Phase transitions: applications to physics beyond the Standard Model

Abstract

Despite their phenomenological successes, the Standard Models (SMs) of particle physics and cosmology remain incomplete. Several theoretical and observational problems cannot be explained within this framework, including the hierarchy problem, dark matter (DM), and the baryon asymmetry of the Universe. The objective of this thesis is to investigate phenomenological and theoretical aspects of the solutions to these issues. We consider two kinds of phase transitions that can occur in the early or late Universe in extensions of the SM, that can be either responsible for dark matter and/or baryon asymmetry production or may be used to constrain possible models of new physics. In the first part we analyze string theory-inspired models where the Universe transitions from matter- to radiation-dominated evolution just before Big Bang Nucleosynthesis through out-of-equilibrium decays of a scalar modulus field. We employ these decays to produce DM and for baryogenesis. We study the phenomenology of these scenarios and its implications for high-scale physics. The second part of this thesis is dedicated to thermodynamic and quantum phase transitions in the early and late Universe, respectively. In the former case, we investigate the dynamics of the electroweak phase transition when the electroweak symmetry is broken down to electromagnetism in the Inert Doublet Model, a simple extension of the SM that can account for DM. Such transitions can generate the baryon asymmetry in a process called electroweak baryogenesis. Some extensions of the SM also predict similar transitions through quantum tunneling that break the colour and electromagnetic symmetries, indicating that our ground state is unstable. We use these arguments to put new constraints on the Minimal Supersymmetric Standard Model.