Thesis

Measurement of Te130 two-neutrino double beta decay half-life with the SNO+ experiment

• Call:

  IDPASC Portugal - PHD Programme 2017

• Academic Year:

  2017 / 2018

• Domains:

  Experimental Particle Physics | Astroparticle 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:

  This PhD research plan aims at the measurement of the two-neutrino double-beta decay (2NDBD) half-life of Te130 with the SNO+ experiment at SNOLAB. This measurement is a crucial step in the search for the 0-neutrino double-beta decay (0NDBD) mode, a process that is sensitive to the absolute neutrino masses and the Majorana nature of neutrinos. Leptogenesis models for the explanation of the matter/antimatter asymmetry of the Universe typically require Majorana neutrinos and the consequent violation of lepton number. 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 is the search for 0NDBD. SNO+ will use the advantages of a large mass and very-low background detector to search for this process by loading large quantities - 3900 kg - of Tellurium (containing 34% of Te130) in the liquid scintillator. The relation between the 0NDBD half-life and the Majorana neutrino mass depends on nuclear matrix elements with large theoretical uncertainties, but these can be reduced by accurately measuring the related 2NDBD half-life. 2NDBD is a rare process giving a continuum spectrum with end-point energy at the decay's Q-value, the tail of which leaks in the energy region for the 0NDBD search, making the 2NDBD one of the dominant (and irreducible) backgrounds for SNO+. Therefore, a measurement of its half-life is also an additional constraint that improves the 0NDBD search sensitivity. After a commissioning period with water (ongoing) and unloaded scintillator (2018), the Te data taking phase of SNO+ is expected to start in 2019. So, in the time-scale of a 4-year plan starting in early 2018, and even including data-taking efficiency, it is expectable to have at least one year of physics data from the Te-loaded phase which, with an expected rate of about 1000 ev/kg/y in the fiducial volume, should provide enough statistics for a competitive measurement. The work plan will consist to a large extent in addressing the main challenges: optimization and thorough understanding of the detector optical model and the energy reconstruction performance, including linearity, uniformity and stability; in-situ determination of the rates of internal background rates, including Te cosmogenics, over the energy range of interest for Te130 2NDBD – 0.5 MeV to 2.5 MeV. Finally, using Monte Carlo simulations, those uncertainties will be propagated into the expected 2NDBD shapes and compared to data. 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.