A model of Leptogenesis in the presence of CPT violating gravitational axion backgrounds

Student thesis: Doctoral ThesisDoctor of Philosophy


In this thesis the role of a CPT violating (CPTV) background field B0 is consid-ered as a mechanism for leptogensis. The benefits of such an approach are that the lepton asymmetry can be produced via tree-level decays of a single heavy right-handed neutrino species that is already present in the see-saw mechanism. This model can then be considered as a minimal/economical extension by building on a current explanation for the light masses of the Standard Model (SM) neutrinos. The fact that the lepton asymmetry can be produced with tree-level processes further shows the simplicity of this approach. The Boltzmann equations are derived for such a system of particles interacting in the presence of the CPTV background and are solved semi-analytically in the high temperature regime above the decoupling temperature of equilibrium dynamics. Although the semi-analytical solution to the Boltzmann equations is only valid in the high temperature regime we are able to improve the validity of this solution to regions outside this regime by the method of Padé approximants. This method increases the confidence of the high temperature regime solution when considering the point of decoupling. Our model of leptogenesis begins by considering a constant CPTV background and shows that in such a scenario a lepton asymmetry is indeed produced. The microscopic nature of the background field is then investigated, by considering the Kalb-Ramond (KR) axion field b(x), part of the massless multiplet of string theory. The CPTV background can be identified as the time derivative of this axion field B0 = b˙. The equations of motion of the KR axion field yields the following temperature dependence for the CPTV background field B0(T)∼T3. Such a temperature dependence causes the field to decrease to negligible levels beyond current experimental detection.
Date of Award1 Aug 2021
Original languageEnglish
Awarding Institution
  • King's College London
SupervisorNikolaos Mavromatos (Supervisor) & Sarben Sarkar (Supervisor)

Cite this