Master Thesis defense by Fabian Depenau

Title: Effective field theory of the lepton number violating processes at colliders

Abstract:

Around the turn of the century it was discovered that neutrinos exhibit an exotic behaviour called flavour oscillations, in which neutrinos of different flavours can turn into each other.

The discovery had two consequences that challenged the otherwise very successful Standard Model of Particle Physics, namely that at least two of the neutrino flavours must have non-zero (but still very small) masses, and that lepton flavour is not a conserved quantity. There are various ways to alter the Standard Model such that it can accommodate neutrino flavour oscillations, but which one is the correct one to describe our reality is now the question.

In this thesis we examine various heavy-particle-extensions to the Standard Model, whose effective contributions to dynamics at particle collider energy levels permit not only lepton flavour, but also lepton number violating processes, namely neutrino-less double beta decay and its collider analogues. Such extensions can also generate contributions to neutrino masses, therefore any lepton number violating operators must be suppressed to keep the neutrino masses small in accordance with observations.

Our end goal is the identification of models that can generate lepton number violating processes without being suppressed by neutrino masses. The list of such theories represent our final result. In particular, we demonstrate that the popular neutrino mass models, type-I, -II, and -III seesaw, do not appear in this list.

A potential loophole in our arguments is a cancellation of contribution of various heavy particles in the resulting lepton number violating observables, akin to the cancellations that appear in the quasi-Dirac limit of the type-I seesaw model.