The Standard Model is the best particle physics theory we h ave\, but there are still phenomena that it cannot explain. In this thes is I have worked on two different projects that connect to two of the bi ggest unsolved questions of the Standard Model.

From observations of neutrino oscillations we know that at least one of the neutrinos has to be massive. But the neutrinos of the Standard Model are massless. The first paper in the thesis investigates a simple extension of the Standa rd Model that realizes a fifth force as a U(1) gauge group. In such mode ls\, extra care has to be taken to not introduce inconsistencies known a s anomalies. It turns out that the simplest way to avoid these problems is to introduce three right-handed neutrinos. Such models can then incor porate neutrino masses in a convenient way. In the second paper we have investigated a twist on this model that does not have neutrino masses\, but which makes other interesting models possible&mdash\;such as a model with gauged lepton number.

The observed asymmetry between matter and antimatter cannot be explained by the Standard Model. One of the mor e popular of the possible explanations is known as electroweak baryogene sis. In this scenario the asymmetry is determined during the electroweak phase transition in the early universe. The second project&mdash\;spann ing the three final papers of the thesis&mdash\;has aimed to improve the approximation methods we use to calculate features of this phase transi tion. Such calculations are plagued by two big problems that seem to com pete with each other. On the one hand\, gauge invariant results seem to demand that a strict loop counting must be enforced. On the other hand\, the loop approximation does not work well close to the phase transition . We argue that the solution is to use a different power counting\, but still be strict about sticking to it.

DESCRIPTION: SUMMARY:Disputation: The Powers of Perturbation Theory: Loops and Gauge I nvariance in Particle Physics LOCATION:Ångströmlaboratoriet\, Polhemsalen och Zoom TZID:Europe/Stockholm DTSTART:20201211T091500 DTEND:20201211T235900 UID:20201211T091500-57366@uu.se END:VEVENT END:VCALENDAR