Thesis

Search for Majorana neutrinos in the SNO+ experiment through the identification of backgrounds from radioactive decays

• Call:

  IDPASC Portugal - PHD Programme 2017

• Academic Year:

  2017 / 2018

• Domain:

  Experimental Particle Physics

• Supervisor:

  Gersende Prior

• Co-Supervisor:

  José Maneira

• Institution:

  Faculdade de Ciências - Universidade de Lisboa

• Host Institution:

  Laboratório de Instrumentação e Física Experimental de Partículas

• Abstract:

  SNO+ is a large volume neutrino physics experiment located in the SNOLAB underground laboratory in Canada. It will replace the heavy water target of the Sudbury Neutrino Observatory (SNO) experiment by liquid scintillator, providing sensitivity to several new low energy neutrino physics measurements. One of the main goals of neutrino physics is the search for the process of neutrino-less double-beta decay (0NDBD). If discovered, it would prove that neutrinos are Majorana particles – their own antiparticles – and would allow the measurement of the absolute scale of neutrino masses. SNO+ will use the advantages of a large mass and very low background detector to search for this process by loading large quantities of Tellurium in the liquid scintillator. During all the phases, SNO+ will also detect anti-neutrinos from nuclear reactors and from the Earth's natural radioactivity, as well as galactic Supernova neutrinos. The detector is now under commissioning with water as a detection medium and data are being currently taken as of May 2017. This phase will be followed by a commissioning run period with pure liquid scintillator excepted to start at the end of this year, before the Tellurium-loaded phase. The LIP group participates in SNO+ since the beginning in several topics of physics simulation and analysis and is responsible for several aspects of the calibration system – PMT and scintillator optical calibration, source insertion mechanism. The broad scope project's goals are to obtain the first neutrinoless double-beta decay limits with SNO+. The quality of the measurement is crucially dependent on achieving the lowest possible backgrounds, either by removing contaminants from the scintillator mixture, or through data analysis techniques. The work plan for this project will target the identification of two types of backgrounds to the neutrinoless double beta decay signal: external backgrounds and internal U/Th decays coincidence background. External backgrounds are produced in regions outside of the liquid scintillator volume (e.g. PMTs, H2O) but propagate into it, while internal background occurs within the liquid scintillator volume. A measurement of the external backgrounds in the water phase will serve as input to data extraction methods. Internal background from U/Th daughters should be identified with high efficiency (> 99.99%) to resolve the neutrinoless double beta decay signal. In order to achieve this goal, a fully efficient algorithm using time-window and spatial cuts will be designed and tested using Monte-Carlo. The full performance of the method will be validated with liquid scintillator data when available. Finally, in order to extract the double-beta decay half-life (and corresponding effective neutrino mass) limit, the knowledge of the backgrounds obtained previously, and models of the detectors response obtained by calibration, will be combined in the full data analysis. This will be carried out with maximum likelihood or multi-variate analysis methods that will use as inputs the reconstructed energy, position and particle identification information, in addition to constraints from calibration and independent background analyses. The thesis will focus on the development and full validation of an original algorithm in order to extract the 0NDBD signal. This project will include the analysis of simulated data as well as real data from the scintillator and Te-loaded scintillator phases. The work plan also includes stays at SNOLAB for the participation to the detector commissioning, calibration and physics data-taking.