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Aspects of fine-tuning in theories with extended scalar sectors

PhD ceremony:Mr R.J.C. (Ruud) Peeters
When:May 07, 2021
Supervisors:prof. dr. D. (Daniël) Boer, prof. dr. R.G.E. (Rob) Timmermans
Where:Academy building RUG
Faculty:Science and Engineering
Aspects of fine-tuning in theories with extended scalar sectors

The Standard Model is a theoretical framework that describes the currently known elementary particles and their interactions. This thesis focuses on physics beyond the Standard Model. While there are plenty of reasons to believe that such new physics is necessary, no clear evidence for a specific model has been found yet. This makes it increasingly likely that there is a large hierarchy between the scale of new physics and the masses of the currently known particles.

But it turns out that such a hierarchy often leads to a theoretical problem: fine-tuning. This problem occurs when the parameters of the theory at high energy need to have very specific values in order to produce the observed behaviour at lower energies. If the values would differ only slightly, the results at low energy would be completely different. Such a situation is generally deemed undesirable.

In this thesis, we first consider fine-tuning at the classical level. We investigate various ways to measure the amount of fine-tuning in a model, and highlight that the usual way in which fine-tuning is determined should be reconsidered. Next, we consider the effect of quantum corrections. We investigate a model with a large hierarchy in mass scales, and highlight an issue that appears in the calculation because of this hierarchy. Using a proposed solution to this issue, we find that the amount of fine-tuning decreases with respect to the general approach, but some fine-tuning remains necessary when a large hierarchy is present.