In this thesis, I present the theory of neutrinoless double-beta decay (0 2 ), particularly the theory of exotic modes of 0 2 involving the emission of one or two Majorons. Alongside this, I summarise the most recent results in the experimental search for 0 2 , including limits on the rate of these exotic processes. I describe the SNO+ experiment and it's physics goals, which include the search for 0 2 . As part of the SNO+ collaboration, I have made a signi cant contribution towards the development of the data quality software that is essential for ensuring SNO+ can achieve its physics goals-including in 0 2 searches. I describe how I developed a software package that performs the high-level data quality checks. Continuing with the software theme, I then describe a python-based limit-setting and tting software package called echidna. I have been a lead developer of echidna as part of my PhD, so I describe the software in detail and how it can be used to set limits on 0 2 signals. By reproducing the sensitivity results of the KamLAND-Zen experiment, in four key Majoronemitting 0 2 modes, I verify the use of echidna as a limit-setting tool for this type of search. Finally, I present the results of a comprehensive sensitivity study, where I determine the potential sensitivity of SNO+ to the same set of Majoron-emitting modes that KamLAND-Zen and other 0 2 experiments have already investigated.

A systematic study of the change in proton single-particle occupancies in two neutrinoless double beta decay candidates, 130Te to 130Xe and 136Xe to 136Ba, has been performed. Final states in 129Sb, 129I, 135I and 135Cs have been populated using the (d,3He) single proton removal reaction. The deuterons were accelerated to 101 MeV using the coupled azimuthally varying field and Ring cyclotrons at the Research Center for Nuclear Physics, University of Osaka. The outgoing ejectiles were momentum analysed using the Grand Raiden magnetic spectrometer. Absolute cross sections were measured for states up to 7 MeV in excitation. Transferred angular momenta were identified through a comparison of angular distributions with those calculated using the distorted-wave Born approximation. Spectroscopic factors were extracted from the experimental cross sections. In addition to the 0v2B decay candidates the reaction was also performed on 128Te, 132Xe, 134Xe and 138Ba as a consistency check. The occupancies of the nuclei were determined from the spectroscopic factors, the reaction model was normalised using a common normalisation factor across all targets. The change in occupancies between the 0v2B decay candidates and their daughters were then determined. The change in occupancies were then compared with those calculated by interacting shell model, interacting boson model and quasiparticle random phase approximation. This comparison showed that whilst the three theories were qualitatively able to reproduce the change in occupancies, quantitatively there are significant discrepancies. These are the same models that are used to determine the nuclear matrix elements governing the rate of 0v2B decay.

The third and final phase of the Sudbury Neutrino Observatory (SNO) experiment utilized a series of 3He proportional counters called Neutral Current Detectors (NCDs) to detect the neutrons produced by the neutral current interactions of solar neutrinos in the detector. The number of neutrons detected by the NCDs, and hence the total flux of 8B solar neutrinos, has been determined using two novel signal extraction techniques which were designed to be robust against potential unexpected behaviour in the NCD background. These techniques yield total 8B solar neutrino flux measurements of 5.04(+0.42-0.40(stat))(+/-0.28(syst))x10E6/cm2/s and (4.40 - 6.43)x10E6/cm2/s, which are in good agreement with previous SNO results and with solar model predictions, and which confirm that previous NCD analyses were not unduly affected by unexpected background behaviour. The majority of the hardware from the now-completed SNO experiment will be reused to create a new liquid scintillator based neutrino experiment called SNO+. An important part of the SNO+ physics program will be a search for neutrinoless double beta decay, carried out by dissolving 150Nd into the scintillator. The sensitivity of the SNO+ experiment to neutrinoless double beta decay has been evaluated. If loaded at 0.1% (w/w) with natural neodymium, after 1 kTa of data taking SNO+ would have a 90%C.L. sensitivity equivalent to a neutrinoless double beta decay half life of 8.0x10E24a or better 50% of the time; if the experiment were run with neodymium enriched to 50% in 150Nd this limit improves to 57x10E24a. Under a reasonable choice for the 150Nd neutrinoless double beta decay matrix element, these half lives correspond to upper limits on the effective Majorana neutrino mass of 112 meV and 42 meV, respectively. These limits are competitive with those expected from all other near-term neutrinoless double beta decay experiments. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2009-09-10 21:07:00.25

