Topological and time based event classification for neutrinoless double beta decay in liquid scintillator

Thesis

The SNO+ experiment is the successor to the SNO neutrino detector, which replaces its heavy water target with a liquid scintillator one. The primary physics goal is the search for neutrinoless double beta decay (0vββ) in ¹³⁰Te, which will be loaded into the scintillator. Fitted with > 9300 photo-multiplier tubes, the SNO+ detector will have the highest photo-cathode coverage of any large liquid scintillator detector. This thesis shows that, at this light collection level, SNO+ is sensitive to differences in the scintillation pulses produced by electrons, positrons and gammas, and that these differences may be used to classify single-site 0vββ events and multi-site radioactive backgrounds which emit γ. This pulse shape discrimination technique (PSD) is applied to background events from radiation originating outside the detector, which limit the experiment's fiducial volume, and potential internal radioactive decays, like ⁶⁰Co, which are otherwise difficult to distinguish from 0vββ. A new signal extraction framework is described and used to perform 2D fits in energy and event radius, which estimate an expected limit on the 0vββ half-life of T^0v_1/2 1.76 x 10²⁶ yr, at 90% confidence, assuming an exposure of 4.0 tonne yr of ¹³⁰Te. The corresponding limit on the effective Majorana mass is m^ββ < 49.7meV, using the IBM-2 nuclear model. Further, it is shown that adding PSD as an additional fit dimension can reduce the SNO+ 3 sigma discovery level on m^ββ from 190meV to 91 meV, assuming the same exposure. The final portion of this work discusses what more could be achieved using a liquid scintillator experiment which can separate scintillation and Cherenkov signals in time. A simulation of a SNO+ style detector, filled with a slow scintillator and equipped with a high coverage of fast, high quantum efficiency PMTs is used to demonstrate separation of Cherenkov and scintillation signals and reconstruction algorithms for electron and 0vββ events are described. Differences in Cherenkov signals are used to distinguish 0vββ from the solar neutrino elastic scattering background, and to demonstrate for the first time that, in principle, the 0vββ mechanism may be determined in liquid scintillator by fitting the angular separation and energy split of the two emitted electrons.

Authors: Dunger, J

• DOI: 10.5287/ora-z6zypyxz1
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Neutrinoless double beta decay; Liquid Scintillator; Particle identification
• Subjects: Neutrinos