the one-loop beta functions of the pertinent couplings of the MSISM specifically for this purpose. We also discuss the phenomenological implications of these scenarios, in particular, whether they realise explicit or spontaneous CP violation, contain neutrino masses or provide dark matter candidates. Of particular importance is the discovery of a new minimal scale invariant model which provides maximal spontaneous CP violation, can naturally incorporate neutrino masses, produces a massive stable scalar dark matter candidate and can remain perturbative up to the Planck scale. It can be argued that the last property, along with the classical scale invariance, can potentially solve the gauge hierarchy problem for this model.

The presence of dark matter is inferred at scales ranging from rotations of galaxies to imprints in the CMB – the Big Bang after-glow. The nature of dark matter is, however, still unknown as no detection other than the gravitational one has been made. This thesis presents two analyses searching for a neutrino signal from dark matter annihilations in the Milky Way. The first analysis searched for an excess of νμ charged current events with directions from the central region of the dark matter halo and, was focused on low energy events, thus probing low dark matter particle masses. Approximately 319 days of data collected with the 79-string configuration of the IceCube detector was used in the analysis. Despite a large deficit in the number of observed events the data were found to be consistent with background and upper limits were set on &lt;σⱴ&gt;. At the time of the analysis these limits were the strongest set by a neutrino experiment below 100 GeV. The second analysis was performed on a data sample originally used in an unfolding analysis of the atmospheric and astrophysical neutrino spectra. The data consisted of contained cascade events above 1 TeV collected with the 79-string configuration and the completed detector in the 86-string configuration during two years of data-taking. The limits set by this analysis were more constraining by up to a factor of 10 compared to previous IceCube analyses, and the most competitive limits are set assuming a Burkert halo profile. These two analyses prompted the development of a signal subtraction likelihood method to address the problem of signal contamination in background estimates based on scrambled data. Additionally a study concerning future extensions of IceCube in the Gen2 project is presented. The cascade reconstruction performance was examined and compared for different proposed detector extensions.

En los últimos años los telescopios submarinos de neutrinos han cobrado una mayor importancia ya que consisten en un nuevo y único instrumento para observar el Universo. Los neutrinos son partículas sin carga e interactúan muy débilmente con la materia que les rodean, pueden escaparse fácilmente de la fuente que los ha producidos y llegar a La Tierra sin ser desviada por los campo magnético y sin interactuar con otras partículas. Esto implica que los neutrinos pueden traer informaciones astrofísicas que otros mensajeros no pueden aportar y abrir una potencial ventana hacia el Universo. Por otro lado, su baja interacción con la materia impone la necesidad de construir un detector de grandes dimensiones del orden de 1 km3 utilizando volumen de agua o hielo y con muchos sensores ópticos para detectar esta interacción de neutrino de alta energía. Un método para detectar neutrinos es a través de la luz Cherenkov emitida por el muon generado después de una interacción de neutrino. Esta partícula, al atravesar el detector con una velocidad superior a la luz en el medio, genera una débil luz azulada llamada radiación de Cherenkov que es detectada por una red de sensores ópticos (fotomultiplicadores). El tiempo de llegada de la luz a los fotomultiplicadores puede ser utilizado para reconstruir la traza del muon y consecuentemente del neutrino que lo ha producido. La precisión en la reconstrucción de la traza del muon depende de la precisión en la medida del tiempo de llegada de la luz y en la precisión en de la posición de los sensores ópticos en el detector. Por esta razón, en telescopios submarinos es necesario un sistema de posicionamiento acústico (APS) capaz de monitorizar el movimiento de los sensores ópticos con una precisión de ~10 cm. Los estudios realizados están enmarcados dentro de las actividades de calibración de posicionamiento acústico en dos colaboraciones europeas para el diseño, construcción y operación de telescopios submarinos de neutrinos en el Mediter / Larosa ., G. (2012). Design and Development of an acoustic positioning system for a cubic kilometre underwater neutrino telescope [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/16877 / Palancia

京都大学 / 新制・課程博士 / 博士(理学) / 甲第22993号 / 理博第4670号 / 新制||理||1670(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)准教授 吉岡 興一, 教授 田中 貴浩, 准教授 髙山 史宏 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

