Naturalness in the Standard Model of Particle Physics and Beyond

Who: Mette Lindharth Als Kristensen (CP3-Origins)
When: Thursday, June 1, 2017

In this master thesis the naturalness criterion, as formulated by K. Wilson and G. ’t Hooft, is used as a guideline, when the Standard Model, and models beyond the Standard Model, are analyzed and discussed. It is shown that the Standard Model is highly unnatural, due to a fundamental Higgs sector. If the Standard Model is to be valid to the Grand Unified Scale the fine tuning required to explain the observed Higgs mass is $latex \frac{1}{10^26}. For this reason the Standard Model appears to be an incomplete description of physics at the electroweak scale and extensions well motivated. Three different models, which all replaces the Higgs sector of the Standard Model with a new strongly coupled sector called technicolor, are discussed. In the Minimal Walking Technicolor Model the massive spin one bosons acquire their masses in a completely natural way. The Standard Model fermion masses arise from four fermion operators, requiring additional new physics beyond technicolor. This can either be a fundamental scalar, which is unnatural per se, or a new strongly coupled gauge theory, which is broken at a high energy scale. In this model framework it is challenging to explain the observed 125 GeV mass of the Higgs boson, and therefore a variant model termed composite Higgs Model is discussed. In this model the Higgs boson is a Goldstone Boson of the strongly coupled sector, but the model introduces new free parameters and requires some degree of tuning of two different mechanisms to be phenomenologically viable. Finally we discuss a recent specific realization of the Composite Higgs framework, where the Higgs boson arises as a mixture of a fundamental doublet and a composite Goldstone boson. We term this the Partially Composite Higgs model. We discuss the shortcomings of this model as currently presented in the literature, and we provide a novel study of the constraint from vacuum stability on the viability of this model framework.