Thesis

Search for neutrinoless double-beta decay in SNO+

• Call:

  IDPASC Portugal - PHD Programme 2014

• Academic Year:

  2014 /2015

• Domain:

  Experimental Particle Physics

• Supervisor:

  José Maneira

• Co-Supervisor:

  Gersende Prior

• Institution:

  Faculdade de Ciências - Universidade de Lisboa

• Host Institution:

  Laboratório de Instrumentação e Física Experimental de Partículas

• Abstract:

  The Sudbury Neutrino Observatory (SNO) is a large volume neutrino detector located in the SNOLAB underground laboratory in Canada. SNO has demonstrated that solar neutrinos do change flavor and thus have a small, but non-zero, mass. The SNO+ experiment will replace SNO's heavy water target by liquid scintillator, providing sensitivity to several new low energy neutrino physics measurements. If it exists, the rare process of neutrino-less double-beta decay 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 upgrade of the SNO+ experiment (scintillator purification system, new acrylic vessel support, new calibration systems, etc..) is currently being completed at SNOLAB, in Canada. Data taking is expected to start in late 2015. A short commissioning run period with pure liquid scintillator will be followed by the Tellurium-loaded phase. In later phases – with pure liquid scintillator again – SNO+ will also be able to measure several components of the solar neutrino spectrum. The LIP group is responsible for several aspects of the calibration system – PMT and scintillator optical calibration, source insertion mechanism – as well as the anti-neutrino analysis. The broad scope project's goals are to obtain the first neutrinoless double-beta decay limits (or positive measurement) 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 the use of short-lived isotope background events identification methods. The workplan for this project will target this aspect and focus on the development of coincidence techniques. The goal of high sensitivity demands a high efficiency > 99.99% to reconstruct these backgrounds. In order to achieve this goal, the tagging efficiency of bismuth-polonium coincidence events from the natural U/Th decay chains has to be fully determined. Time-window and spatial cuts will be tested using Monte-Carlo simulation and the full performance of the method will be validated with data when available. A measurement of the tagging inefficiencies due to the detector dead-time will also be performed and serve as input to the measurement of the total background from the U/Th chain seen in the detector. 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 methods that will use the reconstructed energy, position and particle identification information, in addition to constraints from calibration and independent background analyses. This project will include simulation and analysis of simulation data as well as the water and scintillator data. In the early stages of the thesis, a participation to the in-situ activities in SNOLAB or on the commissioning of the calibration system may also be required.