京都大学 / 新制・課程博士 / 博士(理学) / 甲第25115号 / 理博第5022号 / 新制||理||1716(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)教授 中家 剛, 教授 田島 治, 准教授 WENDELLRoger / 学位規則第4条第1項該当 / Doctor of Agricultural Science / Kyoto University / DFAM

SNO+ is a liquid scintillator experiment whose physics goals include measurements of solar neutrinos, reactor anti-neutrinos, geo neutrinos and double beta decay. During an initial water phase, it will also search for invisible modes of nucleon decay. This thesis investigates methods of improving the detector's sensitivity to the baryon and lepton violating processes of neutrinoless double beta decay and invisible nucleon decay. It does this through an improved scintillator model, allowing the sensitivity of the detector with different loading techniques to be evaluated, through a new background rejection technique, capable of increasing the active volume of the detector, and with the development of improved position fitters, achieving resolutions of approximately 10 cm in scintillator and 25 cm in water. The sensitivity of SNO+ to invisible modes of nucleon decay is explored, predicting, after one month of data, a limit of t > 1.38 x 10³⁰ years on the decay of neutrons and of t > 1.57 x 10³⁰ years on the decay of protons.

This Master thesis aims at designing, assembling and operating a prototypal luminescentbolometer containing a candidate with high Q-value (116Cd and 100Mo)for the study of the neutrinoless double- decay. The crystal is scintillating (with 116CdWO4 and Li2MoO4 compounds). The prototype is designed according to a simple thermal model and cooled down to 18 mK. Data analysis of the 116CdWO4 crystal determines the energy resolution (intrinsicand in the ROI) and the alpha/beta discrimination power. It includes a full interpretation of the background energy spectrum in terms of environmental radioactivity and an evaluation of the crystal radiopurity by the detection of internal contamination of the detector. An evaluation of the potential of a future experiment based on the 116CdWO4developed prototype is performed, ascertaining the feasibility of large scale experiments to search for neutrinoless double beta decay. The use of the thermal model of the detector response to interpret its bolometric behaviour and the study of future optimizations of the detector performance concludes this project.

While neutrino oscillations indicate that neutrino flavors mix and that neutrinos have mass, they do not supply information on the absolute mass scale of the three flavors of neutrinos. Currently, the only viable way to determine this mass scale is through the observation of the theoretically predicted process of neutrinoless double-beta decay (0νββ). This yet-to-be-observed decay process is speculated to occur in a handful of nuclei and has predicted half-lives greater than 10²⁵ years. Observation of 0νββ is the goal of several large-scale, multinational efforts and consists of detecting a sharp peak in the summed β energies at the Q-value of the reaction. An exceptional candidate for the observation of 0νββ is ⁷⁶Ge, which offers an excellent combination of capabilities and sensitivities, and two such collaborations, MAJORANA and GERDA, propose tonne-scale experiments that have already begun initial phases using a fraction of the material. The absolute scale of the neutrino masses hinges on a matrix element, which depends on the ground-state wave functions for both the parent (⁷⁶Ge) and daughter (⁷⁶Se) nuclei in the 0νββ decay and can only be calculated from nuclear structure models. Efforts to provide information on the applicability of these models have been undertaken at the University of Kentucky Accelerator Laboratory using gamma-ray spectroscopy following inelastic scattering reactions with monoenergetic, accelerator-produced fast neutrons. Information on new energy levels and transitions, spin and parity assignments, lifetimes, multipole mixing ratios, and transition probabilities have been determined for ⁷⁶Se, the daughter of ⁷⁶Ge 0νββ, up to 3.0 MeV. Additionally, inaccuracies in the accepted level schemes have been addressed. Observation of 0νββ requires precise knowledge of potential contributors to background within the region of interest, i.e., approximately 2039 keV for ⁷⁶Ge. In addition to backgrounds resulting from surrounding materials in the experimental setup, ⁷⁶Ge has a previously observed 3952-keV level with a de-exciting 2040-keV γ ray. This γ ray constitutes a potential background for 0νββ searches, if this level is excited. The cross sections for this level and, subsequently, for the 2040-keV γ ray has been determined in the range from 4 to 5 MeV.

