Browsing by Author "Niemi, Lauri"

First-order phase transitions in the electroweak sector are an active subject of research as they contain ingredients for baryon number violation and gravitational-wave production. The electroweak phase transition in the Standard Model (SM) is of a crossover type, but first-order transitions are possible in scalar extensions of the SM, provided that interactions of the Higgs boson with the new particles are sufficiently strong. If such particles exist, they are expected to have observable signatures in future collider experiments. Conversely, studying the electroweak transition in theories beyond the SM can bring new insight on the cosmological implications of these models. Reliable estimates of the properties of the transition require non-perturbative approaches to quantum field theory due to infrared problems plaguing perturbative calculations at high temperatures. We discuss three-dimensional effective theories that are suitable for lattice simulations of the transition. These theories are constructed perturbatively by factorizing correlation functions so that contributions from light field modes driving the phase transition can be identified. Resummation of infrared divergences is naturally carried out in the construction procedure, and simulating the resulting effective theory on the lattice allows for a non-perturbative phase-transition study that is also free of infrared problems. Dimensionally-reduced theories can thus be used to probe the conditions under which perturbative treatments of the electroweak phase transition are valid. We apply the method to the SM augmented with a real $\text{SU}(2)$ triplet scalar and provide a detailed description of dimensional reduction of this model. Regions of a first-order transition in the parameter space are identified in the heavy triplet limit by the use of an effective theory for which lattice results are known. We provide a rough estimate for the accuracy of our results by considering higher-order operators that have been omitted from the effective theory and discuss future prospects for the three-dimensional approach.