Marshall, George Geoffrey;
(2025)
First Search for Majorana Neutrinos with LEGEND-200.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
The discovery of neutrinoless double beta decay (0νββ) would revolutionise our understanding of physics. It would prove that neutrinos are their own antiparticles, lepton number is not conserved and matter can be produced without balancing an- timatter. The LEGEND collaboration is pursuing a discovery-oriented 0νββ search using hundreds of high-purity germanium detectors made of Ge-76, a 0νββ candi- date isotope. The running stage, LEGEND-200, is projected to provide the world’s best 0νββ discovery power for the next 5 years. This thesis work focuses on the design and implementation of the LEGEND- 200 analysis pipeline, and its application both to the characterisation of novel detectors and its first physics data. It shows how the design performance of LEGEND-200 has been largely met, in particular in terms of the resolution and sta- bility of the energy scale, livetime, and background rates. The background across the array was thoroughly characterised and proved uniform within the statistical uncertainties. This enabled a single parameter treatment in the 0νββ statistical analysis, significantly reducing its complexity. Building on this optimised analysis pipeline and the detector performance char- acterisation and background contributions extracted through it, a blinded 0νββ analysis was enabled on the first LEGEND-200 data set corresponding to an ex- posure of 48.3 kg yr. In addition to performing the entire data reduction and sam- ple preparation, the work presented in this thesis also covers the definition of the main sources of systematic uncertainties and key performance parameters en- tering the collaboration’s statistical analysis. A background index of 5.3 ± 2.2 × 10−4 counts / (keV kg yr) was measured proving that LEGEND-200 will deliver its physics programme. Combining this data set with those of past Ge-76 exper- iments, a combined constraint on the 0νββ half life of > 1.9 × 1026 yr at 90% CL was derived. This is the world leading limit in Ge-76 and one of the most sensitive ones in the field.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | First Search for Majorana Neutrinos with LEGEND-200 |
| Open access status: | An open access version is available from UCL Discovery |
| Language: | English |
| Additional information: | Copyright © The Author 2025. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request. |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10204049 |
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