L'observation de la désintégration double bêta sans émission de neutrino (0ν2β) fournirait des informations essentielles sur la nature du neutrino et son échelle de masse absolue. Ce processus consiste en la transformation simultanée de deux protons en deux neutrons avec l'émission de deux électrons et aucun neutrino. Cette transition n'est possible que si les neutrinos sont égaux aux antineutrinos (nature Majorana du neutrino). Les recherches pour une désintégration à ce point rare représentent un défi technique complexe, car les expériences de prochaine génération visent des sensibilités de l'ordre de 10^27-10^28 ans afin d'avoir un potentiel de découverte élevé. Cette thèse est focalisée sur les projets LUMINEU et CUPID-Mo, développant la technique des bolomètres scintillants pour la recherche de désintégration 0ν2β avec le radio-isotope 100Mo.Les bolomètres sont des détecteurs cryogéniques mesurant l'énergie des particules déposées via un changement de température dans l'absorbeur. Si des cristaux scintillants sont utilisés comme absorbeurs, les signaux lumineux peuvent être enregistrés avec un bolomètre auxiliaire, sensible à l'énergie totale déposée par les photons de scintillation. Une telle configuration permet de séparer les particules α des γ/β, en rejetant le fond le plus difficile. La technologie des bolomètres scintillants est décrite en détail comme une option pour une future expérience cryogénique à l'échelle d'une tonne, appelée CUPID, qui peut couvrir complètement la région de masses de neutrinos dans la hiérarchie inversée. / Neutrinoless double beta (0ν2β) decay is a process of great interest for neutrino physics: its observation would provide essential information on neutrino nature and its absolute mass scale. This process consists of the simultaneous transformation of two protons into two neutrons with the emission of two electrons and no neutrino, implying the violation of the total lepton number. Such transition is possible only if neutrinos are equal to antineutrinos (Majorana particles). The searches for such a rare decay are becoming a complicated technical challenge, as next generation of 0ν2β experiments aim at sensitivities of the order of half-life at 10^27-10^28 yr. This thesis is focused on LUMINEU and CUPID-Mo projects, developing the scintillating bolometers technique for 0ν2β decay search with 100Mo with Li2MoO4 crystals. Bolometers are cryogenic detectors measuring the deposited particle energy as a change of temperature in the absorber. The use of scintillating crystals allows to perform discrimination of α particles from γ/β ones due to different light output of these two particle types, rejecting the most challenging background. The scintillating bolometers technology is described in details as an option for a future ton-scale cryogenic experiment, named CUPID, which can completely cover the inverted hierarchy region of neutrino masses.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22627号 / 理博第4616号 / 新制||理||1663(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 市川 温子, 教授 中家 剛, 准教授 窪 秀利 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

>Magister Scientiae - MSc / Observation of neutrinoless double beta decay (0 ) is currently the only means by which one could establish the Majorana nature of neutrinos. Additionally, such an observation would determine the absolute neutrino mass scale. However, this requires that the matrix element for a given 0 decay process is accurately calculated. The objective of this project is to provide useful nuclear structure information that aim to improve future theoretical calculations for the nuclear matrix element (NME) of 136Xe 0 decay to 136Ba. We studied neutron pairing correlations in 134Ba using the 136Ba(p; t) reaction to stringently test the Bardeen-Cooper-Schrie er (BCS) approximation in the A = 136 mass region. This is because many theoretical calculations of the NME's for 0 decay are performed using the quasiparticle random phase approximation (QRPA), which uses the BCS approximation to describe the ground states of the even-even parent and daughter nuclei. Our results show a signi cant fragmentation of the neutron-pair transfer to excited 0+ states, implying a breakdown of the BCS approximation in this mass region.