An overwhelming majority of matter in the Universe is dark matter, a substance unlike anything we know. Detecting dark matter particles requires ruling out observed phenomena caused by known particles. This thesis advances efforts toward the detection of dark matter using one of the most sensitive particle detection technologies: the dual-phase liquid xenon time projection chamber. Specifically, data from the XENON1T Experiment, located in Italy, and the Purdue small-scale ASTERiX detector are analyzed. A background of Lead-214 beta decay events can be mitigated by tracing the radioactive Radon-222 decay chain in XENON1T. However, a preliminary reduction of background has a high cost to exposure. Research on several topics was conducted with Purdue undergraduates, including a search for dark matter particles up to the Planck Mass, characterizing backgrounds due to muons, and searching for Boron-8 solar neutrino signals. XENON1T single-scatter dark matter limits were extended to a particle mass of 10¹⁸GeV/c². The ASTERiX detector was modified to characterize a significant background to the smallest detectable energy signatures: single- and few-electron ionization signals. Infrared light was determined to be ineffective at reducing this background, and their rates were observed to decrease inversely with time since an energetic interaction according to a power law. The rates of single- and few- electron backgrounds increase linearly with increased applied extraction fields and increased depth of the initial interaction in the detector. These results indicate that these backgrounds originate at the liquid-gas interface of dual-phase detectors. In exploring a single-photon threshold for initial scintillation signals, a previously unconsidered background of large dark count signals in the photosensors became apparent. The high background of small ionization signals and large dark count signals deterred a search for Boron-8 solar neutrino interactions in XENON1T. These studies are vital to mitigating backgrounds and improving the sensitivity of liquid xenon time projection chambers to new physical phenomena.

Failed supernovae are the implosive final fates of massive stars, where ablack hole is formed. During the collapse, the proto-neutron star emits a huge number of neutrinos, and when the black hole is finally formed, it engulfs theneutrino-emitting material and the signal is cut off. Inspired by the recent work of Nagakura &amp; Hotokezaka (2020), this thesis improves on some parts of theirs imulation work and further explores the neutrino signal from failed supernovae, using a supercomputer to perform Monte Carlo simulations. In particular, we realized the neutrino flux’ time evolution around black hole formation hasn’t previously been studied well, and so it is investigated here, as well as the plausibility of measuring the black hole mass through the shape of the decay. A new component of the signal is presented, an echo of neutrinos emitted before black hole formation that, due to scattering in supersonic material around the black hole, arrive with a time delay of up to 15 ms, and with a significantly higher average energy, for heavy lepton neutrinos around 50 MeV.

