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Lepton Number Violation in the Nucleus and the Universe

Graf, Lukas; (2018) Lepton Number Violation in the Nucleus and the Universe. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Conservation of lepton number is an intriguing feature common for all particle interactions so far observed in Nature and it can be understood as a consequence of an anomalous abelian symmetry of the Standard Model. However, there are certain hints, most importantly the experimental evidence of non-zero neutrino masses, pointing at breaking of the lepton number symmetry at certain high energy scale. Although the new lepton number violating physics can generally lie beyond the reach of collider experiments, it is also possible to probe it at low energies. Most importantly, an observation of neutrinoless double beta decay would provide an evidence of non-conservation of lepton number, shedding light on the origin of non-zero neutrino mass at the same time. In this work we concentrate on the effective description of the non-standard mechanisms of this extremely rare nuclear process, drawing its connection to lepton number violation at high energies. Moreover, assuming its hypothetical observation we discuss some of the possible cosmological implications. Specifically, following a brief review of the long-range mechanisms triggered by 6-dimensional operators, we study in detail the short-range mechanisms contributing to neutrinoless double beta decay as 9-dimensional operators at Fermi scale. After deriving the nuclear matrix elements and phase-space factors involved, we determine limits on the respective particle physics parameters. Constraining these lepton number violating couplings allows to estimate the high-energy scale of the corresponding new physics. Unlike in the standard case, non-standard mechanisms yield energies of order of a few TeVs. Presence of lepton number violation at such low scales implies together with sphaleron transitions a potential washout of a primordial lepton and baryon asymmetry. Consequently, restrictions may be imposed on scenarios of the observed matter-antimatter asymmetry generation. We argue that certain models of high-scale baryogenesis can be generally excluded, if a non-standard neutrinoless double beta decay is observed.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Lepton Number Violation in the Nucleus and the Universe
Event: UCL
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2018. Original content in this thesis is licensed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0) Licence (https://creativecommons.org/licenses/by/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 > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/10063432
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