Thesis

0υββ search in SNO+: calibration and optimization of event reconstruction

• Call:

  IDPASC Portugal - PHD Programme 2015

• Academic Year:

  2015 / 2016

• 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:

  SNO+ is a large volume neutrino physics experiment located in the SNOLAB underground laboratory in Canada. It will replace heavy water target of the Sudbury Neutrino Observatory (SNO) 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 upgrade of the SNO+ experiment (scintillator purification system, new acrylic vessel support, new calibration systems, etc..) is currently being completed at SNOLAB. A short phase of data taking with water is expected to start in the autumn of 2015. It will be followed by a commissioning run period with pure liquid scintillator before the Tellurium-loaded phase. The LIP group participates in SNO+ since the beginning in several topics of physics simulations 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 0NDBD limits with SNO+. The quality of the measurement is crucially dependent on achieving the lowest possible backgrounds. Some of the dominant backgrounds to the 0NDBD signal – Boron-8 solar neutrinos and the Te130 two-neutrino decay mode (rare, but already observed) – can only be reduced via an accurate reconstruction of the event's characteristics, especially the energy. In fact, while the 0NDBD signal is a peak at the Te130 Q-value of about 2.5 MeV, the solar neutrino spectrum in that region is almost flat, and the 2NDBD contribution is a decreasing tail from lower energies. So to improve the signal to background ratio, we need a narrow energy window and therefore a good energy resolution. This, in turn, depends on the light collection efficiency, that is affected by the scintillator optical properties and our knowledge of the detector optical model, that we measure through dedicated calibrations. The work plan for this thesis project will focus on the achievement of an accurate in-situ calibration of the SNO+ detector's optical properties, and on using those to optimize the event energy reconstruction, with the goal of narrowing the 0NDBD analysis window as much as possible. Due to the large dimensions of the detector (diameter of 17 m), the in-situ measurement of these properties is essential, and makes use of several optical sources, from uniform diffusers to narrow beams of laser light. We need to know how the scintillation light is produced, propagated and detected, namely the absorption of light in the different components of the liquid scintillator, on the acrylic vessel, and the details of reflection in the light guides surrounding the PMTs, that are known to degrade over time. The asymmetric distribution of opaque detector materials, such as support ropes, also affects that response. Initially, the focus will be on commissioning the calibration systems, especially the optical sources (the main one is a diffuser for N2-dye laser pulses) needed for energy response calibration, taking calibration data, and developing dedicated software for automated quality control. A second step is to analyse that data to obtain the optical calibration parameters, improving upon the existing methods. All these can be started during the initial water phase of SNO+. Later, the methods will need to be adapted to the scintillator phase. Once the optical calibration is done, the second part of the work will be to use that knowledge to improve the event reconstruction algorithms and narrow the energy resolution. Several possibilities can be explored, such as the use of PMT charge information, or the inclusion of measured asymmetries (in PMT response, acrylic attenuation) in the detector model. Possible additional goals can include the development of event reconstruction algorithms geared towards particle identification, which is a challenge in a liquid scintillator experiments. The distinction of broader energy deposits by gamma events, or the small component of directional Cherenkov light in solar neutrino events, could all contribute further to improve the SNO+ 0NDBD sensitivity. This project will include the analysis of simulated data as well as real data from the water and scintillator phases. The work plan also includes stays at SNOLAB for the participation to the detector commissioning, calibration and physics data-taking.