Topics in particle physics and cosmology

Abstract

The Standard Model of particle physics, together with the Big Bang model of the early universe, constitute a framework which encompasses our current understanding of fundamental laws and beginning of our universe. Despite recent speculative trends, quantum field theory remains the theoretical tool of choice for investigating new physics either at high energy colliders, or in the early universe. In this dissertation, several field theoretic phenomena relevant to cosmology or particle physics are explored. A common theme in these explorations is the structure of the vacuum state in quantum field theory. First, we discuss first-order phase transitions in the early universe, in which the effective vacuum state of the universe shifts discontinuously as the temperature drops below some critical point. We find that the dynamics of a certain type of first-order phase transition can lead to production of primordial black holes, which could constitute the dark matter of our universe. Alternatively, supercooled first-order phase transitions may be the cause of an extended inflationary epoch in the early universe, which is generally regarded as necessary to solve several cosmological puzzles. We derive limits on such scenarios based on nearly model-independent percolation properties of the transition. We also study some nonperturbative aspects of the field theory vacuum. We show that non-topological solitons of a single fermion and Higgs fields can only exist in strongly coupled theories. In particular, we find that at the lowest fermionic excitations in the Standard Model are single fermions, and not bound states of fermion plugs Higgs. Finally, we investigate the intriguing behavior of instanton-induced cross sections. We discover Higgs-Higgs cross sections which increase exponentially with center of mass energy due to the presence of instanton solutions related to vacuum instability.