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Title: Lepton number violation in the nucleus and the universe
Author: Graf, Lukas
ISNI:       0000 0004 7659 8539
Awarding Body: UCL (University College London)
Current Institution: University College London (University of London)
Date of Award: 2018
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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.
Supervisor: Deppisch, F. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available