CUORE est la plus grande expérience qui recherche la double désintégration bêta sans neutrino avec des bolomètres de TeO₂. La découverte de cette transition nucléaire aurait des conséquences décisives sur la scène actuelle de la physique. Les questions suivantes trouveraient une réponse : pourquoi la matière est-elle dominante dans l’Univers? Quelle est la masse du neutrino? Le neutrino est il un particule de Majorana ou de Dirac? Ce travail présente deux approches différentes pour l’amélioration de la sensibilité de CUORE en vue de sa prochaine phase : CUPID. Dans la première partie de ce travail, une étude du modèle thermique pour les bolomètres équipés avec des NTDs est présentée dans le but de mieux comprendre la réponse des détecteurs de CUORE. Les bolomètres sont des détecteurs extraordinaires utilisés pour un grand nombre d’applications en raison de leurs performances remarquables, mais leur modélisation et leur simulation sont loin d’être complètement comprises. Deux mesures ont été effectuées pour évaluer expérimentalement deux paramètres du modèle thermique : la conductance de la colle et celle entre les électrons et les phonons. Dans la deuxième partie de ce travail, la possibilité de détecter la faible lumière Cherenkov émise par le TeO₂ est étudiée à fin de rejeter des événements alpha, le fond principal de l’expérience CUORE. Le défi consiste dans la détection d’un signal de lumière de 100 eV à moyen d’un détecteur équipé avec un NTD qui a normalement un bruit de l’ordre de 100 eV. Cette question peut être résolue grâce à l’effet Neganov-Trofimov-Luke (NTL) qui a permis de baisser le seuil du détecteur de lumière et d'améliorer son rapport signal-sur-bruit. Cet effet exploite la présence d’un champ électrique pour amplifier les signaux thermiques des bolomètres. Le rejet complet du fond alpha a été prouvé avec un photo-bolomètre amélioré par l’effet NTL et couplé à un bolomètre de TeO₂ comme ceux utilisés par CUORE. Une solution convaincante pour le rejet de fond alpha a été démontrée en vue de l’expérience CUPID. / CUORE is the first tonne-scale experiment searching for the neutrinoless double beta decay with TeO₂ bolometers. The discovery of this nuclear transitionwould have decisive consequences on the present physics scene. The following questions would find an answer: why is matter dominant in the Universe? which is the neutrino mass? has the neutrino a Majorana or a Dirac nature? This work presents two different approaches for the enhancement of the CUORE sensitivity with a view to its upgrade: the CUPID experiment. In the first part, a study of the thermal model describing NTD-based bolometers is presented with the objective to achieve a better comprehension of the response of the CUORE detectors. Bolometers are amazing detectors used for a large number of applications because of their impressive high performance, but their modelisation and simulation is far to be completely understood. Two measurements have been performed for an experimental evaluation of two thermal-model parameters: the glue and the electron-phonon conductances. In the second part, the possibility to detect the tiny Cherenkov light emitted by TeO₂ to reject alpha events — the main background of the CUORE experiment — is studied. The challenge consists in the detection of a 100-eV light signal with a NTD-based light detector that usually is characterised by a baseline noise of the order of 100 eV. This issue is solved with the employment of the Neganov-Trofimov-Luke (NTL) effect to lower the energy threshold of the light detector and improve its signal-to-noise ratio. This effect exploits the presence of an electric field to amplify bolometric thermal signals. The full rejection of the alpha background has been proved with one NTL assisted photo-bolometer coupled to a CUORE-size TeO₂ bolometer. A convincing solution for the alpha background rejection has been demonstrated with a view to the CUPID experiment.

The question of the absolute mass scale of neutrinos is of particular interest for particle physics, astrophysics, and cosmology. The KATRIN experiment (KArlsruhe TRItium Neutrino experiment) aims to address the effective electron antineutrino mass from the shape of the tritium β-spectrum with an unprecedented sensitivity of 0.2 eV/c2 . One of the major systematic effects concerns the experimental energy scale, which has to be stable at the level of only a few parts in a million. For its calibration and monitoring the monoener- getic electrons emitted in the internal conversion of γ-transition of the metastable isotope 83m Kr will be extensively applied. The aim of this thesis is to address the problem of KA- TRIN energy scale distortions and its monitoring in detail. The source of electrons based on 83m Kr embedded in a solid as well as the source based on gaseous 83m Kr are studied. Based on the experimental results an approach for the continuous stability monitoring is proposed. 1

Neutrinoless Double Beta Decay(0𝜈𝛽𝛽) is one of the major research interests in neutrino physics. The discovery of 0𝜈𝛽𝛽 would answer persistent puzzles in the Standard Model of Elementary Particles. KamLAND-Zen is one of the leading efforts in the search of 0𝛽𝛽 and has acquired data from 745 kg of ^{136}Xe over 224 live-days. This data is analyzed using a Bayesian approach consisting of a Markov Chain Monte Carlo (MCMC) algorithm. The implementation of the Bayesian analysis, which is the focal point of this dissertation, yields a 90\% Credible Interval at T^{0𝜈}_{1/2} = 7.03 × 10^{25} years. Finally, a machine learning event classification algorithm, based on a spherical convolutional neural network (spherical CNN) was developed to increase the T^{0𝜈}_{1/2} sensitivity. The classification power of this algorithm was demonstrated on a Monte Carlo detector simulation, and a data driven classifier was trained to reject crucial backgrounds in the 0𝜈𝛽𝛽 analysis. After implementing the spherical CNN, an increase in T^{0𝜈}_{1/2} sensitivity of 11.0% is predicted. These early studies pave the way for substantial improvements in future 0𝜈𝛽𝛽 analyses.