Van Goffrier, Graham; (2024) Nuclear and Particle Physics Aspects of Neutrinoless Double-Beta Decay. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Neutrinoless double-beta (0νβ β) decay is an as-yet unobserved nuclear process, which stands to provide crucial insights for model-building beyond the Standard Model of particle physics. Its detection would simultaneously confirm the hypothesis that neutrinos are Majorana fermions, thus violating lepton-number conservation, and provide the first measurement of the absolute neutrino mass scale. This thesis aims to interrogate the status, implications, and improvement of the dominant source of theoretical uncertainties which constrain inference from 0νβ β searches: the nuclear matrix elements (NMEs). We focus our investigation into two distinct strands. The first strand explores the estimation within chiral effective field theory of the so-called “contact term” for 0νβ β-decay, a short-range two-nucleon effect which is unaccounted for in traditional nuclear approaches to the process. We conduct a thorough review of the justifications for this contact term and the most precise computation of its size to date, whose precision is limited by a truncation to elastic intermediate hadronic states. We then perform an extension of this analysis to a subleading class of inelastic intermediate states which we characterise, delivering an updated figure for the contact coefficient with uncertainty reduced by half. Such ab initio nuclear results, especially with enhanced precision, show promise for the resolution of disagreements between 0νβ β NME estimates from different manybody methods. The second strand explores the influence of current and improved NME uncertainties on the ability of planned searches to constrain both standard and exotic 0νβ β-decay mechanisms. In particular, we perform a Bayesian analysis of the multi-mechanism constraints which may be extracted from multi-isotope observations, across isotopes ⁷⁶Ge, ¹⁰⁰Mo, and ¹³⁶Xe. Our results indicate quantitatively the conditions under which two or three of these isotopes can provide simultaneous information on two types of New Physics, especially as the precision calculation of the NMEs continues to improve.

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