Optimization of the energy measurement at the SNO+ experiment, towards the search for Majorana neutrino signals
IDPASC Portugal - PHD Programme 2016
2016 / 2017
Experimental Particle Physics | Astroparticle Physics
Faculdade de Ciências - Universidade de Lisboa
Laboratório de Instrumentação e Física Experimental de Partículas
Double beta decay (DBD), in which the nucleus emits two electrons alongside two (anti-)neutrinos, is a rare nuclear decay that has been observed in several nuclei, among which Te-130. The process of neutrino-less double beta decay (NLDBD), in which only two electrons are emitted by the nucleus, is possible only if the neutrinos can annihilate with each other and so is considered to be the most viable way to show whether neutrinos are Dirac or Majorana fermions. After the discovery of neutrino oscillations, this question remains unanswered. If neutrinos are Majorana, they are their own anti-particles and they can play a decisive role in the origin of the matter-antimatter asymmetry in the early Universe, through the process of leptogenesis. This thesis proposal addresses the experimental search for NLDBD with Te-130 in the SNO+ experiment. The SNO+ detector, located in the SNOLAB underground laboratory in Canada, will re-use the array of more than 9000 photomultipliers (PMTs) of the Sudbury Neutrino Observatory (SNO), but will replace the heavy water target by liquid scintillator, providing sensitivity to several new low energy neutrino physics measurements. SNO+ will use the advantages of a very large detector mass and very-low background detector to search for NLDBD by loading large quantities of Tellurium in the scintillator. A short commissioning phase of data taking with water is expected to start in late 2016, followed by a period with pure liquid scintillator. The Tellurium-loaded phase is scheduled to start in 2018 and last about 5 years. The LIP group participates in SNO+ since the beginning, with responsibilities in several aspects of the calibration system and topics of physics analysis. Analysis of real and simulated data, both from physics and calibration runs in the SNO+ scintillator phases, are the main focus of the project, but it also includes participation in the detector commissioning and data-taking at SNOLAB. The main goal of this thesis project is to contribute to the NLDBD search results with the first year of SNO+ Te-130 data, by using calibration methods to optimize the energy measurement for physics events. The NLDBD signal is a narrow peak in the energy spectrum, at the Te-130 Q-value of 2.53 MeV. A high signal-to-background ratio is strongly dependent on achieving a narrow energy window and therefore a good energy resolution. Energy will be measured in SNO+ by detecting the scintillation light in the PMTs, so the optical effects due to the propagation and collection of light in scintillator, acrylic and water have to be corrected for. Dedicated optical calibration hardware systems have been developed for SNO+, but the full implementation of an optical model for the scintillator-filled detector, and its validation with data, is still needed and is the first major goal of this project. The second goal is to validate the optical model and measured parameters by comparing real data and simulations of events from natural radioactivity present in the detector, and also of events from radioactive calibration sources (gamma-emitters) to be deployed in the detector. This will be a crucial step towards the NLDBD measurement, since it depends strongly on the detector energy response systematic uncertainties, that we propose to determine with in-situ data. After initial results to be obtained in the pure scintillator-phase, the plan will focus on extending and adapting the developed methods to the Tellurium-loaded phase, where the optical properties of the scintillator will be different.