First Step into A New Physics Realm: Search for the Majorana Nature of Neutrinos in the Inverted Mass Ordering Region

Abstract

The search for neutrinoless double-beta decay (0𝜈𝛽𝛽) is the only way to prove the Majorana nature of neutrinos and is thus a major area of interest for neutrino physics. Discovering 0𝜈𝛽𝛽 and measuring its half-life will be the first solid evidence for physics beyond the Standard Model (BSM) and lead to a plethora of new theoretical and experimental investigations.

This dissertation contains both theoretical and experimental work. The theoretical calculation of the non-perturbative nuclear matrix element for 0𝜈𝛽𝛽 is done using lattice quantum chromodynamics (LQCD) to interpret the experimental data. The preliminary results of 𝑛⁰𝑛⁰ → 𝑝⁺𝑝⁺𝑒⁻𝑒⁻ process for both long-range (light Majorana neutrino exchange) and short-range (heavy Majorana neutrino exchange) contributions are obtained.

The experimental work focused on KamLAND-Zen 800. It is one of the leading efforts with data from an exposure of 970 kg·yr of ¹³⁶Xe. Machine learning methods are employed to discriminate background events in data and generate new simulations for future study. With no 0𝜈𝛽𝛽 signal excess over the background expectation, statistical properties are extracted by a Bayesian analysis utilizing Markov Chain Monte Carlo (MCMC) algorithm. The lower limit for the 0𝜈𝛽𝛽 half-life of [formula] at 90% C.I., corresponding to the effective neutrino mass range of 38.4-160.0 meV, which is the first search in the inverted mass ordering region.