Towards the neutrinoless double-beta decay study with SNO+: radioactive background characterisation with SNO+ scintillator data

0νββ is one of the most relevant processes in particle physics, as it has the potential to unveil the Majorana neutrino’s nature, and its mass generation mechanism. SNO+, a multipurpose neutrino detector, will load 130Te into its large liquid scintillator in order to search for this rare process. The aim of this thesis is to characterize all the SNO+ backgrounds that are potentially dangerous for the 0νββ signal in data collected before SNO+ loads Tellurium.

This work included three main steps: (T1) identification of the dataset to use; (T2) selection of variables to remove known backgrounds and (T3) fit of the resulting energy spectrum. (T1) was performed using a coincidence tagging technique, which allows analysing an excess in 214Bi following from Rn ingress. In (T2) backgrounds like instrumentals, muons, externals, coincident and pile-up events have been rejected. The impact of these cuts was evaluated on livetime and efficiency.

In (T3) Maximum likelihood fits were performed on data, collected between May 2022 and March 2023 (228.2 net livetime days). Background PDFs from the 238U, 232Th and 235U chains, muon induced backgrounds and Solar neutrinos were used. Lastly, the time dependence of events above 1.5 MeV, was studied in intervals of 15 days, for the three energy regions: [1.5,1.8], [1.8,2.2] and [2.2,2.6] MeV to see any potential deviation from a constant trend, indicating a background ingress.