Nome
Neutrinoless double beta decay search with the SNO+ experiment

Código
PTDC/FIS-PAR/2679/2021

Entidade Beneficiária

LIP - Laboratório de Instrumentação e Física Experimental de Partículas

Sumário do Projecto

The discovery of neutrino oscillations (Nobel Prize 2015) has proven that neutrinos do have mass, but we still do not understand what generates that mass. For all elementary charged fermions, interaction of the Higgs field with the Dirac fields of each particle appears to be the relevant mechanism. For neutrinos another possibility exists. Since they are the only elementary neutral fermions, they can be Majorana particles and their mass can be generated in different ways.The understanding of this question has far-reaching implications in both Particle Physics and Cosmology. If neutrinos are Majorana particles, this would certainly imply new physics and new symmetries beyond the Standard Model of Particle Physics, and would support the idea of generation of the matter/antimatter asymmetry in the early Universe through charge-parity (CP) violation in the leptonic sector. Experimentally, the Majorana nature of neutrinos can be tested by searching for the neutrinoless mode of the extremely rare double beta decays, that are known to occur for several isotopes in the standard mode with two neutrino emission. The search for neutrinoless double beta decay (0nbb) is therefore a high priority of the global particle physics program, and is also the main goal of the SNO+ experiment and this proposal.In order to search for 0nbb, with half-lifes larger than 10^25 yrs, experiments need to have extremely low levels of background and employ massive quantities of a candidate decaying isotope.SNO+ is an underground, low energy and low background experiment using 780 tons of liquid scintillator contained in an acrylic sphere, shielded by ultra-pure water and viewed by an array of photomultiplier tubes (PMTs). SNO+ will search for 0nbb decay by loading ton-scale quantities of the Te130 0nbb isotope in the liquid scintillator, benefiting from its large natural abundance (34%, the largest of the 0nbb isotopes) which doesn’t require enrichment, a Q-value above most of the natural radioactivity, and relatively large nuclear matrix elements that make the 0nbb more favourable to be observed. As of February 2021, the detector was filled with about 740 tons (95% of the total mass) of liquid scintillator and the Te purification plant had been fully installed underground. The isotope loading is expected to be complete by summer 2023, followed by 5 years of data taking.The primary goal of this research plan is to observe indications of the neutrinoless double beta decay of Te130 or establish a competitive limit for its half-life, with the initial SNO+ data. This will be the first time that the scintillator loading technique will be employed with Te130, laying the groundwork towards the full SNO+ Te loading analysis and the next generation of experiments aiming at the normal hierarchy of neutrino masses.In order to reach these physics goals we will provide a thorough characterisation of the detector response and backgrounds impacting both the two-neutrino (2nbb) and 0nbb decay signals by improving the existing methods for particle identification and by developing new advanced algorithms for event classification.We will focus on identifying decays and reactions in the scintillator volume that can fall in the energy region where the signal is expected, their time evolution and spatial non- uniformities. We will track any potential source of contamination, such as radon ingress, leaching off the vessel surface, and muon induced events. Additionally, we will develop algorithms to separate the 0nbb signal from background events based on directionality - a characteristic of solar neutrinos - , and multiple scattering - a signature of gammas from external sources.This research plan will lead to the publication of the first 0nbb decay search results with Tellurium and the loaded scintillator technique -- a high impact outcome in itself -- and to the development of techniques and capabilities essential for the next generation of experiments.This program builds on the LIP neutrino physics group's long standing experience in 0nbb decay searches, event reconstruction algorithms and background characterisation studies, both within the context of SNO+ and other 0nbb experiments.  

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