Zhang, Zhong;
(2025)
Probing Heavy Neutral Leptons in Direct Searches, Double Beta Decay and Cosmology.
Doctoral thesis (Ph.D), UCL (University College London).
Preview |
Text
Zhang_10203103_thesis.pdf Download (8MB) | Preview |
Abstract
Heavy Neutral Leptons (HNLs) are a popular extension of the Standard Model (SM) to explain the lightness of neutrino masses and the matter-antimatter asymmetry through leptogenesis. This work studies the phenomenology of low scale Seesaw HNLs with their mass between 1 MeV and 1 GeV in direct searches, neutrinoless double beta decay and cosmology. Future direct searches, such as fixed target setups like DUNE, and neutrinoless double beta decay, such as LEGEND-1000, are both expected to probe the regime of the standard Seesaw scenario of neutrino mass generation for HNL masses around mN ≲ 1 GeV. We analyse the complementarity between the two experiments to probe the nature of HNLs, i.e., the Majorana nature and CP-violating phases in the HNL sector. Following an analytic discussion of the complementarity, a statistical analysis is performed in the combined search for HNLs. HNLs in the MeV-scale mass range, are very constrained by Big Bang Nucle- osynthesis (BBN) as their decays significantly impact the formation of the primor- dial elements. We propose here a model where the primary decay channel for the HNLs is to an axion-like particle (ALP) and a neutrino. Consequently HNLs decay much earlier and can evade the BBN bound for much lower masses, provided the ALPs themselves decay considerably later. Further cosmological and astrophysical constraints limit severely the range of validity of the ALP properties. We find that a new parameter region opens for HNLs with masses between 1 MeV and 1 GeV, and active-sterile mixing between 10−9 − 10−6 that is consistent with observations. In such a scenario, current HNL bounds, as well as sensitivities of future direct HNL searches such as at DUNE or PIONEER, will be affected.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | Probing Heavy Neutral Leptons in Direct Searches, Double Beta Decay and Cosmology |
| 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/10203103 |
Loading...
Loading...Loading...Loading...Archive Staff Only
 |
View Item |
