Probing new physics mechanisms in neutrinoless double-beta decay with SNO+

PhDIn this thesis, I present the theory of neutrinoless double-beta decay (0 2 ), particularly the theory of exotic modes of 0 2 involving the emission of one or two Majorons. Alongside this, I summarise the most recent results in the experimental search for 0 2 , including limits on the rate of these exotic processes. I describe the SNO+ experiment and it’s physics goals, which include the search for 0 2 . As part of the SNO+ collaboration, I have made a signi cant contribution towards the development of the data quality software that is essential for ensuring SNO+ can achieve its physics goals—including in 0 2 searches. I describe how I developed a software package that performs the high-level data quality checks. Continuing with the software theme, I then describe a python-based limit-setting and tting software package called echidna. I have been a lead developer of echidna as part of my PhD, so I describe the software in detail and how it can be used to set limits on 0 2 signals. By reproducing the sensitivity results of the KamLAND-Zen experiment, in four key Majoronemitting 0 2 modes, I verify the use of echidna as a limit-setting tool for this type of search. Finally, I present the results of a comprehensive sensitivity study, where I determine the potential sensitivity of SNO+ to the same set of Majoron-emitting modes that KamLAND-Zen and other 0 2 experiments have already investigated